Transcript for:
CHM 103 Ch 2

in this chapter we're going to look at the relationship between energy and matter and how do we approach chemistry from that standpoint i think i've pointed out in the previous chapter that chemistry is the science of matter and that we would go into more detail into what that actually means in this chapter so what is matter i i think i briefly described it as just physical stuff back in chapter one so it's any tangible material something that has mass that takes up space and in chemistry we kind of focus on what exactly that means in terms of what are the pieces that make it up how are they organized and so on so as far as chemistry goes we say that matter is made up of these small particles called atoms now they can be organized into little groups called molecules and we'll go into more detail into how this happens in a later chapter on chemical bonds but for now understand that matter involves these small building blocks called atoms okay so everything's made up of these atoms right so uh like basically how if you could divide matter into smaller and smaller pieces the smallest piece you could get from a chemistry standpoint is an atom an atom is a building block of matter in chemistry now is possible to go to even smaller and smaller pieces in order to understand how atoms work we kind of have to do that so in a later chapter when we understand what atoms are we will look at subatomic particles and if you go into particle physics you'll definitely go into even more detail on this but as far as we're concerned for chemistry the smallest practical units are these atoms so in addition to matter we're also going to look at energy now energy is the capacity to do work in in other words when matter interacts with things it carries out certain processes energy is what's exchanged when these processes happen okay so so anytime there's a physical or chemical change there's some exchange of energy this quantity that we call energy so we'll go into more detail on that later on in this chapter we'll understand what are physical and chemical changes uh very briefly we'll go into more detail on that in later chapters but we'll understand what these physical and chemical changes are and what energy is and how they're related so let's focus on matter for now so i pointed out that all physical material in the universe is matter and we can divide that into certain subcategories uh in the previous chapter i'm passing in passing i mentioned pure substances these are where these are samples of matter where all those pieces making them up are identical okay so if you have a a uniform material where all those you know that you could divide into these repeating units that are all identical so to speak we call that a pure substance okay now if the repeating units are molecules that substance is called a compound whereas if all the atoms of your material are identical we say that that is an element okay um we'll when you have more than one substance mixed together that brings us to mixtures so let's uh not get too far ahead let's look at each of these in turn okay so starting off with pure substances um if you have a for example a glass of distilled water now if you were to zoom in that sample of water you would see these repeating water molecules okay so so we call that distilled water a pure substance because all of those molecules are identical and there's only those molecules present in the sample so we have this repeating unit of these water molecules and that is a pure substance now is that an element or compound we look at those repeating units those water molecules and we see that they're made up of more than one type of atom a water molecule so if you're probably familiar with with the formula of water as h2o so a water molecule has one oxygen atom and two hydrogen atoms so if we were to look at a sort of a cartoon of a water molecule we'd see an oxygen atom and two hydrogen atoms attached to it okay and the fact that there are different types of atoms within this one water molecule tells you that this is a compound okay a compound contains more than one element making up its molecules now it is possible to get pure substances where all of your repeating units are the same type of atom so for example if you had a bar of 24 karat gold okay that bar of gold has only gold atoms in it it's pure gold and since gold is an element on your periodic table we call that sample of 24 karat gold an element okay so the the key thing to uh to determine here is that you can't break down an element into any simpler substance through at least through chemical reactions uh atoms are the simplest building blocks as far as chemistry goes so we can't break them down further at least not through simple ordinary chemical reactions in a compound though it is possible to break those those molecules into their constituent atoms into their constituent elements now it's often a little difficult to do this so for example with water you could separate that into hydrogen gas and oxygen gas if you ran electricity through your sample of water it's a process called electrolysis likewise there are certain chemical reactions that do that are able to carry out these sorts of you know processes it is possible to split apart a compound into its constituent elements this way okay but my point here is that it's not typically easy to do you have to use some sort of chemical method okay now the other interesting distinction with compounds is that your ratio of elements is always fixed and that's what the mole how the molecule is defined so water has the formula h2o because there's two hydrogens for every one oxygen and that is a ratio that makes water water if you had a different ratio of hydrogen to oxygen it would not be water so in order for it to be water you have to have that ratio of those elements okay and that's something that's going to be important to keep in mind when we look at mixtures in a second okay another thing that's interesting to note about compounds that help you determine that it's a compound is that the physical and chemical properties of your compound are going to be very different compared to your elements let me give you an example here sodium chloride table salt is nacl it's made up of sodium which is uh and the na in nacl na is the symbol for sodium and the cl is chlorine okay and that's what makes the chloride ions in your sodium chloride now if you look at sodium sodium chloride table salt i think most of you are probably familiar with it you've seen it's a white crystalline solid but compare that to the sodium and the chlorine that it comes from okay sodium metal is a very silvery soft metal okay and and chlorine gas is this sort of greenish yellow very reactive gas okay there was a very very different from our white crystalline solid that's sodium chloride that has a really high melting point uh the melting points of sodium metal uh and chlorine gas are much much lower than that um i mean in the case of chlorines obvious it's lower than than room temperature so that's why it's a gas room temperature so my point here is that you can tell that you've got a compound because it looks different than the elements that make it up that brings us to mixtures now a mixture is when you have more than one substance together so it's not really a pure substance where there's only one type of substance you've got multiple substances mixed together hence the name mixture okay so this mixing process is a physical process so we'll find that we can separate out the parts of a mixture through a physical process okay for example you could take a mixture of sand and water and you can filter out the sand or you could take some salt water and distill off the water okay now the other interesting thing here that makes mixtures different uh is that there isn't any rule as to what what ratio you mix your parts of your mixture in so for example if i wanted to make a salt water solution i could add a little bit of salt and a lot of water make a very dilute solution or i could add a lot of salt with relatively little water make brine for example both of those are mixtures of salt and water but they're not the same okay so there's no fixed ratio like we see in the fixed ratios of elements making a compound okay so that's that's a very important distinction to make here i think this is a common sticking point for a lot of students who get mixed up with why is something a mixture and not a compound and vice versa okay so just here i'll use sodium chloride here as my as my example of a compound when we want to distinguish things here right so for example i pointed out that we can separate out the parts of a mixture really easily through some physical process like filtration or distilling and so on okay in the case of a compound you need a chemical process to do this you need a chemical reaction or electrolysis you can you can do it with electricity but it's not as simple as just you know a physical method like filtration so in the case of sodium chloride you can't separate out the sodium ions or the sodium atoms from your sodium chloride you need a chemical reaction to reduce that those sodium ions back down to sodium metal okay or you can run electricity to it through it you can make do electrolysis to get the sodium and chlorine back again but it's not as simple process like but you can't just filter out sodium likewise with water you can't just filter out your hydrogen atoms from those because you've got to break chemical bonds to do this okay um the other thing to to notice is the uh i think i mentioned the ratios thing already so uh in a compound there's always a fixed ratio of of elements right so sodium chloride has a one-to-one ratio of sodium to chlorine there's one sodium for every one chlorine but in the case of a mixture you can have variable ratios okay so you can if you make salt water uh you can have it doesn't matter how much salt or how much water you have you can make you can mix them together the uh third way you can distinguish these is by looking at the um the properties of your mixture or your compound compared to the things making it up in i mentioned that with sodium chloride sodium chloride as a compound is very different from the elements making it up sodium and chlorine in the case of a mixture instead of getting new properties your mixture kind of has the sum total of your prop of your parts of the mixture so for example let's say we're looking at salt water okay so you mix salt and water together the resulting solution you get is still wet like water it still tastes salty like salt okay so you see where i'm going with this like you kind of get both sets of properties from all the parts of your mixture okay now when it comes to mixtures just as we can distinguish pure substances into elements and compounds mixtures can also be subdivided into two types we have what are called homogeneous mixtures and heterogeneous mixtures now homogeneous means same phase so the the prefix or homo means same okay that means when you look at that mixture it looks like it's one material this is actually where a lot of students do run into that problem of getting compounds and mixtures um well mixed up they when you look at a homogeneous mixture just looking at it it's hard to tell that it's a mixture that there's more than one thing it looks almost like a pure substance okay so you've got to watch out for for homogeneous mixtures like that so salt water is an example of this so if you dissolve salt in water the salt kind of looks like it's disappearing as it dissolves right you only see the water but we'll talk we'll spend an entire chapter talking about homogeneous mixtures later on uh in the semester it's uh the chapter on solutions and i'll explain how this happens but for now just understand that the key when you have a homogeneous mixture it looks uh like one material it looks like the majority of that mixture a heterogeneous mixture as the name implies has different parts or different phases that's the what the prefix hetero means means different you can see the different parts of your mixture so for example if you have a mixture of sand and water so you have some muddy water just looking at it looking at that cloudiness you can tell like okay there's something in here that's not water it's not clear it's not transparent like water should be there's clearly something in here that's making this cloudy okay uh so that that's an example of that so if you can see the different parts of your mixture or at least like tell like visibly that there are different parts you know it's a heterogeneous mixture okay so here's some examples to keep in mind so you know again like if you know there's more than one substance mixed together and that's a mixture if looking at that mixture it looks like one uniform thing then you know it is a homogeneous mixture so a sugar solution looks like the water making up that solution even though it's a mixture of sugar and water uh 14 karat gold which is a mixture of copper and gold looks like gold right so it's a homogeneous mixture even though it's a mixture of two different things um air air is another good example of this like air is a is a homogeneous mixture where you have multiple gases mixed together okay typically with homogeneous mixtures things are mixed really well that's another way by the way that you can tell that you're dealing with a homogenous mixture it's a very even spread of the parts making it up so for example with air you've got about 79 nitrogen and about 20 oxygen and then a few other gases right so they are mixed very evenly so it's not like uh when you're in a room the air feels any different on one side of the room versus the other right you don't go to one side of a room and say oh it feels like there's more oxygen on this side of the room no the air in that room is very evenly mixed and so it seems like one uniform substance even though it's made up of multiple gases okay whereas with heterogeneous mixtures as i pointed out usually you can look at it and tell immediately like okay there's more than one thing here so for example if you have oil and vinegar salad dressing you can see the globules of of oil floating around in your vinegar right or vice versa i guess depending on which which one you have more off in your dressing okay or like if you've got oatmeal raisin cookies you can see the raisins in your cookies like it's you know those are kind of silly examples but but that's my point with the heterogeneous mixture you can see that there's something else in there yeah blood and milk are examples of other heterogeneous mixtures they're mostly water but clearly they don't look like water there's obviously other stuff in there that makes them more opaque okay so just to summarize what we've learned here when it comes to to matter all matter can be subdivided into two main categories we have pure substances and we have mixtures mixtures can be subdivided further into elements or compounds okay where in an element all of your repeating units are identical atoms whereas in a compound your repeating units are identical molecules and molecules can be split up into their constituent atoms their constituent elements using some chemical method or electricity okay so a chemical reaction or electricity can get your compound molecules to be split up into their elemental atoms okay mixtures can also be subdivided into two categories we have homogeneous mixtures and heterogeneous mixtures okay homogenous mixtures look like one uniform material and that's why they're kind of hard to distinguish from pure substances so you've got to watch out for that and then heterogeneous mixtures you can see the different parts now what distinguishes mixtures from substances well remember with mixtures you've got more than one substance together so that that's why it's not a pure substance but you can separate out a mixture into the pure substances making it up using some physical method so for example if we have our salt water solution if we distill off the water making that solution you know so we boil off the water and condense it the water you get is pure water right though so that's that's a key thing to keep in mind that mixtures are made up of things that by themselves would be pure substances it's just that when you mix them together they well become mixtures okay i alluded to properties of matter earlier now what do i mean by properties properties are a thing or a piece of information that we can pick up about materials okay as we as we look at them or measure them and this can help us identify again it just tells us information about these substances now there are two types of properties we look at we have what are called physical properties and chemical properties okay and how we we measure them or get information from them is slightly different okay now these properties as i pointed out are directly observable they can you can measure them you can observe them sometimes it involves the interaction of other substances with your material to notice this this is especially the case in chemical properties which i'll get into in a second okay so let's talk about physical properties first so physical properties are properties of a material okay there are characteristics or information we're getting about a material that can be observed or measured without changing the material so you can determine a physical property from a substance and it's still the same substance even after you determine that okay now this includes things like color or smell or physical state at room temperature like if it's a solid liquid or gas density so you know like for example like i can look at um you know at a material and say like oh that is red but that doesn't change the material by looking at it right so for example i can look at a bar of iron and say okay that's gray in color but me looking at that bar of iron doesn't change it into something else it's still iron when i'm done looking at it right now out of these melting point and boiling point are also physical properties and this throws off a few students because uh you're probably saying to yourself wait a minute to determine the melting point don't i have to melt the thing i'm checking isn't that changing it isn't going from solid to liquid it's you're changing its physical state yes but it's still the same chemical so for example if i take a block of ice and i melt it to into liquid water to confirm that yes its melting point is zero degrees celsius it's still made up of water molecules whether it's in ice or liquid water form that's still water molecules making up that substance so since the water molecules are still water molecules that is a physical property not a chemical one okay and same for boiling point so if i take my liquid water and boil it into steam and you know note that the boiling point is 100 degrees celsius doing that doesn't make the water molecules stop being water molecules okay so so here's an example so here's some a sample of copper so some physical properties of copper include seeing that it's got this reddish orange color that copper metal is very shiny it's a good conductor of heat and electricity it's got a melting point of 1083 degrees celsius so it's a solid at room temperature and it's got a really high boiling point these are all things you can observe while the copper is still copper okay so determining that it's shiny doesn't stop it being copper running electricity to it or warming it up doesn't stop the copper from being copper it's still copper even after you pass electricity through it okay so if those are physical properties a physical change is kind of similar it's where you're changing a material but it's you're not changing the substance making it up okay so it's some change to a material that doesn't alter the atoms or molecule the molecules making up that um that substance okay now the the simple thing here is that there's really only two ultimately two scenarios where this applies uh you're changing the shape of a material and or you are changing its state of matter from solid to liquid to gas or vice versa okay so for example if i take a sheet of paper and i rip it in half i've changed the shape of that paper right i've it's no longer the same size i've got two smaller pieces of paper but are the pieces of paper still paper yeah there's still paper so that was a physical change for example okay so yeah so changing uh you know like i said changing shape typically is an example of of a physical change so how you can do that in you know multiple ways to do that with metals metals are very malleable you can like hammer them into shapes really easily or draw them into wires that means they're ductile liquids can change their their shape even though they keep the same volume by changing their container so that's again a good example of a physical change if you pour let's say water from one container into another its shape is changing because it's taking up the shape of the container it's in and of course as i mentioned earlier the changing of physical state by melting or boiling those are also examples of of a physical change now to understand what's going on there let's first talk about solids versus liquids versus gases uh there are other states of matter out there um but to be honest with you these three are kind of the main ones and the only ones we'll focus on in this class okay so a in order to distinguish between between these three states of matter the two things i want you to pay attention to are shape and volume okay and ask yourself are these fixed or variable a solid has a fixed shape and a fixed volume okay it maintains its shape and volume without anything being done to it you could leave it alone and it still has the same shape and volume it doesn't need a container or anything to hold it to that shape and volume okay now a liquid on the other hand has a fixed volume but its shape is variable okay so a liquid can be poured into a container from one container and to another and its shape will change depending on the shape of the container okay that's what i mean by it having a variable shape not having a fixed shape however the volume is fixed that it doesn't matter what container you pour your liquid into that sample will keep it same volume okay so for example uh if i took a um you know a container you know let's say a small beaker of water let's say this was like you know i fill this up and this has 50 milliliters of water and i pour it into a larger container it's different shape and this is a hundred milliliters let's say it would take up the shape of its container but the volume marking would it would only fill up to the 50 milliliter point the volume would stay the same okay so that's what i mean by the volume staying fixed even though the shape would change accordingly okay now that brings us to gases gases have neither a fixed shape nor a fixed volume they will take whatever the shape of whatever container they're in and the volume of whatever container they're in that's why you're able to uh it's possible to compress or expand the gas very easily whereas you you can't do that with salts and liquids okay a quick note here about this uh this tendency of liquids and gases to take the shape of their containers this is why it's possible to have liquids and gases flow you can pour them and this is why both liquids and gases are known as fluids all right so you if you've heard that expression that word fluid before uh normally when you talk about fluids most people think of liquids uh and again that's kind of the regular english use of the word fluid in science in chemistry and physics when we talk about fluids we really mean both liquids and gases liquids and gases are both considered fluids from a chemistry and physics standpoint now why do we see this why do solids have fixed volumes and and fixed volume and shape why does a liquid only have a fixed volume but an indefinite shape and why do gases have neither why are they variable for both volume and shape it comes down to the way those molecules are arranged in those substances okay if you look let me use water as an example here so if you have solid water or ice and you look at the water molecules making up ice crystals those molecules are arranged in a fixed repeating pattern like this called a crystal structure uh we'll we'll talk more about that in a later chapter but uh basically it's this set arrangement of these molecules the the fact that they're held in these fixed positions that causes them to have that fixed shape and volume because they can't really move around very much they're kind of you know maintaining that shape and volume of that substance now a quick note here about movement your water molecules in a solid or really any molecules that are present in a solid they're held in roughly fixed position they might vibrate very gently back and forth so there is technically some movement in there as long as you're above absolute zero you know your material has some potential energy and so you know those those molecules are vibrating back and forth very very gently um but they're not moving a lot is so when i say that they're vibrating in fixed positions they're roughly staying in the same place okay and that's why solids are solid liquids on the other hand have are don't have their molecules set in fixed positions however the molecules in your liquid water are still pretty close together so that's something solids and liquids have in common their molecules are relatively close together the only difference is in a liquid these molecules aren't held in place they're constantly sliding past each other moving around randomly okay and that's the key difference that's why they can't maintain a shape right they have a variable shape but because they're so close together and so you can't pull them apart or push them closer that's what gives them a fixed volume okay now gases on the other hand have neither fixed shape nor fixed volume the reason for that is again the arrangement of those molecules in a gas your molecules are very spaced out they're not held in place they're constantly moving around but unlike a liquid where the molecules are all close together in a gas they're far apart and it's this distance apart that allows gas molecules to have a variable volume you can play around with all this empty space that's in between your molecules you can push those molecules closer together because there's a lot of give there there's a lot of room to do that or you can pull them apart and just increase the amount of empty space between them okay there's nothing stopping you there so that's why gases don't have a fixed volume as well as not having a fixed shape so i mentioned that change of state from solid to liquid of gas is a physical change all you're doing is you are just changing the arrangement of those molecules but those molecules are staying the same so be ready for recognizing these physical changes that or change of state when you know and recognize that they are physical changes now the words that we use to describe these changes are probably familiar to most of you so when a solid becomes a liquid we call that melting when a liquid goes to a solid we call that freezing right again probably pretty common words you've heard associated with ice and water likewise when a liquid goes to a gas we call that boiling okay or evaporation and when a gas becomes a liquid we call that condensation okay now a quick note here for those of you who haven't heard these these phrases before um and and you probably won't get asked about this but in case you do or in case you're curious what do you call it when a solid become goes directly to becoming a gas or a gas goes directly to becoming a solid without going through the liquid phase now you're probably saying to yourself like wait a minute does that actually happen uh it actually does and if you go on to chem 111 and 112 you'll study phase diagrams and you'll see how this is possible if you have a high enough pressure it's possible to do this but basically when a solid a solid becomes a gas that process is known as sublimation okay so an example of this would be carbon dioxide uh it's possible to get solid carbon dioxide what's called dry ice uh you may have seen that in uh in packaging for things that need to be kept really cold so if you go to like a ups store or something and you want to send someone food they'll usually pack it in dry ice for example okay with uh and so your dry ice will go directly to carbon dioxide gas so we say it sublimes into carbon dioxide gas okay the reverse process by the way if a gas becomes a solid directly that's known as deposition you deposit a gas as a solid okay so those are physical changes in physical properties right like i said we're we're kind of limited in what can happen here because in order for a physical process to happen or physical change your material has to chemically be the same before and after the process that brings us to chemical changes and chemical properties chemical properties are ones that allow that we observe only when a chemical change happens and for a chemical change to happen your material needs to change into something else okay so it is now a different chemical compound now and this new chemical there are signs that this happens your new chemical compound will have uh new physical and possibly chemical properties uh and usually there's some things you can observe that tell you that this is going on okay so so typically uh when uh when you see a chemical change also known as a chemical reaction uh like i said that you're you're changing your material so you're breaking and reforming new chemical bonds okay so that's what causes one compound to change into another you're you're not just rearranging um you know the the atoms uh the molecules as they are but you're actually changing rearranging the atoms and making new molecules okay so for example you know i i think i gave you an example of tearing a sheet of paper in half and now both halves are still paper that's a physical change well what if i set that paper on fire when i'm done i only have ash left okay that ash is different than the paper i started with right so that is a chemical change my burning of paper into into ash is a chemical change because the ash is different from the paper i started with the chemical property there by the way is the flammability of the paper so the ability of the paper to be burned and form ash that is its um its chemical property that we observe there okay uh and again there are some there are a lot of physical or things you can observe that tell you a chemical reaction is going on so some of these signs uh of a chemical reaction and by the way you're not going to see all of these at once uh you might get more than one of them but usually it's only a couple of them so often times uh there is you know heat and light being given off right there there's a change in temperature a change in temperature that kind of sustains itself you know so it's not like when you're melting something you've got to keep on heating it no it's like it gives off heat by itself as the chemical reaction is going on uh so a good example of this is when you're burning something right so so during a combustion reaction so if i burn my paper it gives off heat and light by itself i'm not continuously heating it up to burn it right it'll continue to burn itself sometimes you'll see changes in physical properties so so smell and odor are and sorry smell and color are physical properties and the fact that they're changing tells you that i've you've made something that has these new physical properties why does it have new physical properties because it's a new substance right it's a new chemical substance that you've made okay so so a change in in smell or change in color tells you that a chemical change has occurred okay so smell and and color by themselves are physical properties right if you're looking at something and it's got a particular smell or color you're observing that you know physically but if you notice something happens and those change that tells you that that change was a chemical change um other signs of a chemical reaction are the presence of effervescence which is just a funny just a fancy word for bubbles okay so if you see bubbles appearing even though you're not boiling the material that tells you that a chemical change is happening and the product is a gas or one of the products at least is a gas and those are the bubbles you're seeing okay so for example when you drop an alka-seltzer tablet into water and it starts bubbling well that that's an example of a chemical change right the alkyl cells or tablets are reacting with water and they're producing carbon dioxide gas okay a precipitate is another sign that a chemical reaction has occurred if you mix two clear solutions and they turn cloudy that cloudy stuff is actually a solid and we call that a precipitate okay so it's any solid that crashes out when you mix together two clear solutions it's the chemical reaction the chemical change that's going on between those those two solutions that causes the production of that precipitate okay so again you're not going to see all of these in a given chemical reaction but the presence of any of them should tip you off that some chemical reaction is happening okay so so make a note of that when you're when you're looking at things happening and and determining whether it's a physical or a chemical change now we'll spend an entire or an entire chapter but a good chunk of a later chapter talking about chemical reactions uh so we'll come back to this in a later chapter but but for now at least be ready to recognize that something is a chemical change versus a physical change so let's try a couple of practice problems with this okay uh if feel free to pause this this video to give you some time to to think about the answers uh before i you know i give them away uh but basically we're gonna look at these three examples here and determine which of these are chemical properties and which of these are physical properties okay so boiling point of ethyl alcohol is 78 degrees celsius okay so so ethanol or ethyl alcohol uh you know is is a liquid and it boils at 78 degrees celsius the fact that we're talking about a boiling point should kind of give this away right because remember um for any physic you know change of state like uh melting or boiling or whatever is a physical change and therefore boiling point is a physical property okay the ethyl alcohol is a liquid when you start boiling it it's a gas when you're done boiling it but it's still ethanol right the chemical molecules making up that liquid and that gas are still the same diamond is very hard okay so this is also a physical property right because when you test the hardness on a diamond you used to scratch something for example or you hit it with a hammer it's still a diamond before and after you do that right so so that's why this is just a physical property okay to observe this property the diamond doesn't stop being a diamond it's still a diamond sugar ferments to form ethyl alcohol so here we have we're starting off with one substance sugar and we're winding up with another substance ethyl alcohol or ethanol now these are not different forms of the same thing it's not like liquid sugars ethyl alcohol they're two completely different substances so this is a chemical change right that we're seeing um so the chemical this is obviously a chemical property and that's the ability of sugar to ferment into ethanol so uh we could try some practice problems here with uh with physical versus chemical changes as opposed to properties but again you're going to approach this very similarly to how we did the uh practice problems on the previous slide okay so so here we're looking for physical and chemical changes uh remember a physical change is probably the easier one to spot right it's either you have a uh change in shape of the material you're looking at but it's still the same material or you'll notice uh words that tend to mark a change of state like melting or boiling or freezing or you know evaporating things like or condensation or you know things like that so so keep that in mind okay so so try each of these again feel free to pause the video if you want to give yourself some more think time okay so that first one is melting of snow again the word melting kind of gives that away right you're going from solid water into liquid water so that's a physical change now burning of gasoline okay so think about what's happening to the gasoline there you start off with you know just liquid gasoline and when you burn it it's you don't have to keep on heating it right you just give it a spark and it ignites it catches fire and it turns into something that's not gasoline anymore right it's turning into carbon dioxide and water vapor which you might not necessarily see but the fact is it's no longer gasoline when it's done so that's a chemical change now uh if you're having trouble like sort of visualizing that like thinking about okay what does gasoline turn into don't forget there are certain signs that a chemical change is happening and so you can kind of look out for those in this case the heat and light that you get from the gasoline being on fire that is a sign of a chemical change so pretty much any time you set something on fire that's a combustion reaction and a chemical reaction as such all right rusting of iron is that a chemical or physical change now think about what that looks like right you start off with this nice gray shiny iron and it turns into this sort of rusty like reddish brown flaky iron oxide so what is what's happening there is a chemical change because your iron is reacting with oxygen in the air to make an iron oxide okay so that's a chemical reaction a chemical change and again just a change in color alone should tip you off that this is a chemical change going on okay so so now that we've talked about matter and what it looks like and how we identify properties of it let's shift over to energy now before we can start discussing energy i first want to talk a little bit more about temperature this is a topic that we brought up in the last chapter and while temperature and heat are not the same thing heat's a form of energy temperature is something that we use to gauge how much heat something contains so even though they're not the same thing and they're often mistaken to be the same thing they are related to each other so we're going to start off by understanding what temperature is okay now i pointed out in the last chapter that we uh while we probably measure things in fahrenheit just um that's probably the way most of you grew up measuring temperature and like that's how you tell the temperature of the weather outside when it comes to science we're going to use dif a different scale actually two different scales we're going to use the celsius scale and we're going to use the kelvin scale now the celsius scale is is uh calibrated using water so the way this is done is a very it's a very metric approach i guess uh where we kind of set the boiling point of water as 100 degrees celsius and the freezing point of water is zero degrees celsius and we calibrate our thermometer based on that we just you know divide the glass of our thermometer into 100 little increments between the point at which water freezes and the point at which it boils okay and that's that's basically how you work out the celsius scale now the kelvin scale on the other hand is just the celsius scale shifted over by 273 degrees now if you remember back in the previous chapter i pointed out why we do this the absolute coldest temperature you can possibly reach which is called absolute zero is negative 273 degrees celsius so if you set that as zero on the kelvin scale all temperatures are going to be higher than that right they're going to be warmer than that so this is why the kelvin scale is also known as the absolute scale it's everything's a positive number on that scale so you just add 273 to whatever your temperature is in celsius and that'll give you the temperature in kelvin so for example water freezes at 0 degrees celsius that's 273 kelvin water boils at 100 degrees celsius that's 373 kelvin okay so notice that we just add 273 to do our conversion here which is probably a little counter-intuitive a lot of conversions involve multiplying a number the kelvin scale is kind of unique in that you could add to the conversion factor okay but yeah so you want to be careful since the absolute zero is the coldest temperature you can go on the kelvin scale you can't have a negative temperature in kelvin so if you wind up writing down a negative temperature in kelvin you've probably made a mistake somewhere so that's a quick way to check your work okay so how do we convert from one to the other well i pointed out that um you know we have a difference in the celsius and fahrenheit scale so this is calculated or calibrated by looking at what temperature water freezes and boils but they both are used to measure the temperatures of things you can still measure the temperature at which water freezes and boils using a fahrenheit thermometer you'd get really weird numbers uh water i believe freezes at 32 fahrenheit uh 32 degrees fahrenheit and boils at 212 degrees fahrenheit and you you know and again that sounds really arbitrary when you complete compared to the 0 and 100 on your celsius scale but the point is that we can convert from one to the other quite easily you can see that what's 0 degrees celsius is 32 fahrenheit so there's a 32 degree difference in our starting point there the other thing to notice is that in the celsius scale the difference between freezing and boiling is 100 degrees celsius but the difference between freezing and boiling water on the fahrenheit scale is 180 degrees so 212 minus 32 is 180. so in other words the size of a degree fahrenheit is different from the size of a degree celsius so if 180 degrees fahrenheit covers the same as 100 degrees celsius that means that there's nine degrees fahrenheit for every five degrees celsius or one degree celsius is the same as 1.8 degrees fahrenheit so if you were looking at the markings on a celsius thermometer versus the markings on a fahrenheit thermometer the markings on a fahrenheit thermometer would be closer to get or sorry it would be a little bit further apart than the markings on your celsius if you were looking at the markings on a celsius thermometer versus the fahrenheit thermometer the markings on your fahrenheit thermometer would be closer to each other they'd be closer together because that'd be two degrees fahrenheit almo almost two degrees fahrenheit for every degree celsius marking on your on your thermometer okay so if you want to visualize that see both of these thermometers here work the same way they still have mercury or alcohol in the bulb of the thermometer there's still a thin glass tube all that's changing here is just where uh you know what the markings are on those glass tubes so if we have both these thermometers in some ice water which is at 0 degrees celsius or 32 fahrenheit notice that the mercury or alcohol is at the same level okay but like i said the what that level is called is different depending on the type of thermometer you're using okay so how do we convert from one to the other if you have the temperature in degrees celsius and you're trying to convert to fahrenheit basically you want to multiply your temperature in celsius by 1.8 because remember 1 degree celsius is equivalent to 1.8 degrees fahrenheit okay sometimes you'll see this written as nine-fifths depending on who you ask for this formula but that's written as written as a decimal that's 1.8 now that helps convert the size of your degrees don't forget the starting point is different so if you want your temperature in fahrenheit you have to add 32 then to that answer to get the temperature in fahrenheit okay so um just a little you know story time here uh i grew up with celsius obviously and uh to this day i still have trouble trying to convert to fahrenheit or when someone tells me the temperature in fahrenheit i have i have trouble visualizing it because i grew up knowing temperatures in celsius so it's uh you know so i still kind of do this calculation in my head well not exactly i do an approximation of this where basically if someone gives me the temperature in fahrenheit i subtract the temperature at 30 degrees from that and divided by two and that gives me a rough approximation of what that temperature is in celsius so uh so so rand i guess if if you ever go let's say to canada or some place that uses celsius and uh and they tell you the temperature in celsius you're trying to figure it out in in fahrenheit so that you understand what the numbers mean to you you would kind of do this in reverse right you would basically take that temperature in celsius and again like the the rough approximation is you double it and add 30 to it okay uh yeah i can't tell you how many uh incidents uh here the little double takes my wife and i have had when telling each other the temperature uh especially um you know in in summer when i say like oh it's going to be a nice day out it's going to be like 30 degrees out and she's like wait that's freezing you know anyway so that's that's the quick approximation but let's practice how to do this conversion now now you will be given this formula um with at least you'll be given this formula in one direction so for example here you're given the the way to calculate the temperature in fahrenheit from the temperature in celsius but not the other way around so be ready to rearrange this formula to solve for the temperature in celsius now for those of you who aren't as comfortable with how to rearrange formulas if if you either haven't taken any algebra or you did and you didn't like it that much don't panic we'll walk through how to do that okay so let's let's do a couple practice problems so a person with hypothermia has a body temperature of 34.8 degrees celsius what is that temperature in fahrenheit okay so again we're given that formula where our temperature in fahrenheit is 1.8 times the temperature in celsius plus 32 degrees now the question going back to the last chapter where i pointed out when you're given a problem like this as a paragraph and you need to look at this and say okay what's the question asking me what's it giving me and then how do i come up with a game plan for this if you're reading this question a person with hyperthermia has a body temperature of 34.8 degrees celsius that's a number that's probably going to be very important what is that same temperature in degrees fahrenheit here the units of what we want to convert to we want to go from degrees celsius so looking at this formula the question is giving us degrees celsius it wants us to find the temperature in degrees fahrenheit so we would plug this number into this formula okay into that formula and go ahead and continue solving it all right so let's try that out so we take our 34.8 and plug it in so we first do the multiplication so for those of you who haven't seen this before in math you generally have to do multiplications and divisions before you can do any additions and subtractions it's just the order of operations in which you do a calculation you may know this through some memory device like uh pemdas or please excuse my dara on sally so what that stands for is parentheses exponents multiplication division addition subtraction so let me write that down again so you we have that down so pemdus or please excuse my dear aunt sally okay so you do uh things that are in parentheses right in brackets uh then you can do things that are exponents right so an exponent is where you have a number raised to another number so like squaring or cubing and so on then multiplication and division and then finally you can do an addition or subtraction so that's the order you go through here so we're going to do this multiplication of 1.8 and 34.8 first which if you do that you get you know 62.6 and then we're going to add 32 to that we do that addition so you wind up with 94.6 degrees fahrenheit okay all right oh in case you're wondering why i only went to one decimal place there since our 34.8 is given to three significant figures it makes sense to give the are also three sig figs uh the other numbers in our formula are exact numbers so they uh are thought to have an infinite amount of precision so we don't use that to determine how many sig figs we go to uh that being said again don't don't stress too much about sig figs here but if you're curious about why i wrote the answer like that that's where that came from okay let's try a question a similar question but let's go backwards here so here in this question they're asking they're telling us that we have a temperature of 350 degrees fahrenheit what is that temperature in celsius so remember we have a formula where we can if we're given the temperature in celsius we can calculate the temperature in fahrenheit but here we're going backwards okay we want to turn try and rearrange this equation to solve for a different variable we want to get in other words we want to get the temperature in celsius by itself okay so how do you move things how do you rearrange an equation all right so those of you who haven't seen this before basically to move something to the opposite side of an equation you kind of have to do the opposite operator that you're seeing on it so for example here we have you know 1.8 times the temperature in celsius plus 32 okay now the 1.8 and the temperature in celsius are kind of tied together in these parentheses okay so for the same reason why when you're doing a calculation you kind of do that first here for moving things you've got to do that last so first we want to move our 32 to the opposite side okay so since it's plus 32 we subtract 32 to move it to the other side here okay what let me do the stepwise so you guys can see how that works okay so if i subtract 32 okay that cancels it on this side but i have to uh you know write it as subtracting 32 on on the left so i have my temperature in fahrenheit minus 32 okay and that's going to equal 1.8 times my temperature in celsius okay by the way if you write two numbers two variables next to each other like this and there's no operator there's no mathematical symbol between them it's understood to be the same as a multiplication sign just so you know okay so now remember i want the temperature in celsius i want it by itself that's what i'm solving for right so how do i move my 1.8 to the other side and leave the temperature in celsius by itself well the opposite of a multiplication is a division so i've got to divide by 1.8 so i take my temperature in fahrenheit minus 32 dividing by 1.8 and that will equal my temperature in celsius by itself okay now you might see me sometimes write this in parentheses just because that shows that this is one term uh if you plug this to your calculator without doing this first you will get the wrong answer because effectively uh instead of dividing both these numbers by 1.8 if you just took the temperature in fahrenheit divide by 1.8 minus 32 you would only be dividing this term by 1.8 and like i said it winds up being the wrong the wrong answer so so something to keep in mind there okay so let's get um move on okay so there's what i've written so now that we've gotten to this point now we can just plug in the numbers like we did before uh the question tells us the temperature at fahrenheit is 350 so we would take we would just plug in that number there we take 350 minus 32 okay and work that out divide by 1.8 and that gives us our temperature in celsius so you get 176.7 degrees celsius that's probably to the wrong number of sig figs but we'll just ignore that for now okay so that's our temperature in celsius now let's look at the second part of this question what is that temperature in kelvin how do we convert from celsius to kelvin remember we've got to add 273 to convert from celsius to kelvin right because negative 273 degrees celsius is 0 kelvin so if we add 273 to our answer in degrees celsius we get an answer of 449.7 kelvin okay so that's all you have to do to answer this question okay so so keep that in mind uh that uh with celsius to kelvin like that last step it's a little counterintuitive because you're just adding a conversion factor you're not multiplying or dividing anything uh by the way if you're ever looking at a change in temperature in celsius or kelvin they actually wind up being the same thing that's because the size of a single degree kelvin is the same size as a single degree celsius you're just shifting your scale you're not multiplying by a scale right it's not like with celsius and fahrenheit where 1.8 degrees fahrenheit is one degree celsius here a change of one degree celsius is the same as a change of one kelvin to give you an example from earlier with our water freezing and boiling water freezes at 0 degrees celsius and boils at 100 degrees celsius that's a difference of 100 degrees if i were to measure that on the kelvin scale water freezes at 273 kelvin and boils at 373 kelvin again that's a difference of 100 kelvin so so something to keep in mind if you're ever looking at a change in temperature in celsius or kelvin there it winds up being the same thing okay it's only when you're being asked for what is the actual temperature then you've got to take your celsius and add 273 to it okay so so let's give you a simple example here uh 25 degrees celsius is standard room temperature so what is the equivalent of that in kelvin so go ahead and add 273 to that and you figure out that it is 298 kelvin which by the way if if later on you're wondering why so many problems uh have a temperature of 298 kelvin uh that's the reason 25 degrees celsius is considered standard room temperature so so a lot of problems are done at 298 kelvin okay so that's temperature what does this have to do with energy right so i mentioned earlier that energy is the capacity to do work when when you have a process happening there's energy involved right now that energy can take multiple forms you have uh just a whole bunch of different types of energy you've got chemical energy you've got nuclear energy electrical energy uh thermal energy which heat energy mechanical energy gravitational potential energy et cetera et cetera there's lots of different kinds of energy out there okay that being said we can break this down into two main two main categories you have potential energy which is energy that's stored somehow and we have kinetic energy that's energy associated with motion or movement okay so any time we have a chemical or physical change there is energy being exchanged in that process all right let me give you an example right let's say you take an object so let's say you're holding your pen right now to write your notes with and you lift it up okay now there your muscles are taking their stored chemical energy and it's doing work but you're giving that pen gravitational potential energy you're raising it up against gravity and by putting it at a higher height you're giving it gravitational potential energy when you let go that pen falls and and that gravitational potential energy is converted into the kinetic energy of the pen falling to the floor now when it hits the floor that kinetic energy is converted into sound and heat from the friction with the floor and so on it's it's lost in certain ways but uh that's basically where that energy goes okay to use another example um you know for if you've ever seen a slow motion video of someone taking a slap shot in hockey i don't know if you've ever done that we we're here in upstate new york so hopefully most of you have but basically if you ever watch it you'll see that the hockey player doesn't hit the puck itself directly they hit the ice right before the puck and if you watch the the video of this happening you'll notice that the stick actually bends backwards when this is going on what's happening there is you're giving the stick uh elastic potential energy it's the same uh type of energy that you see in um let's say in a bow in a bow string when you draw an arrow back on a bow okay and similar concept here but basically that elastic potential energy then gets converted into kinetic energy when the stick snaps forward and hits the puck so anyway my point here is that you a lot of these processes involve changes especially between these two types of energy potential energy versus kinetic energy okay if you go on to take a physics course you'll go into way more detail on this but this is uh you know that physics summarized in a nutshell okay now what does this have to do with chemistry right how do we apply this to chemistry well when it comes to potential energy the most common form you're going to see in chemistry is the form that takes place of chemical bonds we have energy stored in chemical bonds and so that's a form of potential energy when you do chemical reactions you are breaking and forming bonds it costs energy to break those bonds when you form new bonds to make the products of that reaction you're releasing that chemical energy okay now where do we see kinetic energy well i pointed out earlier that molecules uh move constantly if you're in a fluid like a liquid or solid liquid or gas your molecules are constantly in motion even with a solid where the molecules are held roughly in fixed positions those molecules are still vibrating a little bit in place okay even though they're roughly in the same spot they're still vibrating some like back and forth for a little bit and that's you know that's there's some kinetic energy associated with that okay all right so my point here is that in all the chemical and physical processes we've seen energy is involved whether in chemical or potential energy or in whether in potential or kinetic energy okay so how do we measure energy the si unit for for energy is the joule right and the symbol for that is a capital j right the other unit you may come across is the calorie now the calorie which is sometimes abbreviated just cal cal is uh it's you're not going to see that used too often amongst chemists or physicists physical scientists which is kind of odd though because it is if you think about how a calorie is measured it's a very metric unit right it's uh basically it's the amount of energy required to raise one gram of water by one degree celsius right that's a that's a very metric way to define a unit uh but for some reason chemists prefer using joules in you know like physicists um so it's kind of odd that chemists don't use calories as much uh the people who use calories more tend to be uh biologists or people who work in biology related fields like nutritional scientists for example okay now this the other reason i think the calorie isn't as popular is speaking of nutritional scientists uh there is another unit uh that is related to the calorie i'll get into that in a second but uh but if you're curious about how to convert from one to the other one calorie is 4.184 joules now i'm going to ask you to remember that conversion i normally don't ask you to memorize a lot of things but that is something i want you to memorize and you'll see why later okay now quick note here about joules and calories as si and metric units you can use your regular metric prefixes with those so for example a kilojoule is 1000 joules a kilo calorie is 1000 calories now i mentioned that there's a third unit that we kind of use and this is where things get slightly confusing and this is probably why calories aren't as popular and that there is another unit called the nutritional calorie and it's still spelt calorie but it's spelt with a capital c so whether you're brew abbreviating it with you know as cal or you're writing out the whole word calorie you're writing it with a capital c okay so it's really unoriginal in that there is a another unit that sounds exactly like a previous unit we've discussed and this is where this is why it's so confusing one capital c calorie one nutritional calorie is equal to one metric calories or in other words one calorie capital c calorie is the equivalent of one kilo calorie one lower case uh 1000 lowercase c calories okay i agree it's really dumb but it's one of those weird things you've got to watch out for okay so so this is why i think if you go beyond this class you'll notice a lot of the measurements that you're going to come across are going to be in joules and kilojoules and stuff like that that that we don't tend to use calories very often but that being said in this class we will use both joules and calories because uh you know since this is a very introductory course a lot of you are going off into fields where you may need calories so hopefully uh you know just again pay attention to whether or not calorie is written with a capital c or a lower kc okay now i think most of you when you think of calories you usually think of food i mentioned that it's very popular with nutritional scientists again because you can describe energy in this case energy from food in terms of these energy units calories you can still use joules for these as well and so again notice that we can convert from one to the other accordingly right remember one calorie is 4.184 joules so if you want to convert from kilocalories to kilojoules you just multiply by 4.184 these have been rounded off so it's slightly off there but that's basically what we're doing here all right so let's let's practice one of those conversions let's say we have 150 joules of heat energy that can be provided or chemical energy that can provide from a pot of butter as it metabolizes right as it's digested it releases 150 joules of energy how many calories is this equivalent to okay so again if you are looking at a problem like this you're reading this out loud you think to yourself okay what's important in this problem so if i'm reading this i say how many calories okay i know immediately now that's what the question is asking me for right there's how many that should give that away so i know i'm solving for calories when i do this problem are obtained from a pad of butter if it provides 150 joules of energy so that's our starting point right we're starting off with 150 joules and we want to convert to calories so we need some conversion factor that's going to tie together joules and calories okay so think back to that equality i had earlier where one calorie is equal to 4.184 joules okay so that equality is the important one so when we're setting this up remember we want joules to cancel out so we're going to put joules on the bottom of that conversion factor we want our answer in calories so we're going to put calories on the numerator of that conversion factor so that's why one calorie goes on top 4.184 joules goes on the bottom that gets joules and joules to cancel out and we're left with calories so mathematically when you plug this into your calculator 150 times 1 which is still 150 divided by 4.184 gives us 36 to two sig figs okay so you might get a longer decimal than that but they've rounded off to 36 here okay so i mentioned that heat and temperature are often mistaken for each other and they're not the same thing but they are tied together how are they tied together well one way that they're tied together is through specific heat capacity all right when an object absorbs heat or releases heat its temperature will change accordingly right so a material's temperature will generally go up as it absorbs heat materials temperature will go down as it releases heat now the rate at which it a material does this varies from material to material some materials will gain it will will gain some heat and rise in temperature really quickly whereas some don't their temperature changes really slowly even though they get the same amount of heat let me let me give you an example here if you go to the beach think about going to the beach on a very hot day if you stand on the sand your feet get really uncomfortable right they feel like they start burning because the sand is so hot but then you step into the water and your feet feel cold because the water is so well the water's so cold but you've got the same amount of sunshine on both the sand and the water right it's the same sunlight that's shining on both of those things and so why is the water so cold but the sand so hot and the answer is that water has a higher specific heat capacity compared to the sand so a similar example here is if we gave 4.14 joules of heat to a gram of water the water's temperature would rise by one degree celsius so if you were doing this at room temperature if you had 25 degrees celsius water if you gave it 4.184 joules of heat the temperature would rise to 26 degrees celsius however if you did the same thing with the same mass but of gold the temperature would rise by 32 degrees celsius much more significantly higher okay so if you had your gold at room temperature at 25 degrees celsius and you gave it that same amount of heat its temperature would rise up to 57 degrees celsius as opposed to 26. okay so so that's something to keep in mind and the thing that controls this the property of the material that controls this is what we call specific heat capacity okay okay so if we were to define this we would see that a we're relating together the amount of heat sometimes represented with the letter q we're taking the amount of heat we're dividing by the mass of the material in question and the change in temperature because that's what this heat is going to do it's going into raising or lowering the temperature so when it comes to values of specific heat capacity which is represented with the lower case letter c you can get it in units of joules per gram degree celsius or calories per gram degree celsius depending on what you're using for your units of heat okay now a little note here about what we use specific heat capacity for this this makes the assumption that your material is not going to undergo a physical change so your material has to say the same phase so you can only use it for a material heating and cooling you can't use specific heat calculations for a change in phase like for melting or boiling and so on we'll do more problems on that we'll get into that concept in a little bit later on in this powerpoint okay so here's some examples of specific heats some specific capacities um you don't have to memorize this by the way uh the only one you do have to remember is for that of liquid water and this is where things are really convenient but the specific heat capacity of liquid water is 4.184 joules per gram degrees celsius which is the number i asked you to memorize okay so again that makes sense because it would also be one calorie per gram degree celsius so 4.14184 joules is equivalent to one calorie okay so units to watch out for so i pointed out that your units of specific heat capacity which has the symbol c is in joules per gram degrees celsius or calories per gram degree celsius depending on what you use for your units of heat now the formula for this calculation and this formula is given to you by the way usually on a on a quiz or exam you'll usually find this on your formula sheet but q is equal to mc delta t okay now what does that mean all right well q is the amount of heat energy in joules or calories okay our m stands for mass and so that's the mass in grams usually actually it pretty much always is and that's where we get grams in our units okay and delta t is a change in temperature so the greek letter delta it's the thing that looks like this triangle over here okay this triangle is actually the greek letter delta it's like kind of the equivalent of a capital d if you were speaking greek okay so delta t here is one unit by the way so this is one variable okay so you have m c and delta t delta t is the change in temperature so typically it's written in degrees celsius but uh keep in mind by the way a change in temperature in degrees celsius is the same as a change in temperature in kelvin so it is possible you may see this sometimes written with kelvin instead of degrees celsius okay and if you do happen to see it in that way that's okay okay so um like i said the the amount of the substance is important the change in temperature the substance is important uh one thing that you might get out of determining your specific heat is that it can also tell you the identity of your substance so or vice versa if you know what material you're dealing with and you look it up in a table so actually if you go back a couple of slides if you have a table like this you can look up the specific heat capacity for uh for a particular material you're dealing with actually uh while we're at it make a note of the specific heat capacity of iron we're going to do a practice problem later on that involves iron so i'm just telling you this now so you can write it down rather than having to come back to this slide okay so if you have that formula q is equal to mc delta t you can solve for any one of those variables that being said sometimes you have simpler problems where you're just looking at the relationship between things so your heat is going to be directly proportional to each of those other variables because they're on the opposite side of the equation so for example if you have a material and it's you're using the same material so it's the same mass and same specific heat if you double the amount of heat that you give it the temperature will rise by double the amount okay so again that's because your change in temperature and your heat are directly proportional to each other they're on opposite sides of this equation so something to keep in mind if you ever have a a problem that's qualitative you don't have to treat this like a math problem it can be more like a logic problem okay other point that i need to make here based on the fact that your change in temperature and heat are related to each other and that has to do with the sign okay whenever we talk about a change in anything so if you ever see delta something okay it's always the final state minus the initial state so if you have a rise in temperature your final temperature is going to be hotter than your initial temperature so delta t will be a positive number well if mass is a positive number which mass always is and your specific heat is always a positive number okay what is your heat going to be well it will also be a positive number okay so that's something to keep in mind but what if your temperature is cooling what if your object cools down and therefore your final temperature is lower than your initial temperature well remember a number a small number minus a large number gives you a negative delta t so that means that q if your delta t is negative q will also be negative okay so depending on the sign on q that tells you something about whether your substance is gaining or losing heat okay so keep that in mind especially later on when we get to heat transfer problems in a little bit so if something has a negative value for q that means it's losing that heat it's giving out that heat and that's why it's temperature is dropping okay oh a quick note here about how this is written out in this powerpoint i've used sh here for specific heat uh normally i would use the the symbol c for specific heat that's what like the almost everything uh uses as the symbol for specific heat for those of you who have the old timber lake textbook for some reason karen timberlake uses sh for a specific heat instead of the letter c i'm not sure why she does that she's literally the only person i've ever seen do that but for those of you who do happen to have that textbook i left this reference in there so in case you're wondering about that okay so how do we convert things let's let's try some problems out okay so before we begin i'm going to start off with this very simple silly cal conversion like we did earlier with our our pad of butter and 150 joules uh you have a can of pepsi one okay and it has one calorie i think the concept of pepsi one is probably way before most of your times um but it was something that pepsi marketed it very very briefly um anyway so it contains one capital c calorie a nutritional calorie how many joules of heat is that or how many joules of energy is that and i know this is a silly starting off problem i'm going to use this number to do a later calculation okay so feel free to pause this here but think about how you would set this up we're starting with one calorie one capital c calorie that's our starting point and we want to set this up so that we wind up with joules at the end okay so you want to probably find some conversion factor between capital c calories to joules now there isn't a direct way to do this but the middle man here might be a lowercase c calorie so ask yourself how many calories lowercase c calories are in one nutritional calorie okay if you're if you've forgotten it's 1000 okay so one nutritional calorie with a capital c is equal to 1000 metric calories and then we need to convert between metric calories and joules so think about how we do that what's the conversion factor for that okay it's a number i asked you to memorize so ask yourself do i multiply or divide by 4.184 so if you're not sure how to multiply or divide again watch your units here and make sure you're setting up your conversion factors so that the units in one step cancel out in the next step so we're starting off with calories with a capital c so we know we want to divide by calories with a capital c in the next step so we have one nutritional calorie on the bottom and we have 1000 metric calories on the top okay and that gets calories with a capital c to cancel out now in the next step we want lowercase c calories to cancel out so we want calories on the denominator on the bottom and we want joules on top because that's where we want our answer in so again lowercase c calories will cancel out so we have our 4.184 joules on top our one calorie on the bottom and again notice that our units have cancelled out and we're left with just joules so when we plug these numbers in if i take 1 times 1000 times 4.184 divide by 1 which is the same number you get 4184 joules okay so that was just a little practice problem to make sure you're getting the hang of these these conversions let's use that answer to do the next couple of problems so if we have 4184 joules okay and we want to use that that let's say we convert that completely into heat energy how much water can we heat up so let's say we've got water at zero degrees celsius and we're heating it up until the point is boiling now remember boiling is 100 degrees celsius right so think about what this question is giving you and what it's asking you for the question tells you we have 4184 joules we're being asked how many grams of water so that's going to be our ending point we want to get to grams and we're told that the water goes from zero degrees celsius to boiling so there's a change in temperature here we're going from 0 to 100 degrees celsius which is a difference of 100 degrees celsius so something to keep in mind that delta t is going to equal 100 degrees celsius okay so with that in mind you could either set this up like you would uh with any other conversion factors using a table but i'm guessing most of you would probably like that old formula q is equal to mc delta t okay so if we rearrange that you would get this value let me show you how that's done for those of you who are still getting the hang of rearranging formulas using algebra let me show you how to do that to get q to go from q is equal to mc delta t to getting this version that they've got here oops let's uh want to get rid of the uh the pen here okay there we go so we are starting off with q is equal to m c delta t right and we want the mass by itself because we're solving for how many grams and grams is a unit of mass so we want mass by itself so we need to move c and delta t to the other side now remember when variables are just written next to each other and there's no multiplier it's automatically assumed that the multi that the operator is a multiplication symbol okay so what's the opposite of multiplication well division which in this case is shown with a um with the quotient with the the line that you see in a fraction so we move c and delta t we divide those under q okay so that's why they're written on the denominator of this fraction okay by the way it's uh you could treat this if it's easier you could uh treat this as one thing q c and uh delta t as one unit that we just divide by the whole thing and that's why there's still a multiplication sign here because they're both on the denominator but they're multiplied by each other okay so anyway let's we can do it that way or we can plug in uh and think you know set this up like we would any um conversion factor problem so we start off with 4184 joules and so that's our q right so we divide by c which if you do that you'll notice that to divide by something that's the same thing as writing the inverse of it so if the specific heat capacity of water is 4.184 joules per gram degree celsius so let's let's write that out okay so so c for water is equal to 4.184 joules per gram degrees celsius what that means is that's 4.184 joules for 1 divided by 1 gram times 1 degree celsius so if we were to divide by c what that means is that fraction gets flipped okay so if you were to extend this out and say all right that's the same as times 1 gram times 1 degree celsius if i'm dividing by this i'm just flipping this and that's kind of how it's written here we put the 4.184 joules on the bottom 1 gram and 1 degree celsius on top and notice that gets joules to cancel out okay now notice that we're also dividing by delta t and remember delta t is 100 degrees celsius so we divide by 100 degrees celsius and that gets degrees celsius to cancel out now notice that the only unit we have left is our grams over here which is a good thing because that's what we want our answer in so if we plug this into our calculator 4 184 times 1 times 1 divided by 4.184 and also divide by 100 okay or you could multiply 4.184 times 100 first and then take 4184 divided by that answer you'll get the same thing which is 10 grams okay so there's 10 grams of water that's how much water we could heat up with 4184 joules now let's try a similar problem okay so let's say we still had four 4184 joules of heat energy we're using to warm up some water let's say instead of starting off at 0 degrees celsius we're starting off at 25 degrees celsius so very similar problem how many grams of water can you can you heat up with this much heat energy with this 4184 joules okay you're going to set this up very similarly to this the previous problem so feel free to pause this video and then you know continue on as i explain how this is done so we still have we're still solving for grams and we're still starting with 4184 joules the only difference here is going to be delta t because this is still water so your specific heat is still 4.184 joules per gram degree celsius that number has not changed your still solve so the only other variable that's left is delta t and notice that delta t involves your water starting out going from 25 degrees celsius to a hundred that's the difference of 75 degrees celsius which is different from your 100 degrees celsius in the previous problem so we're setting it up the same way q is equal to c divided by delta t or q m is equal to q divided by c and delta t so we would plug these into our our problem the same way we take our fourth house and 184 joules divide by c to get joules cancel out divide by 75 degrees celsius get degrees celsius to cancel out and we'd get our answer in joules or in our answer in grams sorry okay so you should wind up with about 13.33 grams again probably not the perfect number of sig figs but close enough okay why don't you guys try another problem very similar but here we're going to solve for something different so here we've got 50 joules of heat we're applying it to 10 grams of iron and how much will the change in how much will the iron uh warm up okay so in other words what is delta t for this thing of iron okay so when you do a problem like this you want to again you want to kind of set it up the same way as before you're going to read through this and say all right what is this question giving me and what am i solving for and how do i connect them so we're starting off with 50 joules of heat okay remember heat and especially when you see the units of joules that is going to be our q in our equation we've got 10 grams of iron so 10 grams okay that's a mass m by how much will the temperature of the iron increase so in other words what is delta t what's this rise in temperature that we're getting out of our from heating up this iron okay so if we look at the formula we need we have q is equal to mc delta t and the question wants us to solve for delta t so how do we get delta t by itself how do we move m and c to the other side okay so remember we divide by m and c okay so delta t is equal to q divided by m and c now notice that the question gives us q it gives us m we're solving for delta t the question doesn't directly tell us c okay now this were a quiz or exam i would tell you the value of c uh in this case since it's a problem that you're not taking on a quiz or exam you can look that up in the table that was a few slides back but to jog your memories your specific heat capacity of four iron is 0.45 joules per gram degree celsius so so go ahead and plug those numbers in take our 50 joules we divide by the specific heat capacity of 0.45 joules per gram degrees celsius again notice that joules cancels out and we divide by 10 grams get grams to cancel out and you get your answer in degrees celsius up until now we've been looking at a single object gaining or losing heat what happens when there's a transfer of heat from one thing to another right so how where does that heat go or come from so basically if you take a hot object and a cold object and put them together the hot object will lose heat and its temperature will drop the cold object will gain heat and its temperature will rise here's the thing this is what ties them together the heat the same amount of heat that's transferred okay is uniform so the same amount of heat your hot object is losing is equal to the amount of heat that your cold object is gaining okay like from a value standpoint they're the same number now you've got to think back to where we talked about the signs of these these amounts of heat so from the hot objects standpoint this is going to be a negative value because it that object is losing that heat whereas that will be a positive number for the cold object but the point still remains that the numerically those two things are the same now we can take advantage of this to do what is called calorimetry in other words if we take an insulated container and fill it with a material that we know the specific heat capacity of so water in other words we can use it to determine information about an unknown as long as we keep track of the change in temperature okay so so in this example here we we take a hot object okay and we know its mass we know its initial temperature okay and if we know the initial temperature of our water and we drop our hot object into the water okay we can keep track of the change in temperature between the two the water will warm up your hot object will cool down and they'll both keep doing this until they reach the same temperature and stop now this is the final temperature for both your water and your metal object so in other words if you know their initial temperatures you can figure out delta t for both of them you hopefully have already got the mass for both of them okay and since you have the specific heat capacity for water you can figure out how much heat went into the water to raise its temperature now that's going to be a positive value right well that same value is going to be a negative value for your hot metal object okay so you now have q for your metal object you have the mass and you have the change in temperature you can now solve for that specific heat of that unknown object back when i taught high school we actually did a lab like this where we took some paper clips and used that as our hot metal object and dropped into a calorimeter with water and we were able to figure out the specific heat capacity of the steel making the paper clips it was actually surprisingly accurate so coffee cup calorimetry say that 10 times fast is a very useful technique for doing some of these problems that involve heat transfer in fact when you do your energy and matter lab uh we actually will use coffee calorie image coffee cup calorimetry to determine uh you know uh we're gonna basically cool down water with some ice and using a similar method we're going to figure out the specific uh the heat of fusion for the ice as it melts okay but anyway i'm getting ahead of myself why don't we try a problem where we take a hot object drop it in water in our calorimeter and figure out information about that object so we have a 50 gram sample of tin and it starts off at 99.8 degrees celsius it's dropped into 50 grams of water at 15.6 degrees celsius if the final temperature of both the tin and water is 19.8 degrees celsius what is the specific heat of the tin so think about what this question is giving you it's giving you 50 grams as the mass for the tin okay it's telling you that the tin was initially at 99.8 degrees celsius okay so that's the final temperature or sorry the initial temperature for the tin okay and we're told that we have 50 grams of water so it's mass of the water and the water was initially at 15.6 degrees celsius so again that's the initial temperature of the water and you've told that the final temperature of both is 19.8 degrees celsius now you're being asked for the specific heat capacity of the tin what is the specific heat capacity of tin now the question doesn't give this to you but you should know the specific heat capacity of water and so that's 4.184 joules per gram degree celsius okay something you should memorize so let's keep stock of what what we've been given here okay we've been given all these things and we're trying to solve for the specific heat capacity of the metal okay now they've given us final and initial temperatures it might be a good idea to go ahead and calculate what your delta t is for each of these okay so for our tin okay here we have our tin notice that the tin is starting off at 99.8 degrees celsius and it's cooling down to 19.8 degrees celsius so what's the delta t if we take this final temperature and subtract that initial temperature of 99.8 well it's going to be a delta t for our tin of negative 80 degrees celsius because remember our tin is cooling down that's why this is a negative number now look at your water you've got a final temperature of 19.8 degrees celsius and initial temperature of 15.6 so that water temperature rose you're going to get a positive value here when you take 19.8 and subtract 15.6 so delta t for your water is going to be 4.2 degrees celsius positive 4.2 because it's warming up okay so we can break this down into two steps look at the information you're given okay the one that's more complete and you'll notice that the water has has three out of the four variables it's got we've got the mass for the water we've got the specific heat capacity of the water and we've got delta t for the water in other words if we look at our formula q is equal to mc delta t we have the mass we have the specific heat we have delta t for water we can figure out how much heat went into the water now this amount of heat that went to the water is going to be the same amount of heat that left our tin so if we have q for our tin and we have the mass of tin and delta t for the tin we can then solve for specific heat capacity of tin okay so if you want to try this yourselves again pause this video but i will now go ahead and explain how we go about this so like i said you want to start off with your water just solve for water so we know that our water has a mass of 50 grams a specific heat capacity of 4.184 joules per gram degrees celsius and we saw the delta t is 4.2 degrees celsius so we multiply those three numbers grams cancels out degrees celsius cancels out and we're left with a value of around 880 joules now this number is probably rounded off uh to two sig figs because it looks like the temperature was two sig figs so i'm guessing that's why they did that but anyway my point is this is the amount of heat that went into the water so the negative equivalent of this is the amount of heat that left the tin so if you have the formula q is equal to mc delta t oops and we want to solve for the specific heat capacity of tin we have to divide by mass and change in temperature so we take our q which is negative 880 joules and divide by our mass of 50 grams and our change in temperature for tin of negative 80 degrees celsius okay so if you're having trouble seeing that q divided by m and delta t is equal to c by itself okay so when you plug that in you should get .22 joules per gram degree celsius again you can tell the units are going to be that because we have joules divided by grams and degrees celsius and that's going to be our our specific heat capacity for our tin all right i mentioned earlier that the we can only use that formula q is equal to mc delta t when a material stays the same and it does not change phase from let's say solid to liquid or liquid to gas or vice versa right so if we want to keep track of that as well to deep trap phase changes we can keep we can use what's called a heating curve or a cooling curve now uh these work and the same principle is just that with a heating curve you're at a heat with a cooling curve you're removing heat you're cooling down something but what we're going to do here is we're going to plot a graph of how the temperature of a material changes so on the y-axis we have the temperature and on the x-axis we've got heat that we're adding or removing okay so heat's going to be on our x-axis temperature is on our y-axis all right now you're going to notice that the graph will slope in some places and it'll go flat in some other places so the terms we're going to pay attention to here are specific heat which we've already covered but the two flat bits we're going to look at are the heat of fusion and the heat of vaporization these are the heats that are associated with a change of phase heat of fusion is involved with a substance melting or freezing and the heat of vaporization is involved with a substance boiling or condensing so let's have an example let's look at a generic heating curve here for something now by the way you can tell this is the heating curve because as we're one we're adding heat two notice that the temperature is steadily rising okay so that's the sign that we're adding heat and so it's a heating curve if you had a cooling curve it would look like this except it would slope downwards okay but it would be kind of similar where you constantly have a slope downwards in some places you have flat parts in some other places and yeah you alternate between them so what's going on here okay where why does this curve have this particular shape um so let's start off by looking at you know what's going on in this problem let's move along this graph in terms of temperature and heat added so we're starting off here over here on the left we haven't added any heat yet you are at a particular temperature okay so let's say we have a solid that is really cold and so cold it's at a temperature below its melting point so if we have a solid that's a temperature below its melting point what happens when you heat it now your natural instinct is probably to shout out oh it melts you be careful what i said right there it's below its melting point okay so to give you an example here if you had ice at negative 20 degrees celsius and you gave it some heat it wouldn't melt because ice needs to be at zero degrees celsius in order to melt okay so your solid here does not melt because it is below its melting point so when you add heat to this solid that heat goes into raising its temperature and that by the way is why the graph slopes up over here as you add more and more heat the temperature starts rising following that formula q is equal to mc delta t and then you get to this point here now you're at the melting point so what happens when you continue to add heat to a substance that is at its melting point well that heat goes into melting the material but if it's going to melting the material that means it's not going into changing its temperature using specific heat capacity and that by the way is why this graph flatlines while it melts okay so you start off over here with your solid that started to melt by the time you have added this heat over here you now have a liquid at that same temperature in between you have a mixture of solid and liquid so for example if you had ice water that was in where the ice and the water were in equilibrium with each other the temperature of that mixture would be zero degrees celsius exactly okay now once all of your solid has melted into its liquid and you keep on adding heat that liquid is going to start warming up again following q is equal to mc delta t it'll keep on warming up until you get to its boiling point now just like with the melting point once you're at the boiling point the heat you add is going to go into boiling and material so you start off with something that's all liquid and you start heating it and now you start getting a mixture of liquid and gas until finally everything's been converted into a gas now at this point if everything's already a gas no more energy needs to go into changing into a gas because it's already a gas the rest of that energy goes into raising the temperature of the gas and again that's the slopey bit of this graph here where your you can use q is equal to mc delta t if you have the specific heat capacity of the gas okay so notice that there's no temperature change in those two flat parts where we have a phase change where our solid becomes a liquid and our liquid becomes a gas okay so because these parts of the graph are flat you can't use q is equal to m c delta t to do any calculations here because delta t is zero you'd be multiplying by zero so it doesn't make any sense instead what you need is the heat of fusion which is the heat associated with melting and the heat of vaporization which is the heat that's associated with boiling now this these values are given in units of joules per gram so you might get them in other units as well for example joules per mole we haven't talked about what moles are we'll learn about that in a different chapter but my point is that what it's a particular unit of energy per unit of amount whether that amount is mass or some other way of counting it uh so for now we're just going to have the heat of fusion be 334 joules per gram now you don't have to memorize that number on a quiz or exam that we provided to you and otherwise you could look it up here okay so what does that mean right it means that one gram of ice needs 334 joules of heat energy to melt completely into water okay now please note this is this heat of fusion for water only this does not apply to other materials okay so um yeah so that's basically what you would use this for so if you have the mass of your ice in grams you multiply by 334 joules per grams and that'll give you the amount of heat energy required to melt it in joules okay so let's try a problem that involves a heating curve okay so now there might be more than one thing going on let's try reading this problem and see what this question is giving us and how to tackle it how to break it down so calculate the heat needed in joules okay so we already know what we're aiming for we're trying to get joules to melt 15 grams of ice at zero degrees celsius and to heat the water to 75 degrees celsius so we know we're starting off with 15 grams of ice at zero degrees celsius now notice there are two things going on if you're reading this okay you kind of have to get an eye for this but basically notice that we are starting off with ice at zero degrees celsius and it's becoming water at 75 so there's two things that's changed our water our ice is melted into water and the temperature has risen from zero to 75 degrees celsius now if you're having trouble visualizing this and recognizing like oh i'm going to have to do two things to solve this problem it sometimes helps to have a a cooling or a heating curve here to help visualize what's going on okay so basically if you have your heating curve okay now here's a big heating curve and i'm just going to show you on parts of this if you're trying to sketch this out you wouldn't have to draw the whole thing let's say you just started off over here and you say all right i've got ice at 0 degrees celsius that's where i'm starting i'm starting with 15 grams of that ice what happens when i heat it when i heat it it's going to melt from liquid ice at 0 degrees celsius into oh sorry solid ice at 0 degrees celsius into liquid water at 0 degrees celsius okay that's one part of my calculation i need to figure out the heat that's associated with that okay what's the heat that goes from here to here secondly we're going from liquid water at 0 degrees celsius to liquid water at 75 degrees celsius now notice that it's still water during this step but its temperature is changing right our final temperature is 75 degrees celsius our initial temperature was 0 degrees celsius that's a difference of 75 degrees celsius so we have a delta t here and don't forget if we have 15 grams of ice we also have 15 grams of water right that's what the ice melted into so we have an m for r for our sample of water now okay so we have the mass we have the change in temperature and because it's water we also know what c is 4.184 joules per gram degrees celsius so you can use that to solve for the heat that's associated with with this step okay so the key thing is this question is asking us what is the total amount of heat so if we're starting here and we're winding up here what is the amount of heat for this part of the graph well again there's no way to do that directly we have to break this down into two steps we have to figure out okay what's the amount of heat for this step okay using our heat of fusion and what's the amount of heat for this step using our specific heat capacity so break this down into two steps find the answers for each and then just add them together so let's do that first step we're melting 15 grams of ice into liquid water well ice has a heat of fusion of 334 joules per gram so if we start off with 15 grams would we multiply by 334 or divide well you want to multiply it because that way grams cancels out and you wind up with 5010 joules of heat energy required to melt that ice now again remember this is not the final answer this is just part of the answer your 15 grams of liquid water you've made now needs to heat up so again think back if we go back to that graph you can kind of see that here we're going from zero degrees celsius to 75 degrees celsius so that's a delta t of 75 and we have 15 grams of height of liquid water so let's set that up 15 grams times the specific heat capacity of of our liquid water times our 75 degrees celsius notice that grams cancels out degrees celsius cancels out you get your answer in joules okay i've written it up here 4710 joules so what's the total amount of heat just add up those two numbers so we have 9720 joules okay let's look at a heat of vaporization and do similar problems okay so like heat of fusion heat of vaporization also is used to describe one gram of a material in this case one gram of water and we're looking at the amount of heat energy required to boil off that liquid into a gas now notice that this is a very high number it actually costs a lot of energy to rip apart the water molecules and separate them out into a gas right into the gas phase so it's 2260 joules per gram is something you want to keep in mind now again you don't have to memorize this number but in case you're curious that's why that number is so much larger than 334. so similar stopping for workers to continue on from where we stopped and we have water at 75 degrees celsius how much heat do we need to convert it into steam at 100 degrees celsius again notice the difference here that we are starting with water and we're converting into steam so there's boiling going on here also we're going from 75 degrees celsius to 100 degrees celsius which i guess they've written over here as well that's a rise of 25 degrees celsius so keep that in mind for our delta t so again if you want to visualize this we're starting off with 75 degrees celsius liquid water and we're warming it to 100 degrees celsius so notice that we have a you know a change in temperature and this is going to use q is equal to mc delta t to do now we have liquid water at 100 degrees celsius we want to convert it in to water vapor at 100 degrees celsius and so keep in mind that this involves heat of vaporization for our 15 grams of water so just as before split this up into two steps so first calculate the heat that's required to warm up your water and then calculate the heat that's required to boil off that resulting water so to show that out the heating up part is q is equal to mc delta t we know that the mass of water is 15 grams we know c for water is still 4.184 joules per gram degrees celsius and we know delta t for water our change in temperature if we're going to 100 from 75 that's a rise of 25 degrees celsius so get grams to cancel out degrees celsius to cancel out and we multiply those three numbers we get 1570. again this is not the final answer we still have to boil off that water so we have 15 grams of water and we know that the heat of vaporization is 2260 joules per gram so think again about how we're starting off with 15 grams would i multiply or divide by this number to get grams to cancel out well remember since grams is on the denominator you multiply and that gives grams to cancel out so 15 times 2 260 is 33 900 joules a lot of energy required to boil off all that water okay and remember your final answer is just those two answers added together so we take our 1570 from our warming up of our water and then the 33 900 joules in order to boil it and that gives us our total amount of heat for this whole process okay so that's it for this chapter right as always here are some suggested practice problems from the textbook if you want more practice you know we can work on some of the even-numbered problems in your textbook that don't have the answers at the end of the chapter we can we can do that during the uh during our class time um yeah but uh of course be ready for a quiz on this material and uh you know and you'll see some of these concepts in the energy and matter lab that we'll do but as always if you have any questions just let me know and uh yep good luck and preparing for your quiz and lab report