hello everybody welcome back today we're going to start unit 3 which is all about intermolecular forces and properties associated with that and this unit is one of the most heavily covered units on the ap exam so it's it's definitely an important one so let's get started so as usual i always like to start off with a little factoid um and i just find this fascinating you know that something could be so loud um that would be like those of us that are on the east coast if something occurred out in california we would hear it on the east coast that's just incredible to me all right so as i said unit 3 is all about intermolecular forces and we're going to be talking primarily in this unit about liquids and solids and liquids and solids get lumped together because they have a lot of similar characteristics for example they are very very much incompressible okay whereas gases for example are very compressible you can change the volume of a gas liquids and solids not so much okay so liquids and solids have fairly constant density because you can't change their their volume and these similarities are due to the way that the atoms and molecules are held together solids and liquids the particles are very close to one another gaseous atoms and molecules are very far apart and what this unit is all about is what holds one atom or molecule to another and that is inter molecular forces now it's really important that we understand the difference between an intermolecular force and an intra molecular force okay intermolecular means a force that exists between one molecule and another one next door the term intra that prefix intra means within so for example um these are two water molecules in front of you okay if i point to that connection right there that is not an intermolecular force that is a covalent bond that is an intra molecular force or connection inter molecular would be a force of attraction existing between one molecule and another okay between molecules not within them okay and if you'll remember from chem 1 there are three types of intermolecular forces that we're going to focus on london dispersion forces are the weakest then dipole-dipole attraction and hydrogen bonding is the strongest type of intermolecular force so we're going to look at each one independently let's start with london dispersion forces okay first and foremost ladies and gentlemen all covalent compounds experience london dispersion forces between their molecules i want to underline this word experience because what i've gotten in the many years i've been teaching this course is a lot of students will say this particular molecule has ldfs do not use the word has when it comes to intermolecular forces or these molecules have because that makes it sound like you think that those forces exist within the molecule and that is not what we're talking about here we're talking about forces that exist between molecules so the word experience is a nice way to get around that okay now as we said on the previous slide london dispersion forces are forces of attraction but they are the weakest okay if you are dealing with a one of our noble gases or any molecule that is nonpolar and we talked about what what that means remember that means a molecule that does not uh doesn't have a has a balance of charge throughout its shape okay there's no one side that is more negatively charged or positively charged it's balanced okay if we're dealing with a noble gas or a nonpolar molecule london dispersion forces are the only forces intermolecular forces that exist between their molecules now what even is a london dispersion force how can two things that don't have an overall charge experience attraction for one another okay well you know let's say that we're looking at the gas that we are all inhaling o2 okay which is a nonpolar molecule okay it is however guys remember electrons are not static electrons move around and at any given moment in time one side of that molecule just for a moment might have the electrons sort of leaning to one side okay just for a split second because electrons can move maybe the left side of this molecule is slightly more negative and this side is slightly more positive just for a moment that's where that word instantaneous comes in there if that happens then what you're going to see in a molecule nearby because remember intermolecular forces means between two different molecules if this end of that molecule is slightly negative that's going to induce a slight dipole dipole just means a polarity a slight polarity in the molecule next to it now are these are these molecules completely being made to be polar no they're not they're just slightly polar and only for an instant so there is a very weak i know i'm not drawing this very well a very weak force of attraction between the negative end of that oxygen molecule and the positive end of the other but it's a very weak force and it's only for an instant that's why it's called an instantaneous dipole so you get this instantaneous dipole which induces another instantaneous dipole in the molecule next to it and there is a very weak force of attraction that is a london dispersion force now some molecules can be induced into that momentary dipole easier than others and this word polarizability is a really important vocabulary word when you you know write it down in your notes underline it circle it highlight it it's important what determines what makes a molecule more polarizable than another okay is the number of total electrons okay if one molecule is simply larger has a greater molar mass that's going to mean it also contains a greater number of electrons it will more easily be polarized more easily induced into that momentary dipole okay so what that's going to lead to ladies and gentlemen if you have let me actually back up for a second if a molecule is more polarizable it's still only going to have london dispersion forces but those london dispersion forces might just be a little bit stronger than a molecule that is smaller and has fewer electrons okay we're going to look at an example in a mall in a minute so bigger molecules have more electrons and we're going to talk in a minute about how that links to melting and boiling points on occasion this is an old-fashioned term but on occasion you'll see the term van der waals forces that just means the same thing as london dispersion forces same thing just two different names it's kind of an outdated term all right let's talk about the second type of intermolecular force dipole dipole forces or dipole-dipole attraction okay this is a situation where you have forces of attraction between polar molecules okay let's review guys what the word polar means a molecule that has electrons pulled more to one side than the other a molecule that's going to have a positive end and a negative end and opposites attract ladies and gentlemen the positive end of this polar molecule will attract to the negative end of one nearby okay dipole dipole-dipole attraction is a stronger force of attraction than london dispersion forces okay and the more polar the molecule is okay and remember we talked about in the previous unit what makes a molecule more or less polar if electrons are really being pulled to one side of the molecule because there's some element that is very electronegative that means that's a really polar molecule and that's also going to lead to stronger dipole-dipole attractions so let's look at a practice problem here and if you want to just pause the video and try and answer it on your own go for it otherwise i'm going to sort of talk through it so if you want to try it on your own pause the video now but i'm going to go through talk this one through it says the structures of ethane and methanol are shown below one of these two molecules is a liquid at room temperature meaning the other one must be something else probably a gas based on their structure and intermolecular forces predict the molecule which is a liquid at room temperature okay so let's let's talk this one out okay ethane that first molecule if you were to draw that one out ladies and gentlemen you would see that it is a completely symmetrical molecule it is non-polar which means that it has london dispersion forces only that is its only force of attraction intermolecular force of attraction methanol is a polar molecule okay now it is covalent so all covalent molecules have ldfs london dispersion forces but because this one is polar it also has dipole dipole attraction i'm just going to write dip dip don't write that on my test or the ap exam for goodness sake okay so what the question should have said i should have corrected this it should have said at the top one of these two molecules is a liquid and the other is a gas look at what we've just written down here ethane is held together by ldfs only methanol is held together by ldfs and dipole-dipole attraction the methanol molecules are held more tightly together they're stronger intermolecular forces which means methanol is the one that's going to be more likely to be a liquid okay ethane which is held by ldfs only those molecules are not held as tightly together more likely to be a gas okay so this is this is a great example of a free response question all right here's another one let's talk this one through though i don't don't try this one on your own unless you feel like you're a superstar it says the table below summarizes some physical properties of chlorine and hydrogen chloride how can we use our understanding of intermolecular forces to account for the difference in enthalpy of vaporization now you may not know the term enthalpy yet that means heat heat required to boil these molecules guys when you boil something like when you boil a pot of water on the stove you are not pulling the molecule water apart otherwise the vapor that comes out of the pot of water on the stove would be hydrogen and oxygen gas and your you know your kitchen would blow up all you are doing when you boil or vaporize something is breaking the intermolecular forces so let's talk about what intermolecular forces do these molecules have cl2 is totally nonpolar so it has ldfs only hydrogen chloride is polar it's also covalent so it has ldfs and dipole dipole which i'm going to write as dip dip again don't write that on a test okay now look at the little column that says delta h the heat required to boil these compounds now this may look backwards it kind of does at first because this chart is saying it takes more heat to boil chlorine which you might not expect because wait a second it's only held by ldfs wouldn't it be fairly easy to pull those molecules apart from each other why is the heat higher that seems wrong that seems backwards okay but it isn't trust the data okay i want you to look at the question above it did you all notice that the where it says mw below each molecule that stands for molecular weight they have the same molecular weight the same molar mass these two molecules in front of us cl2 and hcl do they have the same molar mass no they do not and which intermolecular force is affected by the size of the molecule london dispersion forces so this is a tricky little question but this is classic on the ap exam even though hcl has two intermolecular forces and thus you'd think oh it must be held more tightly together it's going to be more difficult to boil the data says it isn't the explanation is cl2 has a greater polarized because it's a it has more electrons in the molecule so in this case this is kind of unusual cl2 because it is so it is larger than hcl is more polarizable its london dispersion forces end up being stronger than all of those combined for hcl that comes up on occasion okay and i'm gonna i'm gonna cover that topic again at the end of this lesson so stay tuned all right let's talk about the third and final type hydrogen bonding and i put bonding in quotes because it's not a bond it's just a force between two molecules okay just like the other intermolecular forces okay now hydrogen bonding guys is a type of dipole-dipole attraction but it's more specific dipole-dipole remember means between the positive end of one molecule and the negative end of its neighbor hydrogen bonding is the same sort of thing but it's between the hydrogen of one molecule and n o or f from a neighboring molecule why those three because they're very electronegative so hydrogen bonding is simply a type of dipole-dipole attraction but it's a very strong interaction okay so which that picture in front of you is just showing you some different examples of hydrogen bonding please notice it is not a bond it is simply a force existing between two molecules two different molecules okay now one more thing to mention on hydrogen bonding obviously hydrogen has to be present for hydrogen bonding to take place however it's even more specific than that hydrogen bonding will only occur if hydrogen is chemically bonded to meaning covalently bonded to n o or f okay so let's look at these two examples in front of you all right the example on the left is covalent so let me change color here so it has ldfs it's polar so it experiences dipole-dipole attraction okay the one on the right is also covalent so it will experience ldfs it's also polar so it will also experience dipole-dipole attraction but i'm telling you that one of these molecules experiences hydrogen bonding and the other does not okay maybe you can already guess the one on the right experiences hydrogen bonding why because hydrogen is covalently bonded to oxygen over on the left hand side these hydrogens are not bonded to norf so they do not this molecule over here does not experience hydrogen bonding okay so we're gonna play a little game called is this hydrogen bonding yes or no all right let's go in order let's look at box number one it's pointing to a bond between o and h ladies and gentlemen that is not hydrogen bonding that is a covalent bond hydrogen bonding just like the other types of intermolecular forces are things that exist between molecules not within okay so number one is not number two is this hydrogen bonding yes it is it's an intermolecular force and the hydrogen is bonded to one of those three elements so number two works okay come over to the left hand side number three is that hydrogen bonding it might look like it but it isn't let me tell you why what is the hydrogen physically bonded to is that one of n o or f no it isn't so even though number three really looks like hydrogen bonding it isn't okay let's look over number four up at the top is it hydrogen bonding yes it is it's between molecules and again it's what is the hydrogen bonded to in that interaction it's bonded to oxygen so yes that is an example of hydrogen bonding last one number five no it is not okay now i realize that is all part of one molecule but it is actually and it's not a it's not a chemical bond it's not a covalent bond between those elements so it could be but again what is the hydrogen bonded to nope not norf so that's out okay so you'll see questions like that on multiple choice a lot you know which of the following is an example of hydrogen bonding something like that a little bit later on in this unit we're going to look at um another sort of subcategory called ion-dipole forces when we start talking about things dissolving okay in solutions we're going to see how ions can form forces of attraction between um covalent molecules for example in this picture in front of you on the left-hand side if i dissolve table salt sodium chloride in a glass of water the ions are going to separate those chloride ions are going to attract to the positive side of the water the hydrogen side the sodium ions are going to attract to the oxygen side the negative side okay but we'll talk about that later on when we get to solutions just know that these interactions are stronger than any dipole-dipole attraction but again we'll talk more about it later okay so what are some factors in chemistry that can be explained by intermolecular forces melting and boiling points the temperatures at which things melt and boil are affected by the strength of the imf's because when you melt or boil something you're not ripping the molecule apart you're breaking the intermolecular attractions surface tension okay maybe you'd remember that little experiment maybe you did in elementary school where you can get a paper clip to balance like literally sit on the surface of a glass of water because the water the paper clip isn't heavy enough to break the plane of water surface tension is impacted by the strength of the imf's we already saw a question that involved the amount of heat it takes to vaporize a liquid okay viscosity is a term that means a resistance to flow let me give you some an example or two of a very viscous liquid things like syrup honey motor oil molasses thick liquids that don't flow very quickly are very viscous also tied to the imf's vapor pressure is something we're going to talk about later on when a liquid vaporizes how much vapor is there above it and what is its pressure volatility how easily does a liquid evaporate okay for example when you go and get your flu shots you know the nurse puts a little alcohol swab on your arm to clean the area alcohol you probably noticed it evaporates very quickly alcohol is a very volatile liquid it evaporates quickly the first four these properties increase with stronger intermolecular forces you're going to have higher melting and boiling points greater surface tension higher heats of vaporization thicker liquids more viscous liquids why because they're more their molecules are held more tightly together these two are going to decrease a liquid that has strong imfs is not going to evaporate very easily and so we're not going to get a lot of vapor we're going to have less vapor pressure but we'll talk more about that later on so let me sort of bring this all together now and then we're going to look at an example and then we'll call it a day okay in general if you are being asked to compare molecules that are of similar size similar molecular weights similar molar mass then in general it's london if it only has ldfs that's going to be more weakly held together than something that experiences dipole-dipole attraction which is going to be weaker than something that may also experience hydrogen bonding however and the ap exam loves this if you are comparing molecules that are not the same molar mass not the same size then the question gets a little bit trickier remember that london dispersion forces are the only one that is impacted by the size of the molecule so if the molecules are of different size london dispersion forces can become a more significant factor than dipole or even greater than hydrogen bonding and i'm going to show you an example of what i'm talking about right here again if you want to pause the video and try this one on your own go for it but i'm going to talk through it it says the table above gives the molecular structure and boiling points of cs2 and cos similar molecules but slightly different in terms of types and relative strengths of intermolecular forces in each compound explain why the boiling point of cs2 is higher than that of cos okay well before we even think about that let's determine what the intermolecular forces are that are going on with each molecule first all right cs2 is a totally symmetrical balanced molecule it is totally nonpolar it has ldfs only okay now cos is not a symmetrical molecule it's polar so it is going to experience ldfs and dipole dipole attraction it does not have hydrogen bonding there's no hydrogens in it so we don't need to worry about that but just looking at what intermolecular forces these guys have you would think that cos would have the higher boiling point because its molecules are experiencing stronger forces of attraction you'd think it would be harder to pull apart one cos molecule from its neighbor because they're being held very tightly together there's dipole dipole attraction however i can clearly see that it takes it is we have to heat up cs2 much more to finally get it to boil and that indicates to me that even though that seems contradictory cs2 molecules are held more tightly together and why might that be again they're not the same size okay cs2 is a larger molecule that has more electrons more electrons it's more polarizable which means even though cs2 experiences london dispersion forces only because of its size and its electron cloud its polarizability the london dispersion forces of cs2 end up being stronger than all the intermolecular forces combined of cos just let that sink in for a minute okay the ap exam loves questions like that where the data is contradictory to what you might think and a lot of times guys it is due to the size of the molecule and the polarizability of the molecule okay so i know i've thrown a lot at you but intermolecular forces as i said are one of the most important concepts on the ap exam so it's important that you have a really firm grasp of this concept and can explain the difference differences and characteristics between molecules so i hope you've learned a little something today and i look forward to seeing you next time