so module 3 by now most of you guys already probably through this module you'll know the gist of it so I'll go over the big points the things you need to know going to your exam and make it as high yield as possible we're not going to go into every tiny little detail do every single question because that's up to you guys to do go through past papers um I always say before you go on your exam you should have at least three papers done and some of you would probably give it's scared to that number but if you're not close to that number yet your exam is going to be scary and it's probably not going to go as well as you feel like you'd want it to go I'd recommend if there's one thing to do between now and your prelim even if you only got a week or less is to at least have looked at one exam start to finish look through the market criteria and Mark yourself how you've gone in that and if you do more that would be perfect so what we're going to go through today we're going to go through how you can identify chemical change um all the chemical reactions you need to know how to read metal reactivity series metal reactivity Trends oxidation reduction galvanic cells Redux with going to go through the standard reduction potentials uh and predicting spontaneity and lastly we'll talk about rate of reaction which is really fun and all of that with Collision Theory so lot a lot on the agenda we'll get started straight away as I go if you get lost if you have any questions put them in the chat pull me up because it means I'm explaining it badly and if you don't get it probably means someone else doesn't get it and I'll be going very quickly so pull me back I'll explain things again app to do it twice three times four times even if we need but let me know otherwise I don't know first thing chemical change so we need to know first of all the difference between a physical change and a chemical change physical change is where you just have a change in state usually so you're going from for example liquid to gas or you're going from liquid to solid and this means that you have no change in composition so you might have for example water liquid turning into H2O gas there is no change in composition of this water it's still H2O but it has changed its state it's gone from liquid to gas that's a physical change when we talk about reactions we are not talking about physical change most of the time we're referring to actually chemical change and you need to be able to distinguish chemical from physical chemical change is where you actually do have this change in composition and that's really important to understand any reaction where you see for example um there might be one below any of these so you see a Molly mod kit modeling a chemical reaction you see here ethanol is reacting with com uh with oxygen what type of reaction is this type in chat and throughout it'll be relatively interactive so stay on your feet type things in the chat what type of reaction great Austin combustion perfect first out the first out the gate so we've got chemical change you can see the molecules you start with are different to the ones you end up with and that is chemical change as opposed to physical is everyone with me there chemical physical change CU once you know the difference between them you are able to differentiate them from what you see so when you observe chemical change there are certain signs that you see for example you see the production of bubbles when you see the production of bubbles it means usually one of the substances you've made is a gas and you've made for example in the case of reacting calcium carbonate with HCL this is an acid carbonate reaction your products are going to be salt H2O and CO2 and the bubbles that you get you see are that bubbling CO2 the gas that you've made in this chemical reaction so this is one of the possible indicators indicators if you see bubbles forming does that mean 100% of the time you have a chemical reaction yes or no is that a true statement or false statement if I see bubbles it's a chemical reaction well no good it's not always indicators are things that suggest something's probably happening but are not sure fire signs so there's other places you might see bubbles you might see bubbles from something actually undergoing a physical change from liquid to gas for example when you boil your water you might see vapor coming off that similarly if you smell an odor it might mean you've made a gas that's now being smelt if you change temperature this is a pretty good sign it means you've had an exchange where you've either broke you've put more energy into to break bonds or you've released more energy from making bonds and this is going to lead into a discussion module 4 later today keep those two dot points very close very high yield eight words breaking bonds consumes energy making bonds releases energy and the change in temperature you see in a chemical reaction is because you have an imbalance between the between these two one is greater than the other and they tie into our terms exothermic and endothermic exothermic reactions will release energy because the bonds that you make release more energy than the energy that goes into breaking them in the first place Richard says is that why decomposition reactions are always endothermic um decomp you would be breaking one into two so typically yes but nothing is certain in chem so I won't give the blanket statement but yes that would be the main reason why cuz initially you have to break apart probably put more energy into breaking apart a single compound and when you form two new compounds the energy held in those bonds or the energy released from making those bonds is probably a bit less so yes I would say it's a fair statement but I wouldn't say 100% true nothing's always 100% true in chem other things you'll see precipitate forming so if you make solid from a uh from precipitation reaction you'll see usually a clear solution combining with another originally clear solution and you end up with something that's now opaque you've made a solid precipitate has formed change in color again it's just an indicator for example if you mix two paint cans together it doesn't mean you've made new substances it just means you've combined two things of different colors and now they show a different color um Veta asks is releasing heat the same thing uh releasing energy the same thing as releasing heat yes pretty much when we talk about releasing energy it's typically talking about releasing our heat energy usually we think about it that way makes it a bit easier difficult to reverse is the last thing you can have reversible chemical reactions but for the most part um chemical reactions are harder to reverse Than Physical ones you can't reverse them by simply cooling cooling them back down or changing the temperature so from that we know that chemical reactions you have a rearrangement of bonds you're forming new substances so let's look at exactly how that happens through Molly mod kits what you guys need to be able to do with your Molly mod kits is all Within These syllabus doop points in fact everything I'm going to talk about today is following the syllabus doop points so it's all directly relevant you can be assessed on anything I tell you today pretty much I'll tell you if it's not assessible you need to be able to use modeling to demonstrate the rearrangement of atoms in to make new substances in a chemical reaction as well as the conservation of atoms so this whole idea of conservation is really important when we touched on back in mod one so law of conservation of mass law of conservation of energy what we're talking about here is conservation of matter when we're talking about conservation of atoms it's conservation of matter the number of atoms you start with is the number of atoms you end with in any chemical reaction has to always be true or you violate that law what we talk about in this doop point though is how do we represent a reaction using a model and a model is any depiction description or idea that allows us to make something really complex simple every model has limitations and no model is perfect so what your schools love to test you on in recent years is how does a model suck and you need to be able to look at a model and say how does it not work so we we can all see that this is modeling a reaction we have the balls being atoms and each of these being a molecule and the Sticks being bonds and it's modeling a in this case actually a combustion reaction again how does this model suck type in the chat what is terrible about this model about modeling a chemical reaction you need to be able to use your critical thinking hat in chemistry why does this model suck what are the limitations of this model the Molly mod kit there's many it is too simplistic yes can't see the chemical changes well it depends what you mean by that because the chemical change would just be that we've changed the composition and sort of you can see that but you wouldn't be able to see I guess for example the strengths of the bonds so every single one of these bonds is shown by the same stick so we're not modeling that different bonds of different strengths or we're not showing that when we have to break apart bonds we need to put in different amounts of energy or when we form bonds that releases energy we can't model that using this we can't show accurate distribution of electrons because they don't exist as sticks molecular geometry is not perfect and we can't show transfers of electrons so there's lots of different limitations you can also just say like look this is not to scale this is much bigger than a real chemical reaction between molecules but what you need to be able to do is you need to be able to look at a model that they give you in your exam or they give you in your PRACK exams going forward and be like okay this sucks and reason why be able to critically analyze and evaluate why things don't entirely make sense but that's the main thing I want you to talk about in the modeling section what is a model and how can you analyze models okay writing chemical equations I'm not going to spend too long on this what you do need to be able to do though is you need to know how to write a normal like balanced equation so that's your module one module two stuff you need to be also able to write a complete ionic equation as well as net ionic equations these are the new Concepts for module 3 a complete ionic equation shows aquous ionic compounds dissociated into separate ions so for example if you have the normal balanced equation of potassium bromide reacting with silver acetate making silver acetate making potassium acetate and silver bromide what you'll be required to do to write a complete ionic equation is break apart potassium bromide into its two ions K plus and BR minus both shown as aquous because when we have aquous substance what does it actually mean to be aquous what is an aquous substance type that in the chat or unmute even you're brave what actually does what does it mean to be aquous soluble in water dissolves in water good so when we have potassium bromide aquous yes it exists in solution but it doesn't exist as a lattice anymore once you have dissolved in water you've actually made what we refer to as ion dipole bonds I won't talk too much on those for now but basically every single ion exists independently of the others that it originally was in an ionic lattice with so that's why in a complete ionic equation it's actually more accurate to represent KBR aquous as two separate ions or silver acetate as separate ions same as potassium acetate but you'll notice this silver bromide is still together it's not shown dissociated and that's because it's still a solid in complete ionic in any ionic equation if something is not aquous you don't need to break it up into its ions because this solid will still be in a lattice it will not be dissociated that's the first thing writing a complete onic equation you break up all the aquous ions into independent ions and you leave everything else as is cool ion dioles in prelim syllabus it would be far reaching I would say but some schools do assess it so I would say on the periphery yes but for the most part you need to you need to be able to discuss dispersion dipole dipole and hydrogen bonding if you can do that you've covered 95% of bases if you want the last 5% for an extra 95% effort you go and learn ion dipoles too but it's not strictly speaking needed everywhere okay complete ionic we then move on to net ionic equations and it's actually quite similar you do the first step in finding the complete ionic equation and to turn complete ionic and net ionic you just remove what we call spectator ions spectator ions are ions that exist as aquous ions initially and end up as an aquous ion in the end so I shouldn't have circled silver it's actually potassium starts aquous as an independent ion and then is a product that is still potassium as an Aquis ion and you can see it hasn't changed at all so spectator ions don't react in the equation and we can actually represent our equations without them if we wanted to be more succinct so that's what a net ionic equation is it's a complete ionic equation with all the spectators So k+ ch3 Co just with these all removed and you can see that it's the substance is left over silver bromide makes silver bromide that's the reaction that's happening here and it's the simplest way we can write it a net iic equation in your exam because everyone asked this everyone ask what do I write you write what they ask for so if they ask for net ionic you write net ionic if they ask complete ionic you write complete ionic if they ask for neutral species or they just ask for a balanced equation you usually write um what's at the top here in a written response it's more contextual so I won't strange that you'll have to figure out when you write which one from doing questions but if they ask you to write one they'll usually specify okay so here are some examples we won't go through them but this is what I'd expect in an exam they give you an equation and they say okay now write the net ionic or they might give you the word equation and go okay now write the complete ionic that's usually what you expect okay reaction types there are six categories of reactions to start us off and there are also some metal reactions that come along in the second inquiry question of module 3 so most of you would have seen the general things you would know synthesis what's synthesis in words let's go through these in words type them in the chat what's synthesis in words how do you represent synthesis reactions making new products yes two products create one that's not quite synthesis two smaller compounds actually no sorry no you you correct two products create one my apologies that is correct synthesis is a plus b goes to C in just letters two products create one moving on to decom what's decomposition one reactant broken down into products good I like that wording so you can go a goes into B plus C good so those are just general cases you don't have specific case studies to remember for those reactions but if you see two things combining into one or you see one thing breaking up into two things or more you know that's either synthesis or deom what is combustion one of the worst answered questions across all my classes what is what is combustion so you can keep it really simple and then there's sort of like two branches you need to know great so we've got a few good definitions but I actually like rets the most because it keeps it more General so what is really good about it is that what we need to understand about combustion is it simply burning in oxygen if you remember it as that everything flows from it easier and the things that you can react with oxygen you can either react metal or hydrocarbon or organic compound so hydrocarbon let's just say yeah let's just keep it simple hydrocarbon and hydrocarbon simply meaning a subst that contains H and C only but you can also see organic compounds that might contain oxygen or other things like that so once you know that combustion is burning in oxygen you can combust metal and that makes what if you combust metal good metal oxide metal oxide and if you react hydrocarbon in the chat already it's CO2 and water States being gas liquid for the hydrocarbon it's not black and white um so usually if you want to be safe if the hydrocarbon is less [Music] than believe it's less than five carbons then it is gas and if it's five or more carbons it is liquid but this is only if if it is a hydrocarbon cxh y if you have alcohol where there's an oxygen it changes so take this with a grain of salt okay so that's combustion now onto precipitation what is precipitation two aquous solutions reacting to form a salt and a precipitate good um so that's precipitation Perfect Two aquous Solutions reacting to form a salt and a precipitate I like that and you can represent it in letters as AB aquous plus CD aquous goes to a d let's just say this is the solid plus and I like to be really careful with your letters on the second one I like to say it's CB if you're keeping the order where you're saying that c is the metal and B is the non-metal it just is a little bit more correct but you're not going to be penalized if it's generalized anyway so just something to be careful of but nothing too crazy so precipitation there acid base reactions what does an acid look like what do a base look like that's important because if you can't recognize an acid and base you can't even do these reactions what does an acid look like in its formula and what does a base look like in its formula what do they have acid and base good good good acid is hydrogen base hydroxide yeah nice so your acids tend to have hydrogen with something else whil your bases tend to have something else with hydroxide what's one important base that you do see that doesn't have hydroxide well you're close marrow not quite nh4 NH3 good so ammonia is an important one to keep an eye out for two that's also a base um but apart from that those are your main acid and bases you see and when you react an acid and base what products you make good water and salt let's just say water Plus Salt great and finally acid carbonate so we know what an acid looks like tends to have H so things like HCL h204 H3 you'll learn more about these next year but recognized acids have a h at the front usually what's one acid that has an that is an acid but does doesn't have a h right at the front weird exception what's one acid that doesn't have the H right at the front contains carbon in it yeah carboxilic acid true that's the general way of saying it and AIC acid is the one you guys usually see so acetic acid ch3 Co which are C or ethanolic acid same thing um one to be aware of as well and it's got this H at the end which is different but it's still an acid because of that H very good so those are our reaction types I'll keep it General there actually I didn't finish off with the acid carbonate what do we make acid carbonate makes what good salt water carbon dioxide salt water CO2 okay they're all the reaction types from the first inquiry question and already venturing a little bit into the second inquiry question but we'll well I think pause it there that's pretty much all we need so what you need to do with these reaction types you need to have the word equation memorized or at least understood because in the exam they can ask you to identify a synthesis reaction or identify a precipitation or write out any given one of these word equations so you need to be able to recite the word equation because that will give you the answers in the exam each one of these cases is a different question they can ask you in the exam if you know all these cases in theory you cannot lose the marks okay we won't go through all of these cuz pretty much what I just talked on okay we will focus in on this for a second because this is about to lead into inquir question to so metal reactions we have a few different types but one of which is the displacement reaction so in a displacement reaction what you're doing is you're taking a solution and let's say the solution is copper sulfate copper sulfate is inside of this Beaker and into that Beaker you're going to drop in some zinc based on whether this zinc is more or less reactive than the copper you will observe a reaction or no reaction you will observe a reaction a displacement reaction where the zinc becomes aquous and the copper becomes solid you will observe that if the zinc is more reactive than the copper and this is the premise of a displacement so once you put this zinc in let's say you drop this lump of zinc in not accurate colors just say it's being dropped in you'll see a reaction where the zinc is dissolved into solution and instead the copper sulfate that was in solution already ends up actually I should probably not use blue for zinc that's confusing let's use red since copper is usually actually blue you'll well copper will be brown actually you would see copper coming back out of solution as a as a precipitate and you would instead see the zinc dissolve away into solution and hence you've displaced the copper by adding the more reactive zinc more reactive and this would be less reactive has been displace displaced and this would dissolve and that's your displacement reaction the question is then how do you figure out if two metals one is more reative than the other anyone know the answer where do you go standard potentials good so you you have this list of standard Potentials in your in your data sheet you'll have that in your exam with you in your exam you may get asked I'm adding um ion 2+ as a solid let's say it's Ion 2 solid and I'm adding that into silver solution will I have a displacement reaction happen yes or no Ion 2 solid being added into silver in solution doesn't displace 10 seconds 9 8 7 4 3 2 1 submit your votes we've got mostly yeses and if you said yes you would be correct because Ion 2 all the way up here appears much higher up than silver on potentials meaning it is more reactive as a metal when we see something appear high up on the standard potentials it means it is more likely to oxidize or reduce what do we think if it's higher up more likely to oxidize or reduce oxidize oxidize oxidize good it's more likely to oxidize and as we know when a metal reacts a metal loses electrons and hence when a metal reacts and goes into solution it becomes a positively charged ion the only way it can do that is by losing electrons and so a metal reaction is in essence oxidation and that's a big light bulb moment for a lot of people if you haven't gotten that yet please take that idea that metal reactions are oxidation that is why these concepts are being linked in the same module so linking it back if iron is more likely to oxidize it is therefore more reactive as a metal it is more likely to give up its electrons and that's why iron would displace silver from solution cool I got a question in the chat one quick thing could you quickly explain when the zinc goes into solution uh does it dissolve yes that's correct it will be AIS in solution and that's why you see in your displacement reactions that you see zinc solid become zinc sulfate it would essentially just be zinc ions that are aquous uh in solution okay any questions Okay cool so your precipitation reactions just to be clear you can always figure it out by swapping the two an i or swapping the two cat ions so if this is a these A and C are the cat I and B and D are the anions you can always find the products by swapping around the annion and so you'd end up with a and d and as I said it's probably a bit better to be saying C and B okay solubility rules needless to say if you want to do well you should have these memorized in the nicest way in the nicest way possible if you're trying to get by by just using the data sheet for the solubility rules you're probably not going to be going for the even high range band five or band Six to be in that contention you need to have solubility rules that are pretty much back of the hand and having a list like this memorized and having rules to help you like nag sag PMS castra bear that will mean your exam like speed will just be faster and that's the only way you can make it automatic to get these solubility questions correct that being said if that's not your goal the data sheet does have all insoluble compounds on it that you tend to see so you can use this so make your choice as to where you pitch yourself and where your goals are but I would say if you want to be competing with a higher range marks something like this is necessary um so you should know what is soluble what are the exceptions to what's soluble and what tends to be insoluble as well and what are the exceptions to those cool because then what you'll be asked is questions like this you would be you'll be need you would need to know which of the ions actually forms the insoluble substance so you'd be able to look at question one for example and see okay silver nitrate combining with sodium chloride nitrates always soluble sodium always soluble so when I make sodium nitrate that is going to be soluble and when I combin silver and chloride thinking about PMS Castro bear and nag zag for chloride that's a Group 17 and group 17s are insoluble with PMS s being silver and therefore my silver chloride is going to be solid in my products that has to be automatic or it's going to take way too long to write these equations and you'll be spending too long all right let's keep going so we've done these reactions with acids and a quick summary of those the pyad fruit case study I feel like it's not one of the most assessed things in year 11 and I would be annoyed if I got this in my exam but you basically just need to know that in order to detoxify poisonous food items there are several ways that um abinal toyr is people people were able to do this so you can either use hydrolysis you can wash and leech them you talk this talk about this more in module 5 next year you can even ferment the pyin or you can hook it out but there's different ways whatever your school taught you write what they taught you because that's what they're going to mark for your exam my notes are not going to be as specific as what your school has given you so use what your school said okay reactions of metals we have already talked about some of these but the ones that we need to talk about more in depth now metal with water we've done metal with acid metal with oxygen we technically did combustion but this can appear in different context it makes metal oxide regardless and then with other metal ions and solution we've already talked about this too this is our displacement so what's left is essentially just metal with liquid water and metal with steam metal with liquid water makes a metal hydroxide and hydrogen gas and metal with steam makes metal oxide and hydrogen gas these are important for our reactivity tests we're going to do in a second so keep these in mind but the other reactions we've all seen these already Okay cool so the first investigation that we do you can test different Metals for their reactivity by reacting them with different temperature water and even you can test them all the same temperature of water and then test them at different temperatures too so what you can do is you can react different Metals with liquid water and depending on how vigorously they react or if they react at all you can segment certain Metals as being more or less reactive similar with steam if you have metals that react with that don't react with liquid water you can give that water more energy by making it steam more kinetic energy and encourage more reactions to happen with the less reactive metals and again some metals will react with steam some metals will not so you create these stratifications these sort of cut off ranges between metals where some are more reactive and some are less reactive so much so that there's a cut off where some will react and some will not with different temperatures of water okay and your findings ultimately to summarize it really simply for you guys you find that Metals above calcium on your reactivity series will react with liquid water and everything below will not then from Iron to magnesium they react with steam but not with water and everything below iron does not react with with steam or even or liquid water as well question in the chat is do we have to memorize what reacts with steam or cold water I would say if you want to be very secure in how high Your Mark is going to be this exam I would memorize this these important cut offs because there's actually only two cut offs you have to remember and I'll show you those really simplified in a second but for now note what these cut offs are and it'll become very simple in a second I promise you so the cut offs I want you to remember are the C between magnesium and calcium and the cut off between tin and iron that's all I want you to remember the cut off between calcium magnesium the cut off between iron and Tin all right because with metal reactions you have a similar sort of cut off where every metal reacts with acid except those less reactive than iron so you take that cut off again so you if you reacted all these different Metals with acid all the ones in green will react in decreasing Vig until you reach 10 which will not react and then all those below will not react same with oxygen important cut offs calcium to magnesium ion to 10 because essentially below here you're not reacting unless you provide a lot of heat and do combustion so everything in green reacts with oxygen in decreasing Vigor the cut off between iron and Tin is where you start to require energy to actually pred this reaction so I would just remember that cut off and to keep it really really simple I like this graphic between calcium magnesium between iron and Tin if you have those two cut offs those will carry you through 90% of the procedural questions where they ask you what will I um need to or what will I need to do if I want to separate the reactivity of say barium from zinc from copper and you go okay great I'm going to react it with um cold water and see one will react out of the three which will be barri him and then I'm going to react the remaining two with steam and I'm going to see that one react and that should be zinc and then finally I'm going to react um the last substance with say I mean you very determined at that point you can say well then copper must be the last metal and it is the least reactive of the three so that's the question you need these cards for because they can give you three metals and they say separate the react it describe procedure and you need to have the cut offs unfortunately the notes aren't available for today they are available to all Catalyst students um so if you are interested in having the notes for next term and the four terms going on from here we can give you those from next time going onwards but the notes will not be released today there's two segments of the notes on the booster pack though I think the one coming up there's going to be I think I included galvanic cells it might have been o reduction and for mod four there's one inquire question as well so you have some in that booster pack in the email that will sent to you okay oh yes and I should flag as well as we said before all the metal reactivity order is in your standard potentials so you can always if you know these segmentations you are just drawing lines in your stand of potential and going okay and the only flag is well calcium is out of order in your standard potential so calcium actually goes below magnesium and you would draw a line there and you can see it sort of lines up actually sorry no calcium goes above magnesium like that oh no it's correct it is correct and you draw a line between calcium and magnesium not there you draw it here that's your first cut off and then your next cut off is between iron and Tin so you go find tin you go find ion to your next cut off is there and you have those two lines to guide how you construct your procedure everything else is in order on your standard potentials from most reactive at the top to least at the very bottom in terms of metal reactivity anyway okay and then tying that together you should know your metal reactivity Trends so it relates all back to your ideas of ionization energy in particular as you go down as you go down your ionization energy decreases what's the reasoning behind this as your ionization energy as you go down your ionization energy will decrease why is this the case increasing number of orbital shells this is true but why shielding okay shielding is the word we need shielding shielding shielding and I I be careful saying atoms move further away it's the electrons are further away because the atom is the whole thing together but we need to say that we have an increase in electron shielding because there are now more electron shells between the nucleus and the valence shell so you've got all these inner electrons with a negative charge pushing outwards out out out out and repelling this outer veence shell and that's offsetting the attraction from the nucleus which has obviously it its protons inside so it's always a balance it's always a balance between nuclear attraction and electron repulsion repels repels repels okay and as we go down the group we're adding more shells the strength of that repulsion just gets stronger and stronger and stronger to the point where the electron in the valence shell is held so loosely that it can be ionized very easily and hence that metal becomes very reactive so the most reactive metals are in the bottom left and the least re reactive I mean these aren't Metals anymore but the least reactive in terms of metals would be the ones up here shielding great question what is shielding so shielding is essentially where you have shells of electrons in any atom and because electrons are negatively charged so negative negative negative negative negative negative negative negative negative negative what are they going to do to each other attract or repel negative charges negative charges repel so the whole idea of shielding is that electrons on the inside of a shell that is on the outside that inside shell is going to repel the electrons that are just outside it and that phenomena is shielding because it's shielding the electrons away from the center and that shell that has just been shielded also has negative charge and the one outside it also is negative charge that second shell is also going to be pushing out against the outer one and so you have shielding more shielding and the more shells you put in the stronger the shielding effect becomes and that's what we talk about when we say shielding and that's how you explain ionization energy electro negativity as well as atomic radius it's all about shielding versus Atomic attraction hopefully that helps okay onto Redux Redux we have the idea of oxidation and reduction oxidation is loss reduction is gain hopefully that is at least familiar oil R oxidation is loss reduction is gain and what are we gaining what are we losing we are gaining and losing electrons good so it's all about electron transfers electrons okay we can represent every single oxidation reduction using a half equation so when we talk about reduction oxidation half equations each half equation represents either an oxidation or a reduction can you have a reduction without an oxidation yes or no good you cannot every single time you reduce something something else had to be oxidized in order for that to happen so these are like synchronous processes when one happens the other must happen too reduction happens oxidation happens oxidation happens reduction happens at the same time and in any full Redux equation it is simply the combination of a reduction half equation and and an oxidation half equation hence Redux equation a reduction oxidation equation put together so this one here we have a magnesium that is turning into a mg2+ and giving off two electrons in other words it has lost two electrons but every time this reaction happens and this is a oxidation on the other hand you have these hydrogen positive ions that are taking in these electrons gaining them to become a neutral hydrogen molecule and these have each H+ gains one electron for each reaction that happens and hence this is a reduction cool so starting off simple the first question you're going to get in your exam how do I how do you write the half equations of blah or write them out so you need to be able to look at an equation and figure out what is being oxidized and what is being reduced so if you look at calcium reacting with oxygen to make calcium oxide you can figure out what's being oxidized and reduced in sort of different ways the most foolproof way is by by looking at oxidation States oxidation States this calcium because it is a lone atom an atom that has no charge is going to have an oxidation state of zero so all this oxygen that is neutral at the moment it will have an oxidation state of zero what are the ox oxidation states of calcium and oxygen we'll do calcium first oxidation state of calcium what do you think that is nice it's two plus because it it is a group two Metal 2 plus being group two and for the oxygen what is that 2 minus great so oxygen always 2 minus calcium has to be two plus since group two and it makes sense because if you have one calcium and one oxygen the overall charge is zero and it's all balanced it's great so what's been oxidized what's been oxidized we got one vote for CA anyone in disagreement anyone in agreement second ca ca right okay zero goes to plus two the only way that this could have happened is if the calcium lost two electrons along the way to give it a two plus char charge and hence this calcium has been oxidized and for this oxygen it's going from zero to 2 minus it has had to gain two electrons in order to gain a 2 minus charge so then what's happening in every Redux reaction is that one thing gives up electrons and the other thing accepts those electrons and simply an electron transfer from one substance to another one element to another here the calcium is giving up its electrons two of them being oxidized in order for the oxygen to gain those two electrons and become reduced okay and so when you write the half equations you need to show that calcium turns into C2 plus and gives off two electrons the reason why it's solid here is because the calcium oxide in the end is a solid and for the oxygen in order to make the oxygen gas for O2 you would make two oxygen ions since you have two atoms turn into two ions and the only way you can create two lots of minus 2 charge is by providing the initial oxygen molecule with four electrons since 2 lots of -2 is -4 okay and those are your half equations is it safe to assume that any element this is Sol has an oxidation state of zero uh any the key word there is element it is safe to assume that any element on its own that has no charge is going to have an oxidation state of zero I would say that okay the second question type is combining half equations so if you have two half equations they give you and they say write the Redux write the full equation you'll need to do this by balancing the electrons to cancel them out so you can see in these two half equations if you're reacting aluminium with dilute acid you need need a way to get rid of the three electrons and two electrons by adding these two equations together but if you added them at the moment you would have a leftover electron so the way we do it is we create the lowest common multiple of electrons we multiply the top equation by two and the bottom one by three and that gives us these two half equations which you will now Note have six electrons each so when you add these equations you end up with six electrons as reactants and six as products and that means you can cancel them out because ultimately the net effect is you have no change in the electrons at all so showing it net ionic you get rid of the electrons giving you an overall equation which represents that transfer of electrons without having the electrons in the equation okay oxidizing reducing agent is the oxidizing agent reduced or oxidized okay couldn't get anyone oxidizing agent is reduced reducing agent is oxidized oxidizing agent oxidizes something else so if something else becomes oxidized that other substance had to lose electrons and that means the electrons have to be gained by something else and they go back to the oxidizing agent which gains them and hence oxidizing agent is reduced reducing agent is oxidized the vice versa case as I was going through briefly before oxidation States you should be able to identify oxidation States for any element in a um reaction so there's a few rules when we're assigning oxidation States number one particular atoms have assigned States so Florine is always minus one hydrogen is always plus one except for an specific case Oxygen's always minus 2 Group 1 2 and 7 are + one plus 2 and- one always then this was kind of relating back to the question from before for molecules um with one element only or like single element substances in a neutral form where they have no charge the oxidation state will be zero so diatomic molecules zero sodium magnesium argon neon all have zero because they're just lone elements on their own in a neutral state for molecules and compounds the overall oxidation number can be found by summing the constituent atoms and adding them up to zero so for magnesium oxide carbon dioxide H4 these all have an overall oxidation state of zero because they have a neutral charge they're all got a zero charge and same for ions if sulfate has a 2 minus charge it's oxidation state is minus 2 and if you would add up sulfates number with Oxygen's number numbers say it's Y X + Y should be equal to -2 or y y * 4 since there's four oxygen would be more correct okay question in the chat what about group six oxidation numbers then from my understanding they're not rigid and fixed in the same way that group 7 1 and two are and they can be different from minus 2 so they're usually not Dependable to to like assume first so if you have group one group two or other known valency Metals bonded to Something in group six you would usually use that metal to figure out what the group six oxidation state is I don't think you can assume group six from my understanding it's a good question though would the oxidation state of o4 be 8 minus or 2us good question so the oxidation state of the oxygen one oxygen is minus 2 and I will think about it as the contribution of the oxygens in that sulfate are minus 8 their contribution of four oxyg is -8 to the overall oxidation state of min-2 for the sulfate does that make sense any hopefully let's do one is practice I think this is an important one to do let's say K2 M4 what is the oxidation state of the manganese type it in the chat when you have it e just a little bit longer okay they're returning to where we were okay M4 potassium sorry we're doing K2 M4 potassium is plus one as it's group one but you have two of them so overall it contributes two 04 each oxygen has an ox state ofus two but you have four of them so it contributes minus8 the overall oxidation state of the whole K2 m04 is zero but I was asking for mag for manganese specifically what is its oxidation state and you can find that by doing zero should be equal to + 2 plus- 8 plus mangan's oxidation state and you would end up finding that it is equal oxidate is equal to plus 6 any questions there okay let's keep moving cool we've done that all right final thing is looking at predicting redo spontaneity if you have any um standard potential the rule is if the standard potential is negative the oxidation half equation is spontaneous that's the first rule that governs what you say so if the standard potential is negative the oxidation half equation is spontaneous so that would be for any of these ones up here they want to proceed going that way because at the moment these are all written as reductions you can see that K plus plus a K plus plus an electron going to K solid that is a reduction the potassium is gaining an electron but if we wanted to represent the oxidation we'd simply show the reaction going the other way we'd go from K to K plus plus e minus that would be the loss of an electron instead and that would be the spontaneous reaction for all of these potentials where it is negative because the verse reaction going that way its standard potential would be positive 2.94 instead of negative 2.94 you simply flip the sign if you flip a reduction or oxidation and whenever the sign is positive it means that that direction is spontaneous so it makes sense then for the bottom half if the standard potential on the sheet is positive the reduction half equation is spontaneous since that is now the positive number and as I said before substances that are higher up are more likely to oxidize substances that are lower down are less likely to oxidize and that's why things at the very top are metals we consider those to be the most reactive metals because they are most likely to oxidize now we know all right on to the big one galvanic cells pausing for a second take a 30 second breather get a drink of water if you need but don't go too far 20 seconds question is are they going to be questions like calculate the cell potential of Redux reactions um in a galvanic cell yeah in a galvanic cell there will be and that's a very common question type that we will go through okay let's get started again galvanic cells probably the most feared Topic in all of prelim and HC chemistry everyone who goes out from prelim to HC forgets this and it's very difficult to initially wrap your head around but when you have it it's not too bad so gavan Excel I want you to understand that this is a device that uses spontaneous Redux to create electrical current and it's converting your chemical energy chemical potential energy into electrical energy and it's what we use to make electricity in batteries for example generally speaking in a galvanic cell you have two metal electrodes which are joined together by electrical leads so an electrical lead like this then you have the leads attached to the electrodes which are submerged in a salt solution so these Salt Solutions we refer to as electrolytes and if you want to be more specific the one that the anode is in is the anolyte and the one that the cathode in is in is the catholyte but electrolyte is fine and finally there's a salt bridge that joins the two electrolytes so the four key components are your electrolytes sorry your electrodes firstly your anode and cathode your electrical lead you can mention the volt meter not essential but it's good to say it anyway then you have your electrolytes and you have your salt bridge four things that you talk about in your composition then you need to understand which of the anode and cathode is going to be the oxidizing one and the reducing one and you can remember an ox red cat as your acronym the anode oxidizes the cathode reduces so how can you tell that zinc is going to oxidize and copper is going to reduce if you got these two Gonic cells how do you know that zinc is going to oxidize and copper is to reduce good Austin standard potential so you know that zinc appears higher than copper if zinc appears higher than copper it means it's more likely to oxidize therefore it should be the anode and if copper is lower down it's more likely to reduce it should be instead the cathode and you have your two I guess cases laid out there that's all you need to be able to determine to figure out what is the anode what is the cathode first thing done second thing if you know what's being oxidized what's being reduced we now need to figure out what is actually happening in this whole big circuit I want you to think of it as a loop a looping system where charge just moves around and around and around and before we get into that we have to define conductivity so conductivity you can think of as the ability to have free moving charge particles so if a substance is conductive it means that the it has some sort of charge particle which can move whether that is electrons or it is ions these are your two options so when we said in module one metals are conductive why are metals conductive what that reason that we just memorized why are metals conductive free moving electrons good so you have your C of delocalized C of delocalized electrons and everyone just throws it out there as the reason why but not many people actually understand why if you have a metal and it's the ion like it's got its metallic structure it's all these positive ions and these little electrons that hold together like glue and they can move around freely so they can Bounce Around off the edges and go wherever they want that ability for a Charged particle to move around in a substance is what makes something conductive of electricity and you can also say if if substance can move freely it helps to make things thermally conductive as well we won't get into that we just say if a substance has charged particles that can move freely then it is electrically conductive so then for a solution that we use in our ganic cell C cuz we have in our gavic cell two pieces of metal we have an electro lead which is metal those are allowing for free movement of electrons but we also have a salt bridge which is not strictly a piece of metal and we have these two solutions which are also conductive so what makes those two solutions conductive and the delocalized electrons not the the salt bridge and the electrolytes what makes those conductive yeah I think I asked it really badly let me say it again what makes the solutions conductive charged ions good sorry that's my bad so the solutions are conductive because in those Solutions you have zinc ions for example you have zinc ions zinc 2+ floating around you have sulfate floating around and you have copper floating around and you have sulfate floating around that makes these Solutions conductive because they have free moving charged particles that is what makes something conductive and if you can get that into your head everything we talk about with conductivity in year 11 and year 12 going on from here will be a lot more sensible okay so the galvanic cell you have an external circuit and you have an internal circuit your external circuit is where your oxidation reduction is happening and your electron transfers are happening so as we said the anode is where you have oxidation um why does the pres of free ions conduct electricity because they are able to move freely from one side to the other and you can move charge from one side of a solution to the other side therefore that's as much as you need to to take if you can move charge freely that permits the property of electrical conductivity okay back to here anode is where we have oxidation cathode is Where We R C over time one of the phenomena that you'll see in a galvanic cell is that anod will shrink whilst the cathode will grow and so the anode ends up becoming like this little shrunken shriveled piece of metal whilst the cathode ends up growing like this massive sort of bulb around as it takes up ions around it and that's because the anode is oxidizing turning into aquous ions so all of it is turning aquous whilst the cathode is taking up all the aquous ions and making it into solid so that's what's happening when you usually see the shrunken little one and the Really R out cathode Okay cool so then why does it matter that we have oxidation and reduction and Samar asked just to clarify ions electrons um for what in particular before I answer that question as in what gives conductivity in solution when the cathode is gaining electrons yeah so I'll answer the question as if it's when you uh when you are reducing the cathode is it gaining ions are electrons it is gaining electrons and it is an ion that gains those electrons so to clarify that there does that make more sense it's this copper ion that's in solution that is gaining to electrons and hence becoming solid that's what's happening yeah okay and then as that happens because these two are joined by an an electrical lead the anode has lost electrons whilst the cathode has gained electrons and so the natural flow of electrons is always from anode to cathode because the anode will be losing electrons whilst the cathode will be gaining them and over time that is the natural anod to cathode direction we see that's why it happens and that all happens through the electrical lead in the external circuit to be very clear ions do not move through the external circuit it is only electrons that move from anode to cathode only electrons and electron flows from anode to cathode that's correct okay anode cool over this lead often times you put a voltmeter and it can measure what we refer to as voltage um and this is due to the fact that ohms law states that voltage is current times resistance the lead will have some amount of resistance current is basically how much charge you have moving per second you can kind of think of it that way how many charge particles are moving through per second and if you are moving lots of electrons through then it's going to be nonzero and you will have some voltage formed because of that you don't need to know ohms law but that's just why we're forming a voltage here and we'll talk about the standard cell potential in a second have you encountered problems when they have asked for current flow Direction instead of electron flow Direction I don't think so but I'd imagine it's the same I'm not 100% sure so don't don't quote me on that one and the second question is do the ions from the salt bridge also move um so K plus and NO3 minus we'll get to the salt bridge in a second I'll explain it then Okay so we've got electrons moving over the problem is in this circuit if you move electrons from anode to cathode you're going to have a buildup of charge in either solution because the anode the the anod electrolyte solution is essentially just having positive ions pumped into it as the zinc solid turns into zinc 2+ aquous plus 2 e minus you are essentially just dumping positively charged particles into that solution so you'll build up over time positive charge in this solution positive positive positive and for the other side because you're having copper 2+ and electrons to make copper solid the copper 2 plus the positive ions are being taken up so you're actually becoming more negative on this side instead and if you build up enough charge this whole circuit can stop working because it becomes increasingly difficult to remove negative charge from a positively charged solution and vice versa so this is the big problem that the salt bridge solves does everyone make sense of that problem as you put more positive ions into solution or you take positive ions out you will either become more positive or more negative and build up charge which prevents the movement of electrons and will potentially stop your galvanic cell from working okay and that's where the salt bridge comes in the salt bridge serves the sole function of returning your solutions to neutral charge ask why is positive ions um wouldn't it be negative because it's oxidizing good question where does the negative charge go the negative charge is exiting through the external circuit these electrons but the positive ions are instead going into the solution and that's what's building up building up there the positive Muhammad asks does the cathode solution become light because these ions are depositing on the electrode um yeah you could say that it would become less I mean it would it would lose Mass CU you losing ions from it yeah and Oh you mean lighter in color yeah yeah yeah yes if it if it was colored in the first place because you would be reducing the concentration of ions in solution so yes I agree okay but back to the problem we were saying if we built up this positive and negative charge the only way to remedy it is we bring back everything into neutral charge and so you can do that by using a salt bridge it's NAC here I don't really like that usually you see N3 minus the salt bridge you can think of as a tube that contains a gel inside of it and the gel contains ions that can move freely and that's usually a a soluble salt gel like sodium nitrate or potassium nitrate something very soluble that doesn't precipitate with anything into the positive solution the salt bridge will drop negative ions so nitrate for example and to the negative solution the salt bridge will drop in positive ions and that will bring that solution back to neutrality and wipe out this buildup of charge which prevents the cell from breaking and so you keep going around on the external circuit building up charge salt bridge drops in ions brings back to neutral and you go again and again and again and again and again that's the idea of the salt bridge okay so to summarize and be very clear two electrodes anod under goes oxidation cathod under goes reduction electrons go from anode to cathode the anode will spit positive ions into the solution and this will build up positive charge whilst the cathode takes up positive ions and builds up negative charge that's problematic because if you build up too much oxidation reduction becomes difficult to proceed you can solve this problem by using a salt bridge which will drop in oppositely charged ions to neutralize those buildups and hence keep the circuit going and that is the whole premise of a galvanic cell uh you do not need memorize Color Solutions I would not say for year 11 even year 12 not really yes I can explain that so the question is why is there a build up of negative charge in the cathode in the cathode as you undergo reduction copper 2 plus is combining with two electrons to make copper solid so what's essentially happening is the two electrons that come in through the electrical lead combine with a copper 2 plus that was in solution and hence you are removing a positively charged ion from the solution if your solution becomes less positive it is therefore becoming more negative and that's what's happening there that's why you build up a negative charge because you are losing positive ions does that make more sense tomorrow okay great cool there are three ways that you can break ganic or make it stop working number one your anode runs out so if your anode completely turns into ions you'll have no solid left to fuel that process case number two your catholyte or the electrolyte that your cathode is submerged in if that runs out of ions for you to be taking out then you will have nothing left to fuel the reduction and so it'll break once again and the final way is if you have no salt bridge or your salt bridge runs out of ions and that would build up your charges you have nothing to neutralize the solutions or bring them back to a neutral charge and hence the movement of electrons will stop you need to be able to notate your ganic cells in the referred notation which is anode and anite cathol cathode separated by a single line a double line in the middle to represent the salt bridge and then the catholyte and the cathode okay there are also three types of GIC cells I'm not going to go into too much depth with these but basically you have the type that you have two reacting electrodes this is what we've talked about so far you have the type where you have a non-reacting electrod and this is where you see like Platinum for example um essentially all the Platinum is doing is helping you to transmit the electrons into solution and you will often times in solution have two substances one which is being reduced into the other and that's actually acting as um where the reduction is happening instead so the electron essentially just going straight through the platinum and going into solution and that's a another case that often comes up up and finally you can have the non-reacting electrode with gaseous Redux where you have gas going into solution and if the gas is being reduced in this case in particular that can also function as where you have reduction happening I would worry mostly about your two reacting electrodes case maybe the non-reacting electrode and if you're at James Roose or anything like that you may want to look into the non-reacting electrode with gases Redux as well but otherwise I would say for the mo for the most part that's what you need for the galvanic cell structurally this ties us into predicting Redux reactions and when they will or won't happen so if you get a question such as this a ganic cell is constructed between magnesium um nitrate and silver electrode and silver nitrate there are a few parts you need to do number one and the question is I should read the question write the reduction oxidation half equations and the overall Redux equation determine whether Redux reaction is spontaneous and write the galvanic cell notation um let's just focus on getting the overall Redux equation so if you get two half equations you need to figure out first of all what's being oxidized what's being reduced whatever appears higher up is going to be oxidized whatever's lower down is going to be reduced and so you want to show the oxid equation flipped around therefore to make it oxidation and that's where we get these two equations first then when you want to combine them together you need to make sure you have the correct number of electrons so that's why the bottom equation has been multiplied by two in order to give you two electrons and two electrons so that when you add the two equations together they cancel out and you get your overall equation then the question that you always get followed up with on this is will this be or calculate the cell potential and there's two ways to do this everyone gets a mixed up so both of these methods are correct some depending on who you are and how you think one will be more confusing than the other so I don't want to confuse you but there are two ways to do this and they're both correct you can either use e cell equals e of the reduction of of the cathode minus E of the reduction of the anode this is the version where you don't change any signs you just take what's on the data sheet or you can do e cell equals e of reduction of cathode plus the E of oxidation of the anode both these are correct I would say the slightly more correct one is probably this one if I was thinking of it logically but they're both correct doesn't matter what you do they'll both work so the first one because we know the anode is for magnesium this is the reduction of magnesium minus 2.36 this is the reduction of the cathode 0.80 and to find the overall standard cell potential you do cathode minus anode and you see how the minuses you have a minus minus gives you just a positive so it's essentially 0.80 plus 2.36 see how that resembles this and gives you 3.16 the alternative way the second way is you say that okay let's actually think about the cell potential of the anode as an oxidation because that's actually what's happening and to find the cell potential of an oxidation you flip the sign you flip the sign to give you positive 2.36 and if you wanted to add them together then you get the exact same number so whatever you prefer do that I'm not going to dictate what works better but they're both correct and a lot of people get confused on these because of how similar they look okay questions in the chat what are some reasons why you won't get a read on the voltmeter other than the salt in the salt bridge forming a precipitate with the electrodes [Music] um I mean I would imagine it's the reasons that I said before as well so it' be if you've depleted your anode or you've depleted the cathol you would have nothing left to to actually make electrons move and I think that would give you a volt meter reading of of zero as well and the second question is when the question ask for the name of the cathode and anode do you say the name of the inert electrode or the name of the actual reacting half cell yeah so this is a question I've debated myself actually don't have a solid answer for this one I think that you still say the name of the inert electrode as a cathode um I would do that because that's what you notate it as in the in the galvanic cell notation I would do that yeah exactly how would you answer a question asking identify Bridge great question you just memorize what works and the two things that work for a salt bridge are K3 and N3 every time will always be correct these are your two go-tos you choose one of them because they're always soluble and they will not precipitate with anything the reason why something like NAC might be problematic is because chloride can precipitate with um your PMs and that might block up your Bridge so I would just stick with with these two if you get asked that question okay questions questions questions and we're on to the last inquiry question I'll keep it very brief on rates of reaction because it's a bit more straightforward Collision Theory determines how fast a rate of reaction a reaction will react and there are two things that dictate a successful Collision or how many successful collisions you have per second so the rate of reaction is essentially how many successful collisions per second and there are two things that dictate that yes we'll be doing module 4 um from maybe 707 7 o' was when we no hang on 6 o' was when we planning to do it so how many sexual collisions per second two things that decide it your success rate of collisions and the number of collisions each of these four factors temperature concentration surface area and Catalyst affect either or and or success rate and or the number of collisions and you need to be clear on which does which firstly how would you maximize rate of reaction with temperature would you increase or decrease it what maximizes rate good so everyone's on increase perfect you want to drive it high concentration high or low to maximize rate of reaction High good surface area high or low high high high high high good and lastly Catalyst do you want a catalyst or not yes okay good so very self-explanatory I hope this module ending is a bit nicer than the rest of it but what I want you all to take away and this is sort of the the special source that a lot of people Miss on collision theory is does that factor affect the number of collisions and or the success rate so let's start with temperature does it affect the number of collision per second yes or no yes good so it affects number does it affect success rate you got a few yeses it does affect success rate so it increases success success as well so I just represent success rate as a percentage sign increases number of collision and success rate is the only one of the four factors that does this so keep that in mind concentration does it affect frequency yes does it affect success rate success rate yes or no does not affect success rate only frequency surface area success rate or frequency good it is frequency so increase the number of collisions and lastly catalyst success rate frequency catalyst is Success R not frequency and the last trick one what if you had a solid Catalyst and you broke the Catalyst into smaller pieces is that increase inreasing frequency of success rate if you break a solid Catalyst into smaller pieces are you increasing frequency and or success rate okay we got a few interesting actually I should say let's compare it against a case where you have the catalyst so you have a say a cube of catalyst and you break it into lots of little small particles would you in I I will say you will increase your rate of reaction is this change through an increase in success rate or an increase in frequency yeah good question Austin I should answer that so success rate you can think of it as any one Collision that happens what is the percentage chance that's going to be successful for any one Collision so if you have a 100% success rate it means every single Collision that happens your particles will be orientated correctly and they will have enough energy to overcome activation energy that's 100% success rate every Collision that happens will be successful but that's not true for the most part it will probably more like 20% or 10% something like that so if you have 10% success rate one in every 10 collisions will be successful because it have the right orientation and the right amount of energy and every the nine remaining collisions that happen will be all unsuccessful so when when I say we're increasing success uccess rate what I mean by that is where increasing the percentage of collisions or the percentage chance of a single Collision rather the percentage chance that a single Collision becomes successful and if you're going from a 10% success rate to a 50% success rate that means that you now have five times higher chance of colliding successfully and doing a successful Collision that's what I mean okay cool and I would say that's pretty much the main things on rates of reaction and collision Theory um I've got PRS here but they're not really worth talking through anymore they're all self-explanatory Collision Theory you need to have sufficient energy so overactivation energy be orientated correctly we'll talk more on activation energy in mod four energy profiles again we'll talk more on mod four there are two ways to increase rate of reaction as we said increase the number of collisions or increase the success rate so you increase the energy that particles collide with or you can decrease the activation energy through a catalyst so the answer to the last question was it's only frequency sorry got sidetracked only frequency because you still have the same Catalyst which decreases activation energy so the chance of any single Collision happening with the presence of that catalyst is still the same but you just have more collisions that are able to make use of the Catalyst per second and so you'd have a higher frequency but You' have an unchanged success rate hopefully that makes sense and saying that's the end of module 3 uh yes surface era correct it's because you've increased surface area by breaking up the Catalyst into small pieces and apart from that that's the end of module 3