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AQA A-Level Chemistry - Intermolecular Forces

Jun 05, 2021
Okay, so this video will address the topic of

intermolecular

forces

, it is a unit 1 or

chemistry

1 topic in the AAS specification, so if AQ a comes as part of the link topic, there is a shapes and molecules video, this will be purely just look intimately at

forces

, what they are, how they work within molecules, then you will end up looking at Surma, some exam questions which focused on how you can apply your understanding in an exam situation, so firstly ,

intermolecular

forces, so what does this mean? deliver temecula forces or interactive force actually mean good with anything, if you can break down the term, you can figure out what it really means, then you're halfway there, you've almost won the battle, such intimate forces. the term inter which actually refers to between and places that can be seen, an intercity train is a train that travels between cities, not within its interval, and that is very, very important here, the term molecular is explained by itself.
aqa a level chemistry   intermolecular forces
We're actually talking about molecules with reference to a molecule, so these are forces acting between molecules, not within the internal force, within the molecular force, with a molecular force. It should say things like the covalent bond, the ionic bond, and your metallic bond. talking about the forces that are actually created between molecules and I can't emphasize that you really can't, so what are these forces? Well, there are three that you need to know and it is important that you certainly know what these forces are. Actually, do you know their name? I'm going to start with naming their names before going into detail about each one and said then I finished with some exam questions to get all the ideas together. so the forces are, we have van der Waals forces, let's call them abbreviation V D W, we have dipole-dipole and we have hydrogen bonds, so those are our three forces.
aqa a level chemistry   intermolecular forces

More Interesting Facts About,

aqa a level chemistry intermolecular forces...

There are a few different names for these forces. You could put the permanent term in front of our dipole. dipole here and these could be classified as dipole induced dipole but I'm going to stick with the Vander files dipole dipole just as a general term there's a separate one and hydrogen bond there as a different one there again if you look through a mask I'll see there's also everything kind of other names. I'm not going to go over those other names. These are the terms I'm going to use. These are terms that I'm happy with and I think you know they work well if You can stick with these in an exam, you're absolutely laughing, the important thing here, Australia in the world, the important thing here is that these forces are not equal, particularly the way they function within a molecule, took the strength of forces. the force is not equal, we discovered that I have done this in order like this on purpose, starting here at the Van der Waals forces, this is our weakest force and this one up here is our strongest force.
aqa a level chemistry   intermolecular forces
Now keep in mind that these are not a strong hydrogen bond even here. They're not as strong as our proper bonds, these are intellectual forces, they're much weaker, they have an effect, they have a huge effect and things that you would have done with both of them all through A

level

and GCSE, and you know, ketose makes everything. the rest have a very important effect, but they are not as strong as the proper bonds if you like ionic covalent in metal bonds, so it is usually intellectual forces and the three forces that are normally this force that we have.
aqa a level chemistry   intermolecular forces
Speaking of which, it is very, very important. I'm going to go through them now to break them down into how they work and analyze them in a little more detail, giving some examples and everything else, starting then with van der Waals. forces, so Van der Waals forces, look what color I want to use, we're going to go to Valve, so this is the weakest intermolecular force, as you saw in the previous slide, so these are the weakest ones that we use. you're going to deal with in AAS Unit One, what they really are, although van der Waals forces exist in all molecules, okay, that's key, they exist all molecules are the weakest to intimately inflect the force, but they are there in all the molecules, while the other two are not necessarily going to be there. be there, the way they work is that if you imagine that you have a molecule and we imagine, let's just say that we have to say that we have the iodine split, so that the iodine molecule that is made of one iodine atom bonded to another island through covalent bonds now if we were to actually think what we find here is that around these atoms inside this molecule we have electrons so there are electrons inside the molecule and these electrons are constantly in motion they are not static , they don't sit in these lovely little orbitals, they just sit there waiting for reactions to happen, they're moving, they're active and they're going all over the place, but the key is anywhere.
There are probably more electrons in one place in the molecule than in the others, so, for example, if I talk about the electron density that I could find if I looked around this molecule, maybe I would have something like this and What I mean here is that I have less electrons here and more electrons here, so the electrons have moved inside the molecule and I have a greater weight of electrons on this side than on this side and what that does is create an induced dipole. so I can say that this side because there are less electrons is there, I guess Delta is positive and therefore this side would receive a negative result.
Now the key is that these electrons are moving, so as soon as this becomes apparent, it could flip. and it could go to the other side where we have many here and then a smaller number and again that changes this now becomes Delta negative and this number comes Delta positive the key is in a group when you have many molecules Together, this random change happens all the time. time, so one molecule becomes net Delta, non-negative there and positive Delta there, which is attracted, therefore the next molecule is negative Delta and positive Delta which is constantly changing, there are constant attractions that occur and change constantly, so where there was an attraction and there is no attraction where there was one there is not everything else, but the general effect is that these temporary dipoles basically cause a general attraction and that is really very important, it is really key, so we have a general attraction that I don't worry too much about the theory behind it, but it kind of helps explain it, gives you a little grounding in this kind of what's going on, so the general change in where the electrons inside the molecule means that therefore, they can be attracted to other molecules due to the opposite opposite charges, so Delta positive would be attracted to this Delta negative here, which will attract other Delta positive molecules and everything else passing through this huge molecule and they all change and go towards Again, one important thing is this force, this Van der Waals force, its strength changes depending on the size of the molecule, ultimately this is the number of electrons that are present, so Therefore, the opposite is true: a smaller molecule has fewer electrons, therefore it has stronger Van der Waals forces. weak and you can see this particularly if we look at a graph of something like the noble gases in particular, so these guys here we start with helium, which is the smallest weapon up to the radius, sorry, radon, which is the largest and as you can see the size increases, we get more electrons, we get larger atomic radii in the rest and the boiling and melting point when your lines represent the boiling melting point, they increase. because the forces get bigger and what we're really talking about with the melting and boiling points is that we're separating the forces between the molecules and that's very, very important, so the melting and boiling point is really the energy required to separate the molecules.
We're talking about a kind of partial separation boiling, we're looking at a complete separation where we turn from vapor to gas, where the forces between them are relatively low, but this fits exactly with what I just said. molecule in this case, obviously these are atoms, but again, more electrons, therefore stronger Vander Waals forces ranging from small, relatively small, to relatively large, they are on the other side or down the group, but They range from helium to radon, so that's really very important, this is one way. If you could see this, you could certainly be asked for a test to describe this trend and explain this trend, and all you would have to do is say that as we go down the group from helium to attack, the size of the atom increases, therefore there are more electrons, therefore there are stronger Vander Waals forces, therefore more energy is required to overcome the attractions, overcome the forces between atoms, so those are really the forces of our founder VAR, okay, that's the key to our Van der Vaart forces and while it's important, you and then send the van der waals is that actually within all of this the idea of ​​fusion and we'll put the fusion points /boiling is really what we're looking at in the energy required to separate molecules or in terms obviously what we've done there we could also say atoms, but it's more that, being intermolecular forces, it's more likely to be molecules rather than of atoms in the last part of the van der Vaart to move on to the dipole-dipole question and this is a case particularly when we look at things like fats are a very good example, straight chain saturated fats versus unsaturated fats, saturated fats and unsaturated fats , this term saturated and unsaturated applies to the link that is present in the chains of events.
I'm not going to detail what the facts look like. The really key thing is saturation, they're all modified single bonds with sort of chains that look like this and if we drew the next one it would be like this and the next one like this when we look at unsaturated fats. we tend to find that chains have some sort of kinks and as a result they tend to be like this or this also applies to branched chains. I guess here I'm looking at straight chains and again the Van der Waals forces are coming into play here if this is a section of my saturated fat or my straight chain molecule this is a section of my branched chain or my in this case the fat unsaturated where there is a kink note that when we have our linear chains they can be very close together and therefore because of this the Van der Waals forces are stronger because the chains are very close together when we look at the ones with bright branches , they are not as close to each other and because they are not aligned as close to each other. each other here, what I find is that my Van de Vars forces are actually weaker as a result, so it doesn't just depend on the size of the molecule, but it also depends on the branch and how close together they can be. side by side in this case, branching. makes them weaker because really the force here is less surface area, I guess, for them to be in contact and therefore less overall attraction between the chains and the molecules and the proof really is that you would have seen These saturated fats , like butter, for example, or lard or coconut oil, some of them tend to be solid at room temperature, well or quite solid at room temperature, and that is because they have a high hyperportion of fats saturated inside, and because of this, Straight changes that are close to each other therefore require more energy and the ambient temperature does not have the energy to overcome those forces and as a result, the allowed button tends to be solid unsaturated fats, we are talking about our olive oils or in general. or vegetable oils and in this case they tend to be liquids, they contain more unsaturated fats, the Van der Waals forces are weaker, therefore the energy provided at room temperature is sufficient to overcome the forces and therefore we end up with liquids if we reduce them. you would find certificates, but the key is that at room temperature, the comparison between these two is an application and it is a way for us to see the van der Waals forces in action, particularly the straight chain versus the secondary chain, so that's Van der Vaart forces, okay that's it, he basically made his electrons move causing a general attraction and that's basically it, moving on to the next one, we'll look at permanent dipoles, so permanent dipoles, what are these ?
Well, Van der Vaart. I mentioned the term, the idea of ​​temporary or induced dipoles due to those moving electrons. Well, here we have non-temporary opposites, they are permanent opposites in the name, it is dipole permanent dipole, so we are looking at situations where that Delta negative Delta positive is always present and the classic here is something like a hydrogen halide, so I'll choose hydrogen chloride. There is mythey attract each other using the hardened metaphor established in Part D. I include all partial charges and all lone pairs of electrons. in your diagram now, I actually did a little bit wrong, I made a boo in the previous part of the video where I didn't draw enough lone pairs, so go back and fix that or use this your standalone version, but I should include all the self pairs.
I didn't do it then. I also didn't mean to include them all, so I'm not entirely wrong, but it was also a little difficult for me, but anyway, let's move on to this question: hydrogen fluoride covalent bond between hydrogen. and fluorine three lone pairs in my fluorine none of my hydrogen obviously draw my other model because it specifies two molecules don't start drawing six seven eight nine and it's not necessary you have a couple of ways to draw lone pairs I quite like that double point one, don't just make dots like that because it's very easy to miss them, make them like a meteor looking for a couple of dots or stick that little balloon here while you make shapes and molecules that would be completely adequate too either way I have three lone pairs ask for partial charges here throw partial charge I also have a positive delta region and I have a negative dose region another positive Delta another negative Delta says everything partial charge so don't just do it on one molecule that's ridiculous, do it on all of them, the bond itself is formed between fluorine and hydrogen, so it's the only pair of fluorine and hydrogen that we get, all the lone pair of oxygen, oxygen or nitrogen, whatever, but in this one of us it's got to be them in fluorine between the hydrogen and your three marks, because this means three marks for drawing so much.
It's ridiculous, the brand is for everyone and it bothers me that they are all partial charges, one brand, all the solitary pairs, is the other, so everyone. four partial charges or six free pairs and the correct placement of the bond three brands bish bash Bosh I like this question this is a comparative question it is quite intelligent I have lost the tip of the pen it gives you the boiling points of fluorine and hydrogen soil and you question, it gives you what they are and blah, blah, blah and then it says in Spain in terms of bonds why the boiling point of fluorine is very low, well this is smart because here some will start talking about why Hydrogen fluoride has one so high, respectively or in comparison, now that's not necessary because it's not asked, it's just asking you about Florida, why is the fluoride so low?
Well, fluorine f2 only has Van der Waals forces, nothing more, Van der vials are weak, therefore not much. energy required to break them and that really is probably three points almost worth it, but certainly this one here is definitely one, if not two, points in this question that we have already protected to mark, so it will be a point for that one, one mark one two marks overall lovely job here so not bad so if you want to see the grading scheme of things the 12th January paper was ok let's move on to the next paper our next question and then another very similar type of question. strongest type of inspector force that holds the water molecules together in the ice crystal well with water notes for immediately let's make H bond we are not afraid to write that immediately mark and mark the next state the strongest type again in the methane well Methane, if we extract it, does not contain halogens, does not contain any real difference in electricity between the atoms in the bonds, therefore it does not have dipole-dipole, it has hydrogen, but it has oxygen, nitrogen or fluorine, there are no hydrogen bonds, so all dipole-dipole, so they are just Van der valves. easy grading, however, that grading really very easy, so I made another one there, two other marks that were the document from January 11, if you want to review the marking schemes and the things that follow to keep them going, this is the document from June 11 and this is one here.
I ask for an explanation as to why iodine has a higher melting point than fluorine, so we break it down, think about these are the fluorines from the halogen group at the top followed by the chlorine from the bromine followed by the iodine down here, we have an increase in size and the number of electrons also increases. and this ties back exactly to what I said in the previous part of the video, more electrons, bigger, the size, stronger, the force, so let's say iodine is bigger than fluorine, therefore more electrons and we could use iodine here, the I, it doesn't matter either way. iodine atoms are larger therefore the overall molecule is also larger so I didn't talk about fluorine therefore more electrons therefore requires iodine to be specific.
Iodine requires more energy to overcome, of course, here there is more energy to overcome the Van der Waals forces that it is trying to be. comparable there, you're really trying to compare iodine to fluorine to Marx again, the idea of ​​size and then the idea that more energy is required, the last one here and I like it, this seems like a slightly different way of asking if this time was June 12, telling you there are no hydrogen bonds between the phosphate molecules, if we go up and look at this, you can see phosphine here it has a pH 3, so there's my phosphine, so pxv, why are there no bonds of hydrogen?
Well, I have hydrogen but no nitrile option. that's not enough to say that's not an answer that's not an explanation there is no oxygen nitrogen nor fluorine what about the oxygen nitrogen fluorine that allows hydrogen bonding that is the key so here between my phosphorus and my hydrogen officer I have three of In those bonds there is not enough difference in electronegativity between these two to allow a permanent dipole to form, whereas if it were nitrogen there would be enough and that is the key, if we have exactly the same format here, ammonia nh3, there is hydrogen . bond because this is high enough in terms of electronegativity to create a big difference here, which ultimately leads us to a dipole.
Ultra leads us to, so I said mm in open dipole, which leads to a difference large enough to give us whatever partial charges allow us. For attraction to occur, basically the answer is that phosphorus is not very electronegative, therefore there is not a big enough difference in electronegativity between phosphorus and hydrogen. One brand, there are very few questions that should hopefully be the topic of follicular forces. I hope it makes some sense. If you have any problems please let me know and I will do my best to try to answer any queries again. I hope this was helpful and thanks for watching.

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