YTread Logo
YTread Logo

Intermolecular Forces

Mar 16, 2022
hi my name is chris harris and i am from allerry tutors.com and welcome to this video on

intermolecular

forces

, in this video we will look at the three types of

intermolecular

forces

you need to know in chemistry and we will also look at how we can identify an intermolecular force in a molecule and we'll also go through a worked example and not only that, we'll also show you a method that I hope explains what a van is. der waals force is actually because this is probably one of the hardest to understand what's going on right so let's start by looking at what the meaning or meaning of intermolecular forces means I should say like this meaning inter means between um and a bit like intercity um intercontinental and so on so inter means between and molecular is obviously molecules so these are forces that exist between molecules these are completely different from bonds which are very very strong and require a lot of energy and to break bonds intermolecular forces are significantly weaker compared to a bond and different from a bond so you have to understand and the difference between the two ok so what we're going to do is look at the first type of intermolecular force and this is called the van der waals force now the van der vaal force as you can see from here is the weaker force of the three we're going to look at and there will be a van der waals force where you have a molecule or an atom that has electrons so let's face it that's pretty much all the molecules out there so van der waals forces are pretty common and the idea is that because they have electrons, they create like a little dipole between them. and they are also known as temporary dipoles or induced dipoles or London forces, so there are different terms for them as well, so you might see them mentioned in other places, also the key concept with the van der vaals, which I'll show you which one is.
intermolecular forces
In a minute, um, it's effectively the larger the mr of the molecule, the greater the van der waals force, so if you have a molecule that has a larger mr, then it's more likely to have or will have a larger van der waals force and so it's the melting and boiling point increases now with intermolecular forces what governs the melting and boiling points is the actual type of force the molecule has so we'll get to that when we see the other three as well, okay, let's see what the force der waals is going for you to see here.
intermolecular forces

More Interesting Facts About,

intermolecular forces...

I have some diagrams here and I have a nonpolar chlorine molecule. There's an example, so cl2 now normally the electrons are equally distributed in that molecule, but when another chlorine molecule gets close. another chlorine molecule, so we have two chlorine molecules sticking together, something strange happens to the electrons. I'm going to try to explain this using a bowl of water believe it or not so I hope this actually works so you can see here here is our bowl of water you can see what I've done is draw uh like a molecule of chlorine on the side of the dish, now also what I've done is put some water inside the bowl and the water is there to symbolize the electrons and you can see here that normally if it's chlorine on its own without any additional influence, the electrons are evenly distributed through the molecule as you can see there so let's say if I just hold this carefully not to let go and let's say I have a molecule here and I'm going to push it over to this side so we have our molecule here and let's say that I have another chlorine molecule approaching from this side now this chlorine molecule has electrons just like this one because they both have electrons we get electron repulsion so this electron this chlorine molecule Chlorine, sorry, it's getting closer and what happens is the electrons are going to move to one side, so the electrons are coming from here, the chlorine atom is coming from here and so the electrons will be going to then in this molecule to that side and you can see what we have as a result you can see we have chlorine we have a chlorine atom on this side of the molecule that has a lot of electrons you can see the high electron density on this side and this chlorine atom actually has a lot fewer electrons on its side so what we've done is created a temporary dipole and the dipole is only there because the chlorine molecule on this side has effectively distorted the electrons so we have more electrons on this side than we do on this side and so what this created is a negative delta chloro on this side and the delta positive chloro on this side so we have an induced dipole and because we have a positive delta and a negative delta positive delta and delta negative uh um same thing on the other chlorine molecule too that means we have a little attraction between the two molecules and that little attraction is a van der waals force so you can see as long as you have electrons in it then you'll have this dipole temporarily and effectively an attraction between one end of the molecule and the other, so let's go back to this diagram, okay, you can see here, here's our chlorine atom, we have a negative delta area and a positive delta. this is where the electrons have bunched up to this side and as a result, because this is positive delta, the neighboring chlorine molecule pushes its electrons to the side where the positive delta is obviously because that's where they attract each other and so so much this red line here you can see everything alternates this red line is the weak intermolecular force or the van der waals force that exists between the chlorine molecules now as soon as this chlorine molecule disappears these electrons will go back to the way they were before until they come across another chlorine molecule, in which case they will reorganize again.
intermolecular forces
Now you can imagine a space very full of lots of chlorine molecules. or you have some form of dipole in your molecule therefore these forces will always exist at some point ok looking at the next guy this is a dipole dipole force now dipole-dipole forces occur in molecules where we have a permanent dipole that exists then this is not just a non-polar molecule like chlorine these have a permanent dipole so they are independent of other molecules around them so examples of these molecules could be hcl which we have there and we could have hbr hf ammonia which is nh3 water h2o these all have a permanent dipole and this works in exactly the same way as a van der waals force except the dipole exists permanently the positive end of a molecule will be attracted to the negative end delta of another and this attractive force um is the dipole-dipole interaction again i have drawn the red line in the middle to show the attractive forces between them now dipole d ipoles are stronger as you can see here than van der waals forces and s Therefore, molecules with these forces will generally have a higher melting point than molecules of similar mass that only have van der waals forces.
intermolecular forces
It is also important why we have written the word dipole dipole. It's not because I want to reiterate the point, but when you're in the exam you should write it as dipole dipole you can't just put the word dipole and the reason why it just clarifies the idea that we have a dipole in one molecule and a dipole in the other and because it's these two uh dipoles that stick together we effectively have a force of attraction between them so that's an example of a dipole dipole molecule interaction i should say the last one is the hydrogen bond uh the hydrogen bond is the strongest intermolecular force of the uh three of them we should know for a level so um and this really applies between the three between hydrogen as the word suggests and uh the three most electronegative elements on the periodic table and you can back member that's why it's effective enough so the northern part is the part we're really interested in which is nitrogen oxygen and fluorine so these are the most electronegati elements uh on the periodic table and therefore any interaction with hydrogen between any of these three will be incurring a hydrogen bond now the word hydrogen bond uh could imply that it is actually a bond and it is not, go on being a weak force and it's significantly weaker than a covalent or ionic bond so even though it has the word bond there's no fooling you into thinking this is a real bond it's still a weak intermolecular force that's ok because we're saying that it suffices so it's an interaction between hydrogen and no and f alone so we're looking for molecules with a nitrogen fluid oxygen huh and it must also have a hydrogen so examples of this could be water h2o it could be hydrofluoric acid hf and it could be nh3 ammonia, so any of these would have hydrogen bonds, now one of the key questions on the exam. ons is that you draw uh using two molecules we have here uh an interaction between the two uh and this is a great generator of marks and easy marks you have to make sure you are taking each frame to get all the marks you can the first thing we have All we have to do is draw our base pairs of electrons onto the molecule, so that's what we're going to do.
I'm going to do this in red so it shows, so here's our water molecule. and now water has two lone pairs of electrons on each of the oxygens and its important to show all the lone pairs in your molecule if you ask the question it just shows clarity and you will need to use them when you are drawing your force you can also see that oxygen has six electrons in its outer shell four of them are lone pairs and the other two are involved in bonding with hydrogen that's why we have two lone pairs in oxygen huh and the next step is to draw our intermolecular force and actually before that before intermolecular force uh w We need to draw our dipoles so that the dipoles in this are either negative delta or oxygen so put that negative in there positive delta on the hydrogens negative delta on the oxygen and positive delta on the hydrogen so now we need to draw our force so that our force is an interaction between hydrogen and n or f in this case it is hydrogen and oxygen so we're going to draw our force and we'll do this in red ok so our force will be between hydrogen and oxygen so it's hydrogen there and the lone pair on the oxygen now you can pick either one it doesn't really matter i just did do because it happens to be drawn like this but make sure you draw it from the lone pair of electrons to hydrogen ok the next important point is also that although water has hydrogen bonding it also has dipole-dipole and van der waals forces , it doesn't just have hydrogen bonds so if it has hydrogen bonds it will have the above two above it equally any molecule with a dipole dipole for example hcl will also have van der waals forces so they all will have all three, it's just that the strongest intermolecular force is hydrogen bonding, so be careful about that on the exam if, for example, they ask what intermolecular forces are involved. between water molecules you need to mention all three of them if you ask what is the strongest intermolecular force between water molecules you just need to say hydrogen bond so be very careful about that on the quiz because it might trip people up , so uh like I say be rigid last last thing is just looking at some examples of how we can identify and prove that these intermolecular forces exist and we have two molecules here we have water and methane which is ch4 both have very similar mrs 18 and 16 which means that in terms of van der waals forces they are very similar but if you look at the boiling points between the two water boils at 100 degrees celsius while meta ne boils at minus 164 degrees Celsius there is a big difference in boiling points between them and what governs this difference or boiling point level is actually the kind of force that we have in methane it's just going der vaals which is the strongest intermolecular force there is and while with water we have hydrogen bonding, because water has a stronger force in addition to van der vaals then its melting point increases significantly so this is the explanation behind this and make sure you can link the boiling points of substances to the type of intermolecular forces they have because this governs the melting point and the end point is also to mention that you don't talk about bonds when we are talking about or you talk about bonds when we are talking about boiling points in melting points the only time you talk about bonds is when you talk about breaking a giant structure for example breaking diamonds or graphite or r breaking a giant covalent structure or in ionic terms you don't really would apply here uh are you talking about uh breaking an ionic bond which requires an incredible amount of energy to break an ionic substance a uh giant ionic substance so that's the The only time you talk about bond breaking bonds and melting points is when you talk about breaking giant structures at any other time molecular moleculesSimple ones like this are always intermolecular forces and that's the key now if you want to see videos to do with giant covalent and giant ionic structures just to emphasize that point then if you click on the links below you can see the videos there too and other than that those are your intermolecular forces make sure you know all three of them and make sure you can answer those questions very carefully that's it bye

If you have any copyright issue, please Contact