 All right, so this collection of lectures is on intermolecular forces. The first kind of intermolecular force that we've been talking about is London dispersion forces. And these are basically the weakest attractions between molecules. But there are other kinds of forces, there are other kinds of attractions between molecules and we're going to talk about them, you know, step by step. So we're going to talk about a different kind of intermolecular force and these are going to be called dipole interactions. So we've already talked about permanent dipoles and temporary dipoles. Mostly we're going to be talking about permanent, that is what we're going to be talking about, permanent dipole interactions. So this is a diagram of a water molecule and there's a lot of fancy arrows here. But basically, if you remember from previous semester, there's an imbalance of electrons on the water molecule and the oxygen atom tends to, you know, hold on to the electrons a little bit more than the hydrogens, so this side of the water molecule is a little bit negative. And if you want to do this the proper way in chemistry, what you do is you write the Greek letter delta and you write a minus. And this little delta and a minus means partial negative electrical charge. And because one side of the molecule is imbalanced with a little bit of a negative, the hydrogen sides have partial positive charges and you do the same thing, you write the Greek letter delta which looks like a weird little D and then you write a plus sign. So there's a partial negative and a partial positive charge. And this arrow here with a little bar through it that's called the dipole moment arrow and basically all it does is it points in the direction where the negative charge is. So this water molecule and this is true for all water molecules. There's an imbalance. It has a, I'm going to remove the deltas and just kind of simplify it, there's an imbalance. Part of the molecule is a little bit negative, the oxygen side is and the hydrogen side is a little bit positive and this is true for all water molecules and you say that water has, water molecules have a permanent dipole because that imbalance doesn't really go away. But because you have those partial charges and positive and negative electrical charge things tend to be attracted to each other, if you have a bunch of water molecules they will be attracted to each other and they will be attracted to other things that also have electrical charges. Another thing, so this bullet point here says that this is, water molecules are called polar molecules and the reason for that is that there's little electrical poles on either end. There's an imbalance of negative and positive charges on either side of the molecule even though the negative and positive charges are partial charges. And so some types of interactions that you can have are the following. You can have ion-dipole interactions. So if you remember from the refresher video, ions are atoms or molecules with charges and dipole is a molecule that has this imbalance of partial charges. So an ion-dipole interaction is an interaction between things that have charges. So this is supposed to be a little bit of table salt, NaCl, and if you remember the sodium or the Na has a positive electrical charge and the chlorines have a negative electrical charge and these are not partial charges, they're full blown positive and negative charges. And if you throw water onto your table salt, these are the hydrogens and if you remember from the previous page, the hydrogens have a little bit of a positive charge, partial positive charge and those positive charges like to interact with the negative charge of the chlorine and that is called an ion-dipole interaction because the chlorine is an ion and these water molecules with their partial charges have permanent dipoles. And basically this sort of helps to explain why water dissolves table salt and the reason why water dissolves table salt is that if you throw enough water molecules onto your table salt the little partial positives of the hydrogens on the water can interact with the negative charge on the chlorine and if you have enough of them they can basically pull all of these chlorines off of the positively charged sodiums and pull them away and separate them and you can also have, here's a different water molecule, just another one that came into view and the partial charge on the oxygens can pull away the positive charges of the sodiums and also pull them off of the little cube of table salt and if you throw enough water on there all of these things, all the sodiums and chlorines go flying off and start to interact with the water instead of interacting with each other and those are called ion-dipole interactions. Ion-dipole interactions are weaker than ion-ion interactions so if I have just a sodium with a plus charge and a chlorine with a minus charge and those are full-blown charges the attraction is very strong and that's called an ion attraction and that's very strong and what I'm showing you here is what I'm showing you here is called an ion-dipole interaction and it's not as strong as the attraction here but if you have enough of them, if you throw enough water onto your table salt it will overwhelm the stronger interaction and it will win out and dissolve your table salt and ion-dipole interactions also tend to be weaker than covalent bonds and then over here well here those green dots you see those green dotted lines are ion-dipole attractions so here the negative charge the partial negative charge on the oxygen from the water gets to interact with the positive the full-blown positive charge of the sodium in the table salt. Same thing is going on here those are also ion-dipole attractions and then the blue lines that you see those are called dipole attractions and what that means is there's a little bit of a negative charge partial negative charge on that oxygen from the water molecule but if it bumps into a neighboring water molecule there will be a little bit of an attraction between the partial negative of one water molecule and the partial positive of a different water molecule as they bounce around near each other and that attraction is being shown to you here with these blue lines and that's called a dipole-dipole attraction and that's even weaker than ion-dipole so if I had to rank them LDF is the weakest dipole-dipole is the next weakest ion-dipole is stronger than the other two ion-ion and covalent those are basically the strongest compared to the other three that I listed as far as which one is stronger ion-ion or covalent it basically depends on the situation so I'm not going to really rank those without getting more specific so at this point now that you know what a dipole-dipole interaction is that's basically an attraction between molecules because of these permanent partial electrical charges you the point that I'm trying to make on this slide is that dipole-dipole interactions can also affect boiling points because dipole-dipole interactions affect whether the molecules like to stick to each other so the way that I'm trying to point out that this is true or correct is that you can compare two sets of molecules you can compare this molecule here which is sort of shown cartoon form up here this is called methyl propane and this molecule is relatively non-polar no permanent dipole in other words the electrons are more or less evenly distributed there's no permanent imbalance of electrons so there's no permanent partial electrical charge on any side of the molecule if I have a bunch of methyl propane molecules and I you know throw them all together into a container there is still some attraction between a bunch of methyl propane molecules because there can be temporary dipoles which will cause a London dispersion forces so a bunch of methyl propane molecules will be attracted to each other but if I take a similar molecule that has roughly the same size so these molecules the one on the left and the one on the right they have about the same weight this molecule over here is called acetone but acetone does have a permanent dipole and this side that's red here is going to have a partial negative charge and the other side will be a little bit more positive so if I throw a bunch of acetone molecules together they will also have London dispersion forces just like the methyl propanes do on the other side but in addition to the London dispersion forces they are also going to have dipole dipole attractions because the little negative the partial negative charge of one side of the molecule can be attracted to the partial positive charge of another acetone molecule that's bouncing around nearby it so not only are the acetone molecules attracted to each other because of LDF but they're also a little bit more attracted to each other because they have dipole dipole attractions so even though the methyl propane and the acetone molecules are similar in size the acetone molecules are harder to boil away because they have both London dispersion forces and dipole dipole attractions and so if you look at their boiling points acetone has a lower boiling point 261 kelvin and acetone has a much larger boiling point 329 kelvin and the reason it has a larger boiling point is because of these dipole dipole attractions which make acetone molecules if you throw a bunch of them together make them like to stick to each other more strongly than the molecules of methyl propane at this point I want to talk about what I'm calling basically a special kind of dipole dipole attraction which is a hydrogen bond the only reason I mention this is that it is it shows up a lot in biology and medicine and basically this is caused when one of the parts of the attraction is a hydrogen atom that has a partial positive charge and it is attracted to some other atom on a on a separate molecule that has a partial negative charge in the classic example that people give of this is with if you have a bunch of water molecules so the hydrogens on the water molecules will have these partial positive charges but if you get enough of them together the partial positives on the hydrogens will be attracted to the partial negatives on the oxygens of water molecules that are bouncing around nearby and there will be a dipole dipole attraction but this particular dipole dipole attraction is called a hydrogen bond for historical reasons and the way that hydrogen bonds are typically drawn is they're drawn with a dotted line and that is basically to say you know when you draw covalent bond you draw solid line that means covalent bond so I assume that the reason it's drawn with a dotted line is to tell people that it's weaker than a covalent bond and like I said hydrogen bonds show up a lot in biology and medicine these are supposed to be pieces of DNA or bases of DNA you don't really need to know what bases of DNA are but you can see that there are hydrogen bonds that connect parts of DNA to each other and so that's what a hydrogen bond is you can think of it as kind of a special kind of dipole dipole attraction here is another thought exercise or you know something where you read the question pause the video and try to answer the question so the question says ethane so this is ethane over here and hydrazine over there have similar molecular structures they kind of look roughly the same and they weigh about they have about pretty similar weights but hydrazine says high the boiling point of hydrazine is over 200 Kelvin greater than that of ethane and basically the question is why why is that true so you can pause the video and think about that so on pausing that the reason for this is that ethane the attachments in the molecules that there's there's no permanent dipole here in other words there's no partial negative or positive charges on these molecules so if you if I throw a bunch of ethane molecules together there will be some London dispersion forces so they'll be attracted to each other a little bit because of temporary dipoles but when you compare it to hydrazine there basically is a these permanent partial electrical charges on hydrazine so this this blue or purple purple blue atom is a nitrogen and it basically has a partial negative electrical charge and because of that the hydrogens on on hydrazine have these partial positive charges and because I have these partial negatives and positives if I throw a bunch of hydrazine molecules together those partial negatives and positives will attract to each other so a bunch of hydrazine molecules will have LVFs that make them stick to each other a little bit but they will also have dipole dipole attractions that make that make them stick to each other even more and that means they're going to be harder to boil away when you compare them to ethane and that means the boiling point is going to be higher so 387 kelvin versus 184 kelvin for ethane and so that's sort of the answer to that question these drawings down here showing you the a little snippet of the periodic table is supposed to help you understand why there are no or little partial electrical charges on ethane and there are on here this is from periodic table that I made and you know distributed last semester I'm not sure we covered this though these numbers up at the top of the boxes those are called electronegativities and basically people can rank people have ranked the elements based on whether they like to hold on to electrons or not even when they're sharing so the bigger the number means they like to hold on to the electrons more which means they're more electro more electronegativities and basically if I have two atoms like carbon and hydrogen that have roughly similar electronegative numbers then they're going to share the electrons equally and there probably won't be a permanent imbalance in the electrons and it probably won't be a permanent partial electrical charge and that's true with ethane because ethane has carbons and hydrogens but if you look at nitrogen it's electronegativity is higher substantially higher than the electronegativity of hydrogen which means these are nitrogens over here which means they're going to hold on to the electrons more even though they're sharing with the hydrogens and because of that because they like to hold on to electrons more they're going to have these partial negative charges so that's basically how you're supposed to figure out that molecule like hydrazine is going to have partial charges and molecule like ethane won't but I'm like poison I'll break up all over you