Intermolecular Forces

let's talk about inter molecular forces. So in a molecular forces are forces that exists between molecules, not within the molecules. Okay, if it's within a molecule that's actually just called a chemical bond, right? That would be like one Adam attached to another atom. But with inter molecular forces, What I'm talking about is one molecule that is loosely associated with another molecule. Meaning they just like to stick together a little bit. And it turns out that EMFs are what make molecules sticky. Okay, so I know that that kind of sounds weird that molecules would be sticky, but they actually are. It turns out that if these molecules did not stay together in some way, everything in the universe would be a gas. Okay, So in order to have solids in order to have liquids, these molecules have toe aggregate. They need to They need thio, remain somewhat close to each other or they're just going to disperse. And that's what inter molecular forces do. Okay, so they don't actually change the compound. The compound is affected by bonds, but the way that the state of the matter is affected by and molecular forces, so I just want to show you guys this really quick diagram. This shows you three different molecules that have about the same molecular weight. Okay, so, theoretically, if they have the same molecular weight, you would think that overall the type of substance would be similar. But actually it turns out to be very different. So here we have a three carbon atom. Here we have a two. I said Adam, I met molecule. Sorry. Here we have a two carbon molecule with one oxygen, and here I also have a two carbon molecule with one oxygen. All right, so these are very similar looking molecules and they're very similar shapes, and yet they're bowling points are vastly different. So if you look at propane, propane is like a propane gas. Why do we keep it in a gas tank? Because it's gas. Like there's no way that you could just put propane in a bowl or in a cup. It would immediately turn into into gas. Okay. And then we have dimethyl ether, which just by adding one oxygen in that location, we've increased the boiling point a little bit. Okay? And remember that ether is one of those re agents that you're going to use a lot in lab. Remember that it's very reactive and one of the reasons is because it turns into a gas very easily. So it's very volatile. Okay, and then the last one here is ethanol and I've brought up ethanol before. You guys are very familiar with it. That's like vodka. Alright. And if your vodka was a gas, it probably wouldn't be as easy to consume, right. But it turns out that ethanol doesn't boil until 173 degrees Celsius. That's crazy. Look how much different this boiling point is from this one. Okay, what's the difference? Why is it such a how much higher boiling point? Why does ethanol exist as a liquid in room temperature, whereas propane and diet methyl ether are gonna exists as a gas? And the reason has to do with inter molecular forces. So whenever you get questions about boiling points or melting points, which are that's the way that professors like to ask these questions. It always has to do with the strength of EMFs between molecules. So Professor ever asks you list the following molecules in order of increasing boiling point or something like that. You know that they're talking about EMFs

So there's three main inter molecular forces that we want to know, and I'm gonna teach you them in order from strongest to weakest. So we're gonna start with the strongest, most important inter molecular force. And then we're gonna end off with weakest one. The strongest one is H bonding, hydrogen bonding. So you guys learned about this in Gen. Kim? Maybe you guys can tell me which atoms are the ones that air ableto hydrogen bond. Do you remember? It was small, highly Electra negatives. That was N O and F. Okay, these are the three atoms that if they're attached to an H, they're able to hydrogen bond. And the way hydrogen bonding works is that, for example, it keeps water together. Water is here, and then you wind up getting is that another water molecule gets close, and between one of the O's and one of the h is you end up getting in a loose a pretty actually pretty tight interaction. And that interaction is called hydrogen bonding. Okay, and now this was drawn a little bit weird, but you can see that pretty much every oh can interact with an H and every in every age can interact with an O Pretty much okay, They can all arrange themselves tightly together. Okay? And that's what keeps molecules like water, like ethanol. Uhm, that's what keeps them as liquids because they're ableto get really close to each other and interact and and attract each other. So hydrogen bonding is gonna be the most important in a regular force. If any molecule has this force, we're going to say that that one is gonna have the highest boiling point or the highest melting point, Whatever.

So now let's go to the second most important, the second strongest. That's gonna be called the dipole Dipole Force or what I like to call the net dipole force. Why? Because if you know how to draw die poles and if you know how to find net die polls, that's all you need for this. So, for example, a molecule like acetone, which is one that I brought up, um, earlier in our lessons, asked one. Looks like this. Does it have a die poll? Yes, it does. It actually has several dipole moments. It would have dipole moments pulling this way in this way because of the lone pairs. And it would have a major dipole moment pulling that way because of the double bond. But overall, we would just say that it has basically a partial negative up here and a partial positive down here. Does that make sense so far? Because basically, what's gonna end up happening is that I'm just gonna get a big die poll pulling electrons. Oops. I'm gonna get a big die poll pulling electrons towards the Oh, Okay. So I'm gonna get a partial negative and a partial positive. Okay, remember that we use the lower case Delta to represent partial. And it means that I don't know exactly what the number is. I just know that it's more than the other. Okay, well, check out what can happen when I have a Net Die poll. Another acetone molecule can arrange itself so that the partial negative from one of the molecules orients with the partial positive of the other. And by that, in that way, they wind up sticking together. And you can imagine that I could have a bunch of acetone molecules all neatly arranged so that all the negatives link with all the positives. Okay, this is only possible, though, if you have a net dipole, If there's no net dipole, then you're not gonna able to form this force. Does that make sense? So this is what we call the second strongest force. It's not quite as strong as hydrogen bonding, but it still is a pretty strong force.

So then we move on to our last one, which is our weakest force. And it turns out that all molecules contain this force groups all molecules possess Vander Wal's. In case I'm just gonna put Vander Wal's okay, but they don't all possess it to the same extent. What that means is that some molecules are gonna have higher Vander Wal's and some are gonna have lower. Okay, So what makes Vander Waals forces increase? The very first thing and most important thing is the size. And that has to do with the molecular weight of the molecule. The higher the molecular weight, the stronger the vander wal's pretty easy, right? Cool. Then we've got the second most important thing. Or the second, um, indicator is gonna be the shape. Okay, The shape of the molecule and the shape has to do with how neatly they could be arranged and how neatly they could be ordered. So what I'm gonna do here is I'm gonna take I'm gonna draw three different molecules and you tell me which one would have the highest Vander Waals force. So here I've got a ring that's a six member dring here, have got a six member chain and then here I've got another six member chain. Okay? And what I want to know is out of these three, which of them is gonna have the highest Vander Wal's? First of all we would look at is the size are all the size is the same. Yes, they all have six carbons. So actually, in terms of size, they're fine. Okay, then I would look at okay, the shape which one has the shape that can arrange the neatest and can can stack the best. And the answer is the ring, because check it out the ring. Actually, since it's so symmetrical, it can have a bunch of rings stacked on top of each other. So if I wanted, I could keep drawing these rings stacked on top of each other. You know what that means? That there's inter molecular Vander Waals forces all in between that keeping them stuck together does that makes them so far. Then with the change, the change is still pretty good, but they're not quite as good as the rings. So here I'm drawing an example of Vander Waals forces here. These are okay, but they're not quite as good as the rings. Does that make sense? And then finally, we've got the branch, which I'll try to move out of the way because I know that it's right on top of me. But with the branch, what we find is that there's not really a great way for them to stack together. They wind up kind of having a lot of space in between them. Does that make sense? So the Vander Waals forces here are gonna be very small compared to the Vander Waals forces here for the rings. Does that make sense? Cool. So, basically, I hope that this clarifies inter molecular forces. The first thing we look for is the type of force. Hydrogen bonding is the best. Vander Wal's is the worst. Die poles in the middle, and then we kind of break Break it down from there. Okay, so now what I want to do is do a few practice problems to get comfortable with this, and I'm gonna try to trick you a little bit, so be prepared. All right, let's go.

PRACTICE:Which of the following pairs of molecules would have the highest boiling point?

• A. Compound on the left
• B. Compound on the right

PRACTICE:Which of the following pairs of molecules would have the highest boiling point?

• A. Compound on the left
• B. Compound on the right

PRACTICE:Which of the following pairs of molecules would have the highest boiling point?

• A. Compound on the left
• B. Compound on the right

PRACTICE:Which of the following pairs of molecules would have the highest boiling point?

• A. Compound on the left
• B. Compound on the right