Intermolecular Forces

We are going to say when we are looking at any type of molecular substance, we have to realize that there were two types of forces at work.
So if we take a look at water, you are going to discover that there are two major forces. We’re going to say that the first major force is called intramolecular force. Intra means that the force operates within the molecule. So inside of water, the force is an intramolecular force. Intramolecular forces influence the chemical properties of water.
Remember, if we were drawing water, oxygen goes in the center. It has 6 valence electrons because it’s in group 6A. Hydrogens are on the outside because they never go in the center. They have only one valence electron because they are in group 1A and they only make one bond. So water would look like that. So let me draw it down here actually. So remember, we’d say that the forces operating inside the water are intramolecular forces.
Now, other than intramolecular forces, we have intermolecular forces. Now, intermolecular forces are the forces between different molecules. So here we have water and it can interact with the compound we are just going to say is B, compound B. Their interaction is an intermolecular force interaction. And we are going to say, intermolecular forces don’t influence the chemical properties of a compound, they influence the physical properties.
What do I mean by physical properties? I mean like boiling point, something called vapor pressure, freezing point, melting point. These are the kinds of properties that intermolecular forces interact and influence. We’ll see those later on but just realize the difference between intra and inter. intra is within; inter is between, between different compounds.
We are going to say that there are five major types of intermolecular forces that you should be aware of. And here we are going to go from the strongest intermolecular to the weakest intermolecular. 

First intermolecular force is the force that exists between an ion and a polar compound, so this is called ion-dipole. Now, an example of ion-dipole is we could have NaCl, remember this is sodium chloride, it’s ionic, and we throw it into water. Now, ionic compounds are polar compounds. And they’re highly polar because, remember, we’ve talked about this before, we say if your difference in electronegativity is 0.4 or greater, you are going to be polar. If it’s greater than 1.7 and higher, you’re going to be ionic. So the difference in their electronegative numbers is very high, so it’s going to be a polar type of compound. All ionic compounds are polar.
And we’re going to learn that water, which is called the universal solvent, it’s called universal because it’s highly polar, so it dissolves a lot of polar things. So polar ionic compound, polar solvent, which is water, polar and polar will interact. So the polar water is going to split this up into its ions. Na is group 1A so it’s going to be Na+ aqueous plus Cl- aqueous . Remember Cl is in group 7A, so it’s -1. Now, what does aqueous mean? All aqueous means is that the water is actually wrapping itself around the ions. This is the ion-dipole interaction.
Now if we wanted to see what it looks like, let’s say we took the sodium ion, now aqueous again means that the water is surrounding it, but which parts of the water are surrounding it? We should realize that oxygen is very electronegative. This compound here is polar if we follow the rules. Because it’s polar, we are going to say that one end is partially positive; the other end is partially negative. Oxygen has a larger electron negative number so this end will be partially negative. The hydrogens are less electronegative, so they are going to be partially positive.
Sodium is positively charged, and remember which side of the water is going to be attracted with it? Opposites attract. So the negative portion of the water will be attracted to it, the partially negative portions. It’s not going to be one molecule surrounding the ion, it’s going to be multiple water molecules surrounding it because we are in solution, it’s in a bucket of water. There’s hundreds and hundreds of water molecules all around, so they all try to surround the ion.
So it’s the interaction between the ion, the positive ion, and the partially negative end of the water molecule. This little dash green line means that it’s not a full bond, it’s just an interaction. They are communicating with each other; they’re partially attracted to each other.
Intermolecular forces are not bonding forces. Remember, we are going to say that intramolecular forces are bonding forces; intermolecular forces are not bonding forces. They are not bonding; they are just attracted to one another. So these little green lines are just an attraction for the positive ion to the partially negative oxygen.
And if we drew chlorine, we’d show that the partially positive ends, the hydrogens, will be attracted to the negative chlorine. That will be our ion-dipole interaction.
So here is an example of ion-dipole and we are going to say basically, fundamentally, we have ion-dipole anytime we have an ionic compound dissolving in a polar liquid, like water. Water is polar based on the rules that we learned.

Now, the next one is easier for us to grasp. This next force is the force that exists anytime H is connected to F, O, N. not fun exactly but F, O, N - fluorine, oxygen, nitrogen. So anytime H is connected to those 3 elements, it’s going to be hydrogen bonding.
So what are some examples of hydrogen bonding? Well, let’s think of some. Water is an example of hydrogen bonding because H is connected directly to oxygen. HF, H is directly connected to fluorine. NH3, H is connected directly to nitrogen. So these three examples would also be hydrogen bonding. And, again, hydrogen bonding is not a real bond; it’s just an interaction.
So a good example is we could have water forming an interaction with another water if we wanted, or even NH3. That dotted red line is not a real bond it’s just an attraction that the molecules have for one another. That would be hydrogen bonding.
Now hydrogen bonding is an extremely important force because water has hydrogen bonding. Water has a very high heat capacity, specific heat capacity. That means that it takes a lot of energy to break water into its molecules. And the reason it takes so much energy to break it up is because water has hydrogen bonding. Hydrogen bonding makes the molecules stronger because all of these attractive forces after first to be broken up before we can start to boil water.
So if you ever wanted why water takes so much time to boil? It’s because of these intermolecular forces. All the water molecules are partially connected to each other, partially attracted to each other. So as a result, it’s going to take a lot of energy to break those partial attractions first before we can begin to vaporize the water.

Now, the force exists when we have two polar covalent compounds attracted to one another. We are going to say here that this is called dipole-dipole. Now, the word dipole means polar, so dipole-dipole means polar-polar. So the name is kind of telling us the reaction. It’s telling us we have a polar A attracted to a polar B. So dipole means polar, so dipole-dipole, again, means polar-polar.
So if we think about it, we can just think of a polar compound, maybe one that we drew earlier. One that we drew earlier would be SiBr4 2-, we know this compound was polar when we drew it. And let say it’s attracted to itself or maybe even attracted to something else that’s polar, PH3. So their attraction to one another, because they are both polar, will be dipole-dipole.

The fourth intermolecular force exists anytime we have a nonpolar covalent compound having a partial interaction with a polar covalent compound. Now this is called dipole/induced-dipole. Now, we said that the word dipole means polar. Now, the word induced-dipole means nonpolar. So dipole/induced-dipole means polar interacting with non-polar. That’s all that means.
So a good example here, something’s that polar and nonpolar interacting, we could have let’s say, water, which we know is polar, interacting with CCl4. Carbon is in the center there. It’s surrounded by the same chlorines all the way around, so it’s a nonpolar compound. It has no lone pairs, so we are using row 1 to determine if it’s polar or nonpolar. Based on the rules for row 1, this compound will be nonpolar. So this compound is nonpolar; this compound is polar. And the interaction that they have with one another is dipole/induced-dipole. 

We’re going to need some room to do this last one, so I’m going to remove myself from the image, guys, so we have more room to work with. So actually, I’m coming back. So actually I’m going to stand in the image guys so that we can work on this.
So the last force is the force that exists when two nonpolar covalent compounds are interacting. So we have two non-polars. Now, this last force goes by a ton of different names. One name that we usually give to it is called London-dispersion forces, that’s one name that it can have. Another name for it is van der Waals force. And then a third name that it could have is induced dipole-induced dipole.
So just remember, this last force can be called any one of these three. The main name is London-dispersion but it can also be called one of the other two. We are going to say that this force exists anytime we have nonpolar compounds. So we could have CCl4 interacting with CH4.
And there’s a few things we need to know about his last force. One, that this force is the weakest force. Also, we need to realize that every compound, no matter what it is, has this force. This is a force that’s present in all compounds.
So water, water's main intermolecular force, strong intermolecular force, would be hydrogen bonding but it also has London-dispersion. Because London-dispersion is weaker, when we scan water we will see that the vast majority of its force comes from H-bonding and a very, very small percentage of its force comes from London-dispersion. So again remember, all compounds fundamentally have London-dispersion forces.
And what we should realize here when it comes to London-dispersion forces, we are going to say that compounds with London-dispersion as their only force, we are going to say that the more they weigh, the greater their London-dispersion will be. So these two compounds are both nonpolar, so they both, fundamentally, have London-dispersion as their only force.
CCl4 London-dispersion will be greater than CH4 because CCl4 weighs more. The more you weigh, the greater your London- dispersion. We are going to say the more you weigh, the more polarizable you are.
Because when it comes to London- dispersion, the way it works is, let’s say that this blob here represents a compound and its main force is London-dispersion. Now how can two things have an interaction to each other if they are both nonpolar? The way it works is polarizability.
What’s going to happen is electrons are going to spontaneously align themselves on one end of the compound. So electrons, for a split second, will move to the left side. They just randomly do that. They are all going to move at the left side. So if all the electrons move to the left side, this side is going to become negative, partially negative. If all the electrons are leaving the right side, this side going to become partially positive.
Then what happens here is polarizability. So the compound that’s nonpolar, usually, for a split second, can become polar. It then comes near a compound that’s strictly nonpolar. So CCl4 comes near CH4. Because this guy here becomes polar for an instant. It’s going to make this compound become polar for an instant as well. This partially positive end here is going to attract electrons. So it’s going to cause the electrons to shift in compound B to this side so they’re close to the partially positive charge. Then moving to the left will cause this side over here to become partially positive.
Now this happens for only an instant. And then the molecule returns back to being nonpolar. But that instant that this happens, that’s when we get this interaction between nonpolar compounds. It happens for just a moment, but in that moment, that’s where we have the London-dispersion happening. And remember, all compounds have London-dispersion. Compounds that are nonpolar have it as their main and only force.
So remember these are your major types of intermolecular forces that your professor will expect you to know, and a majority of them comes from fundamentally knowing how to draw.
Ion-dipole, you don’t really need to know how to draw because ion-dipole exists anytime we have an ionic compound. So if we have an ionic compound and its dissolving in a polar solvent, like water, it’s going to be ion-dipole. H-bonding, we kind of don’t need to really know how to draw either because if H is connected to F, O or N, it’s automatically H-bonding.
It’s when we get to dipole-dipole and London-dispersion where it’s important we know how to draw because you have to learn how to draw this compounds to be able to determine, is it a polar covalent compound or is it a nonpolar covalent. That is where it becomes essential to know what kind of structure we have and go over the rules for polarity. 

Why was it so important that we identify a compound as polar or nonpolar? Because we're going to say that compounds with the same intermolecular force, or polarity, will dissolve into each other to form a solution. Now we're going to say that if you have a polar and a polar, they're going to mix together well. If you have a non-polar and a polar, their polarities are different, so they won't be able to dissolve into each other to form a solution.
We're going to say according to the theory of likes dissolve likes, basically, the two compounds have to have the same intermolecular force. If they have the same intermolecular force, they have the same polarity. But they could also have different intermolecular forces.
Let's say one compound had hydrogen bonding and the other one had dipole-dipole, that's okay because hydrogen bonding and dipole-dipole are both polar forces. Because they're both still polar, they'll be able to dissolve with one another. But let's say one had dipole-dipole and the other one had London-dispersion. Dipole-dipole is polar. London-dispersion is non-polar. Because of their differences in polarity, they will not mix.
Also, we're going to say that there's a difference between a mixture and a solution. We're going to say mixtures, we've talked about this so many weeks ago, mixtures come in two types. We have homogeneous or heterogeneous. We're going to say homogeneous mixtures mix together. They dissolve into each other. We're going to say that heterogeneous mixtures do not mix.
Oil and water is a good example that we've talked about. They won't mix because why? Oils are non-polar solvents. They're non-polar. Water, on the other hand, is polar. As a result, polar and non-polar do not mix. That's why oil and water don't mix together at all. Mixtures come in these two types.
A solution, all solutions, are just homogeneous mixtures. Remember the difference. Mixtures come in two types. They can either be homogeneous, where they mix together, or heterogeneous, where they don't. All solutions are just homogeneous mixtures. In a solution, we can dissolve both things into each other, so they do mix.