Intermolecular Forces - Online Tutor, Practice Problems & Exam Prep

Intermolecular forces (IMFs) are crucial for determining the physical state of substances. The strongest IMF is hydrogen bonding, occurring between molecules with N, O, or F atoms bonded to hydrogen, significantly raising boiling points. The second is dipole-dipole interactions, where molecules with net dipoles attract each other. The weakest are Van der Waals forces, influenced by molecular size and shape. Understanding these forces helps explain why substances like ethanol remain liquid at room temperature while others, like propane, are gases.

How IMFs are related to melting and boiling points.

Let's talk about intermolecular forces. So, intermolecular forces are forces that exist 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 atom attached to another atom. But with intermolecular 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 IMFs are what make molecules sticky. Okay? So, I know that it might sound weird that molecules would be sticky, but they actually are. It turns out that if these molecules did not stick together in some way, everything in the universe would be a gas. So, in order to have solids, and in order to have liquids, these molecules have to aggregate. They need to remain somewhat close to each other or they're just going to disperse. And that's what intermolecular forces do. Okay? They don't actually change the compound. The compound is affected by bonds. But the way the state of the matter is affected by intermolecular 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 the overall type of substance would be similar, but actually it turns out to be very different.

So here we have a 3 carbon atom. Here we have a 2, I said atom, I meant molecule, sorry. Here we have a 2 carbon molecule with 1 oxygen. And here I also have a 2 carbon molecule with 1 oxygen. Alright? So, these are very similar looking molecules and they are very similar shapes yet their boiling 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. There's no way that you could just put propane in a bowl or in a cup. It would immediately turn 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, ether is one of those reagents that you're going to use a lot in the lab. Remember that it's very reactive. 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. I've brought up ethanol before. You guys are very familiar with it. That's like vodka, alright? 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 Tbp=173 °C. That's crazy. Look how much different this boiling point is from this one. Okay? What's the difference? Why is it such a much higher boiling point? Why does ethanol exist as a liquid at room temperature whereas propane and dimethyl ether are going to exist as a gas? And the reason has to do with intermolecular forces.

So, whenever you get questions about boiling points or melting points, which are the ways that professors like to ask these questions, it always has to do with the strength of IMFs between molecules. So, if a 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 IMFs.

How hydrogen bonding works.

So there are three main intermolecular forces that we want to know, and I'm going to teach you them in order from strongest to weakest. We're going to start with the strongest, most important intermolecular force, and we're going to end off with the weakest one. The strongest one is hydrogen bonding (H-bonding). So, you guys learned about this in general chemistry. Maybe you guys can tell me which atoms are the ones that are able to hydrogen bond. Do you remember? It was small, highly electronegative. So that was nitrogen (N), oxygen (O), and fluorine (F). Okay? These are the three atoms that, if they're attached to an H, are able to hydrogen bond.

And the way hydrogen bonding works is that, for example, it keeps water together. Water is here, and then what you wind up getting is another water molecule that gets close, and between one of the O's and one of the H's, you end up getting a loose but actually a pretty tight interaction, and that interaction is called hydrogen bonding. Okay. And now this one's drawn a little bit weird, but you can see that pretty much every O can interact with an H and every H can interact with an O. Okay? They can all arrange themselves tightly together. Okay? That's what keeps molecules like water, like ethanol, that's what keeps them as liquids because they're able to get really close to each other and interact and attract each other. Okay? So, hydrogen bonding is going to be the most important intermolecular force. If any molecule has this force, we're going to say that that one is going to have the highest boiling point or the highest melting point or whatever.

How dipole-dipole forces work.

So now let's go to the 2nd most important, the 2nd strongest, and that's going to be called the dipole-dipole force or what I like to call the net dipole force. Why? Because if you know how to draw dipoles and if you know how to find net dipoles, that's all you need for this. For example, a molecule like acetone, which is one that I brought up earlier in our lessons. Acetone looks like this. Does it have a dipole? Yes, it does. It actually has several dipole moments. It would have dipole moments pulling this way and that 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 going to end up happening is that I'm just going to get a big dipole pulling electrons. I'm going to get a big dipole pulling electrons towards the O. Okay? So, I'm going to get a partial negative and a partial positive. Okay? Remember that we use the lowercase 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 dipole, 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. 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 going to be able to form this force. Does that make sense? So this is what we call the 2nd strongest force. It's not quite as strong as hydrogen bonding, but it still is a pretty strong force.

How Van der Waals forces work.

So then we move on to our last one, which is our weakest force. And it turns out that all molecules contain this force. Oops. All molecules possess Van der Waals forces. Okay, so I'm just going to put Van der Waals. Okay? But they don't all possess it to the same extent. What that means is some molecules are going to have higher Van der Waals forces and some are going to have lower. Okay? So what makes Van der 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 Van der Waals forces. Pretty easy. Right? Cool. Then we've got the second most important thing or the second indicator is going to be the shape. Okay? The shape of the molecule. And the shape has to do with how neatly they can be arranged and how neatly they can be ordered. So what I'm going to do here is I'm going to draw three different molecules and you tell me which one would have the highest Van der Waals force. So here I’ve got a ring that's a 6 membered ring. Here I've got a 6 membered chain. And then here I've got another 6 membered chain. Okay? And what I want to know is out of these three, which of them is going to have the highest Van der Waals forces? First of all, what we would look at is the size. Are all the sizes the same? Yes, they all have 6 carbons. So actually, in terms of size, they're fine. Then I would look at the shape. Which one has the shape that can arrange the neatest and can stack the best? And the answer is the ring because check it out, the ring actually, since it's so symmetrical, You other. You know what that means? That there's intermolecular Van der Waals forces all in between that keeping them stuck together. Does that make sense so far? Then with the chains, the chains are still pretty good, but they're not quite as good as the rings. So here I'm drawing an example of Van der 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 having a lot of space in between them. Does that make sense? So the Van der Waals forces here are going to be very small compared to the Van der Waals forces here for the rings. Does that make sense? Cool. So basically, I hope that this clarifies intermolecular forces. The first thing we look for is the type of force. Hydrogen bonding is the best. Van der Waals is the worst, dipole is in the middle, and then we kind of break it down from there. Okay? So now what I want to do is do a few practice problems so you guys get comfortable with this, and I'm going to try to trick you a little bit, so be prepared. Alright? 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

Here’s what students ask on this topic:

What are intermolecular forces and why are they important?

Intermolecular forces (IMFs) are forces that exist between molecules, as opposed to intramolecular forces, which are the bonds within a molecule. IMFs are crucial because they determine the physical state (solid, liquid, or gas) of a substance. Without these forces, all substances would exist as gases. IMFs are responsible for properties like boiling and melting points. For example, hydrogen bonding, the strongest IMF, significantly raises the boiling point of substances like water and ethanol, keeping them in liquid form at room temperature. Understanding IMFs helps explain why different substances have different physical properties despite having similar molecular weights.

What are the different types of intermolecular forces?

There are three main types of intermolecular forces, listed from strongest to weakest: hydrogen bonding, dipole-dipole interactions, and Van der Waals forces. Hydrogen bonding occurs between molecules containing N, O, or F atoms bonded to hydrogen, significantly raising boiling points. Dipole-dipole interactions occur between molecules with net dipoles, where partial positive and negative charges attract each other. Van der Waals forces, the weakest, are present in all molecules and are influenced by molecular size and shape. Larger molecules with more surface area have stronger Van der Waals forces.

How do hydrogen bonds affect the boiling point of a substance?

Hydrogen bonds significantly increase the boiling point of a substance. This is because hydrogen bonds are strong intermolecular forces that require more energy to break. For example, water (H₂O) has a high boiling point compared to other molecules of similar size due to the hydrogen bonds between its molecules. These bonds form between the hydrogen atom of one water molecule and the oxygen atom of another, creating a network of interactions that hold the molecules together, thus requiring more heat to transition from liquid to gas.

What is the difference between dipole-dipole interactions and Van der Waals forces?

Dipole-dipole interactions occur between molecules that have permanent net dipoles, meaning they have regions of partial positive and partial negative charge. These interactions align the molecules so that opposite charges attract, creating a relatively strong force. Van der Waals forces, on the other hand, are weaker and occur in all molecules, regardless of whether they have a permanent dipole. Van der Waals forces arise from temporary dipoles created by the movement of electrons and are influenced by the size and shape of the molecules. Larger molecules with more surface area have stronger Van der Waals forces.

How do molecular size and shape affect Van der Waals forces?

Molecular size and shape significantly affect Van der Waals forces. Larger molecules with greater molecular weight have more electrons, leading to stronger temporary dipoles and thus stronger Van der Waals forces. The shape of the molecule also plays a role; molecules that can stack neatly, like rings, have stronger Van der Waals forces because they can pack closely together, maximizing the interactions. In contrast, branched molecules have weaker Van der Waals forces due to less efficient packing and more space between molecules.