Van der Waals Forces

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle to start. Take hydrogen chloride, HCL, and hydrogen iodide, H I, two molecules from the same family. HCL is the more polar one; it has the bigger dipole moment. Polar molecules attract each other, so HCL should hold together more strongly and boil higher, right? But no! H I boils higher. How is that possible? To answer this, we need the weak forces that act between molecules, the van der Waals forces. They decide boiling points and even solubility, and questions on them come up often in JEE, so let's begin.

0:42London forces: between all molecules

So, let's start with the simplest case. Take two molecules with no charge and no dipole at all, completely non-polar. Can they attract each other? Yes, they can! These attractions are called London forces, or dispersion forces, the second name in brackets. And remember this: they act between all molecules, polar or non-polar. Okay? Here is how they arise. The electrons in a molecule keep moving. At some instant, a few more of them sit on one side, and the molecule becomes a temporary dipole. The double arrow on screen means, leads to. This temporary dipole induces a dipole in the next molecule, and now the two attract. Got it? Now think. Which electron cloud is easier to push around: a small one, or a big one with many electrons? The big one, exactly! More electrons make the cloud easier to distort, and we call this polarisability. So more electrons, stronger London forces. Right? Second, surface area. Molecules with a larger surface touch their neighbours over more area, so the attraction is stronger. Keep this in mind; it will save you in example two. Clear? And these forces are short range. Their energy of attraction falls off as one over r to the power six, where r is the distance between the molecules. So double r, and this energy becomes sixty four times smaller. With me?

2:12Forces with dipoles and ions

Now, polar molecules. Each one has a permanent dipole, a positive end and a negative end. The positive end of one molecule pulls the negative end of its neighbour. These are dipole-dipole forces, and they act between polar molecules. Okay? The symbol that looks like a little fish lying on its side means, proportional to. For fixed dipoles, the energy of attraction goes as one over r cubed. But molecules keep tumbling around, and for rotating dipoles it falls off as one over r to the power six, just like London forces. Right? Now a mixed pair. Put polar water next to oxygen gas, which is non-polar. Oxygen has no dipole of its own. So, is there any attraction between them? Yes, there is! The dipole of water induces a dipole in the oxygen molecule, O two, the small two below meaning two oxygen atoms. Then the two attract. This is dipole-induced dipole attraction, between a polar and a non-polar molecule, and it is the main reason a little oxygen dissolves in water. Clear? Last one: ions. Take a sodium ion, N a plus, sitting in water. The small plus sign up top is its charge. Which end of the water molecule does it pull towards itself? The negative end, the oxygen end, correct! The ion attracts the oppositely charged end of the water dipole. These ion-dipole forces hold ions in water, and we call this hydration. Got it?

3:41Comparing boiling points

Now let's put these forces to work on boiling points. Stronger attraction between molecules means a higher boiling point, simple. Look at acetone and butane. M is the molar mass, and both have M equals fifty eight. Acetone has a polar C double bond O group, the equals sign on screen, while butane is non-polar. So which one boils higher? Acetone, yes! Acetone boils at fifty six degrees Celsius, the small circle is the degree sign, and butane at minus zero point five degrees Celsius. Same mass, so similar London forces, but acetone adds dipole-dipole forces on top. See the difference? So, rule one: for molecules of similar size, the polar one boils higher. Note the condition, similar size. We will come back to it in a minute, okay? Rule two: in a series of similar molecules, the heavier one, with more electrons, boils higher. More electrons, stronger London forces. Right? Rule three: branching lowers the boiling point. Branched molecules are more compact, with less surface area, so fewer contacts and weaker London forces. Clear?

4:51The trap: HCl versus HI

Now, back to our puzzle from the start. HCL has a larger dipole moment than H I, so it is more polar. By rule one, HCL should boil higher, right? No, no, careful! H I boils higher. This is the trap. Rule one works only for molecules of similar size. And these two are not similar in size at all. With me? Iodine is a much bigger atom than chlorine, with far more electrons. So H I is much larger, and much more polarisable. Its electron cloud distorts easily, okay? So its dispersion forces outweigh the stronger dipole-dipole forces of HCL, and H I boils higher. Puzzle solved! Here, dispersion forces dominate. So never compare only dipole moments; check the size first. Got it?

5:39Example 1: boiling points of the noble gases

Alright, example one, nice and easy. Arrange helium, neon, argon, krypton and xenon by boiling point. The noble gases, okay? Pause the video, and try it first. Done? Okay, let's check. These are single atoms, completely non-polar. No dipoles, no ions. So only one force acts between them: dispersion forces. And dispersion forces grow with the number of electrons. Helium has two, neon ten, argon eighteen, krypton thirty six, and xenon fifty four. So which one boils highest? Xenon, exactly! The order: helium, less than neon, less than argon, less than krypton, less than xenon. The pointed sign on screen means, less than. More electrons, stronger forces, higher boiling point. Easy, right?

6:34Example 2: boiling points of the pentane isomers

Now example two, one step harder. Arrange n pentane, isopentane and neopentane by boiling point. The n in n pentane means normal, the straight chain. Pause and try. Hint: think about shape, not just size. Done? Okay, let's check. All three are C five H twelve; the small five and twelve below count the carbon and hydrogen atoms. Same formula, so the same number of electrons. So all three boil at the same temperature, right? No, no, wait! Remember rule three, branching. n pentane is a straight chain, isopentane has one branch, and neopentane is the most branched. More branching makes the molecule more compact, with a smaller surface area. And a smaller surface area means less contact with the neighbours, so weaker dispersion forces. With me? The data agree. n pentane boils at thirty six, isopentane at twenty eight, and neopentane at just nine point five degrees Celsius. So the order is n pentane, greater than isopentane, greater than neopentane. The sign opening the other way means, greater than. Same formula, different shapes, different boiling points. Beautiful, isn't it?

7:51Quick revision: London forces, polarity and branching

So, quick revision. London forces act between all molecules, and they grow with more electrons and more surface area. Polar molecules add dipole-dipole forces on top, and branching lowers the boiling point. For molecules of similar size, the polar one boils higher; but a bigger, more polarisable molecule can win, like H I over HCL, remember? Now go and try the practice problems on this page. You are ready for them. All the best!