So in this tutorial, we're going to focus on hydrogen bonding. So when you hear the words hydrogen bond, what do you think about? What is a hydrogen bond?
A hydrogen bond is a dipole-dipole interaction. In fact, it's a very special type of dipole-dipole interaction. It's a very strong type of dipole-dipole attraction force. Hydrogen bonds occur whenever hydrogen is directly attached to nitrogen, oxygen, or fluorine. Let's compare the HF bond with the carbon-oxygen bond.
Both of these bonds are highly polar. The electronegativity value of hydrogen is 2.1, and of fluorine is 4.0. For carbon, it's 2.5, and for oxygen, it's 3.5. So for example, The carbon monoxide molecule is a polar molecule.
It has dipole-dipole interactions. Now, HF is also polar, but it has a special type of dipole-dipole interactions known as hydrogen bonds. So, why are hydrogen bonds a lot stronger than dipole-dipole forces, even though they work the same way?
Well, for one reason, the HF bond... is more polarized than the CO bond. The electronegativity difference is a lot bigger in HF.
It's 1.9 compared to 1.0 in CO. So hydrogen bonds are highly polarized. That's one reason. And the second is that hydrogen is a very small atom. Hydrogen is very small compared to carbon.
The average atomic radius of hydrogen is 37 picometers, and for carbon it's 77 picometers. So those are the two reasons why hydrogen bonds are a lot stronger than dipole-dipole interactions. It's because those bonds are highly polarized, there's a huge electronegativity difference, and also because the atoms involved are very, very small. Now to illustrate the effect of size on forces of attraction, let's say if we have a positive charge and a negative charge separated by a distance of, let's say, 1 centimeter.
And let's say that the force of attraction that these two fill, we're going to call it F. What happens? if we bring these two charges closer together.
So let's say if we decrease the distance from 1 cm to half a cm. So we reduce the distance by a factor of 2. What's going to happen to the electrostatic force of attraction? Force is inversely related to the square of the distance.
So you can write it like this if you want. Where r is the distance between the center of the two ions. Q1 and Q2 stands for the charges of the ions.
So if you decrease the distance between the two charges, the force will greatly increase. And notice that it's squared. So 1 divided by half squared is 1 over 1 fourth, which is 4. So therefore, the force will increase by a factor of four if you reduce the distance by a half. So because hydrogen is so small, the distance between the nuclei will be reduced, and so the forces of attraction between HF molecules will be a lot stronger than the forces of attraction between CO molecules. And so that's why size is important when dealing with electrostatic forces of attraction.
If you decrease the size, the force will increase. So now let's go back to the HF molecule. Where is the hydrogen bond that exists among the HF molecules? The hydrogen bond is not this bond that you see here.
This is a covalent bond. It's an intramolecular force of attraction that... Brings the hydrogen and fluorine atoms together. The hydrogen bond exists between molecules.
It's an intermolecular force. Inter means between. Intra means within.
So don't confuse an intramolecular bond with an intermolecular hydrogen bond. So that's the H bond. So make sure you understand the difference between an intermolecular force and an intramolecular force. Hydrogen has a partial positive charge. And fluorine has a partial negative charge.
So the fluorine of one HF molecule is attracted to the hydrogen of another HF molecule. And so that force of attraction between HF molecules is the hydrogen bond. So water also has hydrogen bonds. The oxygen atom of water carries a partial negative charge, and the hydrogen atoms are partially positive.
So water molecules are attracted to each other. So the oxygen of one water molecule is attracted to the hydrogen of another. And so that is the H bond within water.
So hopefully... This makes sense to you because it's very important that you understand that. So this is a covalent bond. It's an intermolecular force of attraction.
And this is an intermolecular bond or hydrogen bond. So that's the hydrogen bonds that exist between water molecules. So now at this point, I want to compare the energy differences between breaking a covalent bond with breaking a hydrogen bond. In order to vaporize liquid water into steam, you need to break apart all of the hydrogen bonds that hold the water molecules together. So in liquid water, the molecules are relatively close to each other because the hydrogen bonds keep them connected.
So in order to vaporize liquid water into steam, you need to break apart all of these bonds. and keep in mind to break a bond it's an endothermic process you have to put heat energy into liquid water to break the hydrogen bonds and once you break the hydrogen bonds the liquid water molecules will be far apart from each other in which case they will be in the gas phase so that's when you have steam likewise when gaseous steam condenses back into liquid water molecules It regenerates this bond by releasing heat energy. So anytime a bond is formed, it's an exothermic process, heat is released.
But to break a bond, it's an endothermic process. That's why if you want to boil liquid water into steam, you've got to put it on a stove. You have to add heat energy to it. Whereas whenever steam condenses, heat energy is released.
So how much heat energy is needed to break one mole? of all of the hydrogen bonds in order to vaporize liquid water into steam. So this is known as the enthalpy of vaporization, and it's about 40.7 kilojoules per mole.
So if you have one mole of liquid water, let's say at 100 degrees Celsius, you need to add 40.7 kilojoules of thermal energy in order to break all of the hydrogen bonds so that all of the liquid water molecules could vaporize into steam. So that gives you a good idea of how much energy is required to break a hydrogen bond in order for it to be vaporized into the gaseous phase. Now what about a covalent bond? The average bond association energy of an OH bond is 467 kJ per mole.
So if you want to break one mole of the OH covalent bond, you need to add 467 kilojoules of heat energy. Now water has two oxygen hydrogen bonds and so if you want to break up all of the covalent bonds that's found in one mole of water it's going to be 467 times two which is 934 kilojoules of thermal energy. So let's compare that to the enthalpy of vaporization which is about 40.7 kilojoules per one mole of water.
If we divide these two numbers, let's divide 934 by 40.7. This is about, if you round it to the nearest whole number, 23. So the covalent bond, that is the OH bond, is about 23 times stronger compared to the hydrogen bond between water molecules. So as you can see, a covalent bond is a lot harder to break than a typical hydrogen bond.
So intermolecular forces are a lot stronger than intermolecular forces.