Hydrogen molecules, H₂, have a single H–H bond. This bond has an energy of 432 kilojoules per mole. What does this mean?
The statement we have to explain is that a single hydrogen–hydrogen bond has an energy of 432 kilojoules per mole. Let’s start by imagining a hydrogen molecule. Here’s the bond. And here are the hydrogen atoms. A chemical bond is a lasting attraction between atomic-scale species, be they atoms, ions, or molecules. That’s not to say a chemical bond will last forever. If energy is put into the system enough to counteract the attraction, the bond will break. So the breaking of bonds requires energy. So the opposite releases energy.
So what does it mean to say that a bond has an energy. Well, let’s have a look at an energy diagram. At high energy, we have H plus H. At low energy, we have H₂. The difference between them is said to be the bond energy, the amount the system is stabilized by the bond. Now, what about the specific energy of the hydrogen–hydrogen bond, 432 kilojoules per mole? There are three parts to this. We can look at the meaning one by one.
Let’s start with kilojoules. Kilo means 1000. So kilojoules means 1000 joules. The joule is the SI unit of energy. It takes about 4.2 joules to raise the temperature of one gram of water by one degree Celsius. Per mole is another way of saying per 6.02 times 10 to 23 of something. This is simply a huge number, called Avogadro’s number, which brings things on the atomic scale up to the human scale. So kilojoules per mole means that for every moles worth of something, you have that many kilojoules.
432 is just a number of kilojoules for every mole of bonds. So we’ve established that if we have a moles worth of hydrogen–hydrogen single bonds, we need 432 kilojoules of energy to break them. To put that in context, 432 kilojoules would heat two litres of water by about 52 degrees Celsius. So how could we put all this into words. So what is meant by a single hydrogen–hydrogen bond having the energy of 432 kilojoules per mole is that 432 kilojoules of energy is required to break one mole of hydrogen–hydrogen bonds.
It’s important not to be confused by the meaning of the word single in the first sentence. Single refers to the type of bond between hydrogen and hydrogen not the number of hydrogen bonds in total. If 432 kilojoules of energy were required per bond, if you had two grams of hydrogen, that’s one mole of hydrogen–hydrogen bonds, it would take over 10 minutes for the entire output of the sum to break those bonds. So be careful, the bond energy is given per mole of bonds and not per bond. So again, our answer is 432 kilojoules of energy is required to break one mole of hydrogen–hydrogen bonds.