A Guide To Endothermic Reactions

Hi, I'm Donald Sinclair. I'm a science teacher with Greater London Tutors and today, we're going to be looking at a few topics in chemistry. This is a guide to endothermic reactions.

Chemical reactions can be either exothermic or endothermic. This describes how heat is either taken in or given out during the reaction. Exothermic reactions are probably the ones you're most familiar with; any reaction that gives out heat is exothermic.

For example, most things combusting or burning is an exothermic reaction. Endothermic reactions on the other hand take in heat from their surroundings. They have a cooling effect.

Chemical ice packs for example, use an endothermic reaction to give a cold sensation. The reason that exothermic reactions heat up and endothermic reactions cool down their surroundings is to do with how bonds are either being broken or formed in the chemical reaction. In an exothermic reaction, a chemical bond is being created.

Since two atoms bound together have less energy than two separate atoms, that excess energy is being given out. You can think of two magnets suddenly being pulled in against each other and giving out some energy. In that case, it would be in the form of sound.

You can imagine it here as heat energy being given out. In endothermic reactions, the opposite is happening. A bond is being broken, and two atoms are being separated.

Now, to break a bond obviously requires energy, so energy is being taken in from the surroundings which is why it has a cooling effect. Again, you can think of two magnets stuck together. They require energy to be separated and to be put on their own.

Let's look at an example that involves both bond-breaking and bond-forming. To find out whether our reaction is exothermic or endothermic, you need to calculate how much energy is needed to break the bonds and how much energy is given out when the new bonds are formed. Let's look at the reaction between methane and fluorine to form fluoromethane and hydrogen fluoride.

Now, every bond has an energy associated with it. This is the energy that is required to put into the into the atoms “into the molecules“ to break the bonds, or it's the energy being given out when the bonds are formed. For example, the carbon-hydrogen bond is an energy of 413 kilo-joules per mole.

That means that one bond being broken per methane molecule - in one mole of methane would take four-hundred and thirteen thousand joules. So, if we look at the structural formula, we can see that one hydrogen is being removed from the methane and being replaced with a fluorine which comes from the fluorine gas. So, what bonds are being broken on the left-hand side? Well, we have one carbon-hydrogen bond being broken.

This requires us to put in +413 kilo-joules per mole. The plus sign is there because we're putting energy into the chemicals; energy is being put into it. We also have to break the fluorine-fluorine bond because a hydrogen is going to come in later on, so we also add +158.

This means that the total energy to break the bonds in the first place is 413 + 158, which is 571 kilo-joules per mole. What bonds are being formed? Well, we have one carbon-fluorine bond, which has an energy of 495. Because the bond is being formed, that means energy is being given out; energy is being lost by the chemicals so we put a minus sign.

Also, a bond being formed is the hydrogen-fluorine bond, which has an energy of 565. This is the energy being lost by the chemicals; this energy is being given out and will be absorbed by the surroundings. Therefore, this is an endothermic reaction because energy is being taken in; the surroundings cool down, and this is an exothermic reaction because energy is being given out; the surroundings heat up.

We can see that the total energy being lost by the chemicals is 1060 kilo-joules per mole. Now, because the amount of energy being lost by the chemicals is greater than the amount of energy bein