 In this video, we'll look at how we can classify different kinds of enthalpy, and then try out a few calculations that combine stoichiometry and enthalpy. Before we explore how to calculate enthalpies, it's worth doing a quick classification. Every single chemical reaction that occurs has an enthalpy of reaction, a change in energy as the reaction occurs. This is because if a reaction is occurring, then somewhere bonds are being broken, which absorbs energy, and made, which releases energy. Usually, the enthalpy will be expressed in kilojoules of energy released or absorbed per mole of reactant. Because we can classify different kinds of reactions, we can also classify enthalpies for these reactions. For instance, if you're measuring enthalpy for an acid-base reaction, in which the acid and the base neutralise each other, you would call it an enthalpy of neutralisation. And if you're measuring the heat released when fuel is burnt, you would talk about an enthalpy of combustion. These two examples can also be classified simply as enthalpies of reaction, because they are chemical reactions. However, we have some other enthalpies that are not so obvious. When a substance is dissolved in a solvent, there is an energy change associated with that too, since as the solid dissolves, the bonds that hold the atoms or molecules or ions in a solid are broken, and new bonds are formed with the surrounding solvent molecules. Most enthalpies of solution are fairly small, and you don't notice them on a day-to-day basis, but there are some compounds which have large enthalpies of solution and can produce or absorb a lot of energy. Lastly, there are enthalpies that relate to physical processes rather than chemical processes. These enthalpies are sometimes called latent heats. When you melt a solid, the attractions that hold the molecules in solid form have to be broken, so the molecules can move freely as a liquid. This requires energy. In the opposite direction, when a liquid freezes, energy is released as the attractions are forming between molecules and they're locked in place as a solid. This means that melting is an endothermic process. It needs energy to be put in. Think of warming ice up to melt it. And freezing is an exothermic process. That freezing is exothermic may seem counter-intuitive because we put things in the freezer and make them cold in order to freeze them. But in fact, the very purpose of making them cold is to suck out that heat from the exothermic process so that the molecules can form the bonds.