 Remember about exothermic and endothermic reactions? We looked at them a few videos ago. We said that the amount of energy released or absorbed during a chemical reaction was called the change in enthalpy. The change in enthalpy is the difference in energy between the reactants and the products, so the change in enthalpy can be positive or negative. If it's positive, it means that the product contain more energy than the reactants, and that means that energy must have been absorbed from the surroundings during the reaction. This will cause the surroundings to cool down. We call this kind of reaction endothermic. If the change in enthalpy is negative, it means that the reactants must have contained more energy than the products, and that means that energy must have been released into the surroundings during the reaction as the reactants are turned into the products, and this will cause the surroundings to heat up. We call this kind of reaction exothermic. Now the actual value of the change in enthalpy depends on what the reaction is, what is actually reacting, but it also depends on how much of the chemicals react. For instance, if you burn five litres of fuel, you've got a combustion reaction happening, you will get five times as much heat out as if you burnt one litre of fuel. So when we talk about enthalpies, we somehow need to take into account how much of the reactants have been used. So the way we express the enthalpy for a particular reaction is like this. The change in enthalpy and the units of that are kilojoules per mole equals the amount of energy that's been either released or absorbed, measured in kilojoules, divided by the amount of reactant in moles. So the energy is in kilojoules. You could also express it in joules, but enthalpies of reaction tend to be quite large values, so we use kilojoules, and it can be positive if the energy was absorbed or it can be negative if it was released. So the equation for enthalpy is enthalpy equals energy over moles, and you can see that in the unit, kilojoules per mole, energy per mole.