I could have just kept to the more general term "energy", but I prefer to be accurate. I have labelled the vertical scale on this particular diagram as enthalpy rather than energy, because we are specifically thinking about enthalpy changes. You will remember that the enthalpy change is the heat evolved or absorbed during a reaction happening at constant pressure. Enthalpy change is simply a particular measure of energy change. Note: It is possibly confusing that I am switching between the terms enthalpy and energy. However many stages the reaction is done in, ultimately the overall enthalpy change will be the same, because the positions of the reactants and products on an enthalpy diagram will always be the same. If you go via the intermediates, you do have to put in some extra heat energy to start with, but you get it back again in the second stage of the reaction sequence. In either case, the overall enthalpy change must be the same, because it is governed by the relative positions of the reactants and products on the enthalpy diagram. In one case, you do a direct conversion in the other, you use a two-step process involving some intermediates. This shows the enthalpy changes for an exothermic reaction using two different ways of getting from reactants A to products B. If you look at the change on an enthalpy diagram, that is actually fairly obvious. Hess's Law is saying that if you convert reactants A into products B, the overall enthalpy change will be exactly the same whether you do it in one step or two steps or however many steps. The enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs. ![]() Hess's Law is the most important law in this part of chemistry. This page explains Hess's Law, and uses it to do some simple enthalpy change calculations involving enthalpy changes of reaction, formation and combustion. HESS'S LAW AND ENTHALPY CHANGE CALCULATIONS Hess's Law and enthalpy change calculations
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