ΔH is the enthalpy change for the reaction. There is a simple equation for the entropy change of the surroundings. So far, you know how to work out the entropy change of the system for a given reaction if you are told the entropies of all the substances involved in the reaction. You should be able to judge what is acceptable to your examiners by looking at how this is presented in your syllabus.Ĭalculating the entropy change of the surroundings. That is because we shall be using this equation under non-standard conditions.

Note: I have deliberately left out the 'standard' symbols in this equation. What matters is the total entropy change, which is the sum of the entropy changes of the system and the surroundings. An endothermic reaction will cool the surroundings, and so the entropy of the surroundings decreases. The reverse is true for an endothermic change. And so increasing the temperature increases the entropy of the surroundings. If you add more energy to the surroundings, the number of different possibilities for arranging the energy over the molecules increases. Heat is given off to the surroundings, and that extra heat increases the entropy of the surroundings. If you only calculate the entropy change of the reaction (the entropy change of the system), you are leaving out an important factor. If you do need to read this page, make sure you have read the page explaining how you calculate the entropy change of the system first. You will still probably have to be able to work out the feasibility of reactions, but that will be done by the rather less confusing use of an equation based on Gibbs free energy. Note: If your syllabus doesn't specifically mention entropy change terms like system, surroundings and total, you could safely ignore this page. It goes on to look at how you can use the total entropy change to decide whether or not a reaction is feasible.

This page considers various entropy changes: of the system, of the surroundings, and the total change.

Change in entropy formula