Concept preview

What you will be able to do

1. 01Explain the two driving forces of change
2. 02Predict the sign of an entropy change
3. 03Use ΔG = ΔH − TΔS to judge spontaneity and its temperature dependence

Why does any change happen at all? Two natural tendencies push every physical and chemical process. Systems tend toward lower enthalpy — releasing energy to become more stable — and toward higher entropy — becoming more disordered and spread out. Sometimes both tendencies point the same way and the change is strongly favoured; sometimes they oppose each other, and the winner depends on temperature. Learning to weigh these two forces is the key to predicting whether a reaction will go.