Entropy & Gibbs Free Energy (A2)

Entropy

Entropy $S$ measures the disorder (number of ways energy and particles can be arranged) of a system; units $\text{J K}^{-1}\text{mol}^{-1}$. A more disordered system has a higher entropy.

Entropy increases when:

a solid → liquid → gas (gases have the highest entropy by far);
the number of gas molecules increases in a reaction;
a solid dissolves;
temperature rises (more energy to spread among levels).

The standard entropy change of a reaction:

$$\Delta S^{\ominus} = \sum S^{\ominus}(\text{products}) - \sum S^{\ominus}(\text{reactants})$$