Topic 15: Energetics/thermochemistry - 15.2 Entropy and spontaneity


Essential idea: A reaction is spontaneous if the overall transformation leads to an increase in total entropy (system plus surroundings). The direction of spontaneous change always increases the total entropy of the universe at the expense of energy available to do useful work. This is known as the second law of thermodynamics.

Entropy (S) refers to the distribution of available energy among the particles. The more ways the energy can be distributed the higher the entropy.

Gibbs free energy (G) relates the energy that can be obtained from a chemical reaction to the change in enthalpy (ΔH), change in entropy (ΔS), and absolute temperature (T).

Entropy of gas > liquid > solid under same conditions.

Applications and skills

Prediction of whether a change will result in an increase or decrease in entropy by considering the states of the reactants and products.

Calculation of entropy changes (ΔS) from given standard entropy values (Sº).

Application of ΔG° = ΔH° - T ΔS° in predicting spontaneity and calculation of various conditions of enthalpy and temperature that will affect this.

Relation of ΔG to position of equilibrium.


Examine various reaction conditions that affect ΔG.

ΔG is a convenient way to take into account both the direct entropy change resulting from the transformation of the chemicals, and the indirect entropy change of the surroundings as a result of the gain/loss of heat energy

Thermodynamic data is given in section 12 of the data booklet.