AP Chemistry 2025 – 400 Free Practice Questions to Pass the Exam

The spontaneity of a chemical reaction is determined by the balance between enthalpy and entropy changes associated with the reaction, which can be influenced by various external conditions. Temperature and pressure play a crucial role in this process.

When evaluating spontaneity, the Gibbs free energy equation (ΔG = ΔH - TΔS) comes into play, where ΔG represents the free energy change, ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔS is the change in entropy. At different temperatures, the contribution of the entropy term can change, potentially affecting whether the reaction is spontaneous. For example, a reaction that may be non-spontaneous at lower temperatures could become spontaneous at higher temperatures if the positive entropy change (ΔS) is sufficiently large.

Pressure also affects the spontaneity of reactions, especially those involving gases. Changes in pressure can influence the concentrations of gaseous reactants and products, potentially shifting the equilibrium position according to Le Chatelier’s principle, which can directly affect the reaction's Gibbs free energy.

While the concentration of reactants, the initial energy of the system, and the identity of the reactants may have an influence on the rate of a reaction and can even affect equilibria