As temperature changes, which statement about the equilibrium constant is true?

• A. It remains fixed for a given reaction regardless of temperature.
• B. It depends on the total pressure of the system only.
• C. It depends on the concentrations of species only at a fixed temperature.
• D. It changes with temperature.

Answer: (D)

The value of the equilibrium constant is set by temperature because it is tied to the standard free energy change of the reaction: ΔG° = -RT ln K. Since ΔG° depends on temperature through ΔG° = ΔH° - TΔS°, changing the temperature changes ΔG°, and thus changes K. In other words, K is not fixed when you vary temperature. Qualitatively, raising temperature favors the endothermic direction (increasing K) and lowers the exothermic direction (decreasing K).

The other ideas aren’t correct because K is not determined by total pressure in general, and at a given temperature K is a constant that relates the activities (or concentrations) of products and reactants at equilibrium, not something that directly follows from the instantaneous concentrations.