At chemical equilibrium, the Gibbs free energy change is equal to

• A. Positive
• B. Negative
• C. Zero
• D. Undefined

Answer: (C)

At constant temperature and pressure, the driving force for a reaction is the Gibbs free energy change, which depends on how far the system is from equilibrium through the reaction quotient Q. The relation ΔG = ΔG° + RT ln Q shows that as Q changes, ΔG changes. At equilibrium, Q equals the equilibrium constant K, and ΔG° = -RT ln K, so ΔG = ΔG° + RT ln Q becomes ΔG = -RT ln K + RT ln K = 0. This means there is no net driving force to move the system toward more products or more reactants—the forward and reverse rates are equal, and the Gibbs free energy is at a minimum for the given conditions. If ΔG were positive or negative, the system would shift in the direction that reduces ΔG, but at equilibrium those tendencies cancel and ΔG is zero.