Which expression correctly describes the nonstandard Gibbs free energy change?

• A. ΔG = ΔG° + RT ln Q
• B. ΔG = ΔG° - RT ln Q
• C. ΔG = ΔH° - TΔS°
• D. ΔG = -RT ln Q

Answer: (A)

Nonstandard Gibbs free energy change reflects both the baseline energy under standard conditions and the current mixture composition. The correct relation is ΔG = ΔG° + RT ln Q, where Q is the reaction quotient for the present concentrations. This shows how deviations from standard state shift the driving force: if the mixture favors products more than standard state would, ln Q is positive and ΔG becomes more positive; if it favors reactants, ln Q is negative and ΔG becomes more negative. At equilibrium, ΔG = 0, which implies ΔG° = −RT ln K, linking the standard change to the equilibrium constant.

The other forms omit essential pieces or use the wrong sign. ΔG° = ΔH° − TΔS° describes the standard Gibbs energy in terms of enthalpy and entropy, not the nonstandard ΔG with current Q. A form like ΔG = −RT ln Q is missing the baseline ΔG° term entirely and only applies in a special case. A form with ΔG° − RT ln Q would double-count the Q contribution in the wrong way.