If reaction quotient Q equals 1, what is ΔG relative to ΔG°?

• A. ΔG equals ΔG°
• B. ΔG equals -RT ln K
• C. ΔG equals 0
• D. ΔG equals ΔH° - TΔS°

Answer: (A)

The key relation is ΔG = ΔG° + RT ln Q. Since Q = 1, ln Q = 0, so ΔG = ΔG°. This means the current free-energy change equals the standard free-energy change because there’s no additional driving force from the actual concentrations relative to the standard state. The other options don’t fit: ΔG = -RT ln K would be tied to the standard state via ΔG° = -RT ln K, not ΔG itself; ΔG = 0 only at equilibrium when Q = K; and ΔG = ΔH° − TΔS° is not the general expression used for relating ΔG to Q.