If E°cell is positive, what is the sign of ΔG° at 25°C?

• A. Negative
• B. Positive
• C. Zero
• D. Depends on the temperature

Answer: (A)

The key idea is how standard cell potential relates to standard Gibbs free energy. The equation ΔG° = − n F E°cell links them, where n is the number of electrons transferred and F is Faraday’s constant (a positive value). If E°cell is positive, then n F E°cell is positive, and the negative sign makes ΔG° negative. Under standard conditions at 25°C, this negative ΔG° means the reaction is spontaneous and can release free energy. If the temperature changes, the standard relation still ties ΔG° to E°cell at 25°C; the sign of ΔG under nonstandard conditions would depend on the actual conditions, but for a positive E°cell at 25°C, ΔG° is negative.