For the reaction CH3COCH3(g) + 4 O2(g) → 3 CO2(g) + 3 H2O(l) at 25 °C with ΔH' = −184 kJ/mol and ΔS' = −236 J/mol·K, what is ΔG'?

• A. −70 kJ/mol
• B. −114 kJ/mol
• C. 0 kJ/mol
• D. −184 kJ/mol

Answer: (B)

Gibbs free energy change is found from ΔG = ΔH − TΔS. At 25 °C, T = 298 K. Convert ΔS to kJ: ΔS = −236 J/(mol·K) = −0.236 kJ/(mol·K). Then TΔS = 298 × (−0.236) = −70.3 kJ/mol. Now ΔG = (−184 kJ/mol) − (−70.3 kJ/mol) ≈ −113.7 kJ/mol, i.e., about −114 kJ/mol. Since ΔG is negative, the reaction is spontaneous at 25 °C. The negative ΔS indicates a loss of disorder (gas → liquid water), but the exothermic enthalpy dominates at this temperature, giving a negative ΔG.