What is the standard Gibbs free energy change for the formation of one mole of ammonia from its elements under standard conditions?

• A. -16.5 kJ/mol
• B. 66 kJ/mol
• C. -33.0 kJ/mol
• D. -66.0 kJ/mol

Answer: (A)

This is asking for the standard Gibbs free energy change for forming ammonia from its elements, which is defined by the standard Gibbs free energy of formation. At standard conditions, the formation reaction is 1/2 N2(g) + 3/2 H2(g) → NH3(g). Elements in their standard states have a standard Gibbs free energy of formation of zero, so the overall ΔG° for this reaction equals the ΔG°f of NH3(g). That value is about −16.5 kJ per mole of NH3 formed. The negative sign means forming NH3 is thermodynamically favorable under standard conditions, though kinetics may affect how fast it occurs.