Gibbs free energyEdexcel A-Level Chemistry: Mind map

Exam questions on Gibbs free energy

1. The Haber process, N₂(g) + 3H₂(g) → 2NH₃(g), has ΔH = −92 kJ mol⁻¹ and ΔS system = −199 J K⁻¹ mol⁻¹. A student uses these values, assumed to be constant with temperature, to investigate how feasible the reaction is at different temperatures.
   The Haber process has a negative ΔG at 298 K but a mixture of nitrogen and hydrogen does not form ammonia at an observable rate at this temperature. Explain why.2 marks
2. Magnesium carbonate decomposes on heating: MgCO₃(s) → MgO(s) + CO₂(g). The reaction has ΔH = +117 kJ mol⁻¹ and ΔS system = +176 J K⁻¹ mol⁻¹. Both values are assumed to be constant with temperature.
   Explain why the decomposition becomes feasible at high temperatures.2 marks
3. A student relates the Gibbs free energy change of a reaction to its equilibrium constant, K, using ΔG = −RT ln K. She uses R = 8.31 J K⁻¹ mol⁻¹ and T = 298 K.
   A reaction has ΔG = −32.7 kJ mol⁻¹ at 298 K. Calculate the value of K for this reaction.3 marks