Gibbs free-energy change and feasibilityAQA A-Level Chemistry: Mind map

Exam questions on Gibbs free-energy change and feasibility

1. Whether a reaction is feasible at a given temperature depends on the balance between its enthalpy change and its entropy change, expressed by the equation ΔG = ΔH − TΔS, where T is the temperature in kelvin. A student has values of ΔH in kJ mol⁻¹ and ΔS in J K⁻¹ mol⁻¹ for several reactions.
   A reaction has a negative ΔH and a positive ΔS. State the sign of ΔG at all temperatures and explain your answer.2 marks
2. Dinitrogen tetroxide decomposes reversibly: N₂O₄(g) → 2NO₂(g). For this reaction ΔH = +57.0 kJ mol⁻¹ and ΔS = +176 J K⁻¹ mol⁻¹.
   Use the equation ΔG = ΔH − TΔS to explain why increasing the temperature makes the decomposition of N₂O₄ more feasible.2 marks
3. In the Haber process, N₂(g) + 3H₂(g) → 2NH₃(g), the enthalpy change is ΔH = −92 kJ mol⁻¹ and the entropy change is ΔS = −199 J K⁻¹ mol⁻¹ for the reaction as written.
   Calculate ΔG for the Haber process at 298 K. State whether the reaction is feasible at this temperature.3 marks