Gibbs free-energy change and feasibilityAQA A-Level Chemistry: Flashcards
What these 12 flashcards ask
- State the Gibbs equation.
- What units should T have in the Gibbs equation?
- What is the condition for a reaction to be feasible?
- How must ΔS be converted before use in ΔG = ΔH − TΔS?
- What is the sign of ΔG at all temperatures if ΔH is negative and ΔS is positive?
- What is the sign of ΔG at all temperatures if ΔH is positive and ΔS is negative?
- When is a reaction with ΔH and ΔS both positive feasible?
- When is a reaction with ΔH and ΔS both negative feasible?
- How do you find the temperature at which a reaction becomes feasible?
- What are the gradient and intercept of a graph of ΔG against T?
- Does a negative ΔG mean a reaction will be fast?
- Convert 1019 K to °C.
Exam questions on Gibbs free-energy change and feasibility
- 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
- 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
- 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
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).