Gibbs free energyEdexcel A-Level Chemistry: Flashcards
What these 13 flashcards ask
- State the equation for Gibbs free energy change.
- When is a reaction feasible in terms of ΔG?
- What units are used when calculating ΔG from ΔH and ΔS?
- How do you find the temperature at which a reaction becomes feasible?
- For ΔH positive and ΔS positive, when is the reaction feasible?
- For ΔH negative and ΔS negative, when is the reaction feasible?
- When is a reaction feasible at all temperatures?
- State the equation linking ΔG and K.
- What does a large K show about ΔG?
- What does ΔG = 0 correspond to?
- Why might a reaction with negative ΔG not occur in practice?
- Give an example of a feasible reaction that does not occur at room temperature.
- What can make a kinetically inhibited reaction occur?
Exam questions on Gibbs free energy
- 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
- 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
- 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
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).