Quick answer: Entropy measures how many ways particles and energy can be arranged; it rises from solid to liquid to gas. This Chemistry 11-12 test on entropy Gibbs energy has 8 questions worth 20 marks, to do in 45 minutes: meaning of entropy, predicting the sign of ΔS and steam reforming: entropy. The mark scheme is on a separate page; free printable PDF.
This test covers entropy, entropy changes of reactions, the Gibbs equation, a ΔG–T graph and the link to K. You have 45 minutes and may use a calculator. Show your working, convert ΔS to kJ before using it, and use kelvin.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R1.4
Question 1 Meaning of entropy (2 marks)
Explain, in terms of particles, why the entropy of water increases when it boils.Question 2 Predicting the sign of ΔS (3 marks)
State whether ΔS is positive or negative for each change, with a reason.- 2KClO₃(s) → 2KCl(s) + 3O₂(g) (1 mark)
- 2NO₂(g) → N₂O₄(g) (1 mark)
- Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s) (1 mark)
Question 3 Steam reforming: entropy (3 marks)
In steam reforming, CH₄(g) + H₂O(g) → CO(g) + 3H₂(g). S° values in J K⁻¹ mol⁻¹: CH₄ 186.3, H₂O(g) 188.8, CO 197.7, H₂ 130.7. Calculate ΔS°.Question 4 Steam reforming: feasibility (2 marks)
For the reaction in the previous question ΔH = +206 kJ/mol. Calculate ΔG at 298 K and state whether the reaction is spontaneous.Question 5 Steam reforming: temperature (3 marks)
Calculate the minimum temperature at which the reaction becomes spontaneous. Give your answer in kelvin.Question 6 Reading a ΔG–T graph (3 marks)
The graph shows ΔG against T for a reaction.- State the signs of ΔH and ΔS, with a reason. (1 mark)
- Read the temperature at which ΔG = 0. (1 mark)
- State the temperatures at which the reaction is spontaneous. (1 mark)
Question 7 From ΔG to K (2 marks)
A reaction has ΔG° = −17.0 kJ/mol at 298 K. Calculate the equilibrium constant (R = 8.31 J K⁻¹ mol⁻¹).Question 8 Entropy of the surroundings (2 marks)
Explain why an exothermic reaction increases the entropy of the surroundings.
