Quick answer: Entropy S measures the dispersal of energy and matter, and the Gibbs free energy change ΔG = ΔH − TΔS predicts spontaneity: a reaction is spontaneous when ΔG is negative. This AP Chemistry test on Gibbs free energy has 8 questions worth 20 marks, to do in 45 minutes: entropy basics, entropy of a decomposition and Gibbs calculation. The mark scheme is on a separate page; free printable PDF.
Answer all questions and show your working. A calculator is allowed. Use R = 8.314 J/(mol·K) and T = 298 K unless the question states otherwise. Convert ΔS to kJ/(mol·K) when combining it with ΔH.
Level: AP Chemistry · Curriculum: AP Chem Unit 9
Question 1 Entropy basics (2 marks)
Define entropy, give its unit and state the sign of ΔS for evaporation. (2 marks)Question 2 Entropy of a decomposition (3 marks)
Calculate ΔS° for 2KClO₃(s) → 2KCl(s) + 3O₂(g). S° (J/(mol·K)): KClO₃ 143.1, KCl 82.6, O₂ 205.1. (3 marks)Question 3 Gibbs calculation (3 marks)
A reaction has ΔH = −120 kJ and ΔS = −250 J/K.- Calculate ΔG at 298 K and state whether it is spontaneous. (2 marks)
- Calculate the temperature above which it is no longer spontaneous. (1 mark)
Question 4 K from ΔG° (3 marks)
ΔG° = −15.0 kJ at 298 K. Calculate K and state what the size of K tells you about the equilibrium position. (3 marks)Question 5 Sign combinations (2 marks)
Explain why a reaction with ΔH positive and ΔS positive is spontaneous only at high temperature. (2 marks)Question 6 Reading a graph (3 marks)
- Read ΔH from the graph. (1 mark)
- State the temperature at which ΔG = 0. (1 mark)
- Calculate ΔS in J/(mol·K). (1 mark)
Question 7 Non-standard conditions (2 marks)
A reaction has ΔG° = −5.0 kJ at 298 K and the reaction quotient is Q = 50. Calculate ΔG and say in which direction the reaction proceeds. (2 marks)Question 8 Comparing entropies (2 marks)
Which has the greater entropy at 298 K: (a) 1 mol H₂O(l) or 1 mol H₂O(g); (b) 1 mol CH₄(g) or 1 mol C₃H₈(g)? Explain each. (2 marks)
