Entropy Practice Questions PDF

1. What is entropy a measure of? * A) The energy of a system. * B) The disorder or randomness of a system. * C) The enthalpy change of a reaction. * D) The rate of a reaction. 2. Which of the following has the *highest* entropy? * A) A solid * B) A liquid * C) A gas * D) They all have the same entr...

1. What is entropy a measure of? * A) The energy of a system. * B) The disorder or randomness of a system. * C) The enthalpy change of a reaction. * D) The rate of a reaction. 2. Which of the following has the *highest* entropy? * A) A solid * B) A liquid * C) A gas * D) They all have the same entropy. 3. What are the standard units of entropy (S)? * A) J mol-1 * B) J K-1 mol-1 * C) kJ mol-1 * D) K mol-1 4. What happens to the total entropy of the universe during a spontaneous process? * A) It decreases. * B) It stays the same. * C) It increases. * D) It depends on the process. 5. Which of the following changes results in an *increase* in entropy? * A) Condensation of a gas * B) Freezing of a liquid * C) Dissolving a solid in a solvent * D) Formation of a solid precipitate 6. How do you calculate the entropy change of a reaction (ΔS°)? * A) ΣS°(products) - ΣS°(reactants) * B) ΣS°(reactants) - ΣS°(products) * C) ΣS°(products) + ΣS°(reactants) * D) ΣS°(reactants) / ΣS°(products) 7. What is the symbol and de nition of Gibbs free energy? * A) H, Enthalpy * B) S, Entropy fi * C) G, Overall energy change of a reaction * D) T, temperature 8. What is the equation that relates Gibbs free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS)? * A) ΔG = ΔH + TΔS * B) ΔG = ΔH - TΔS * C) ΔG = TΔS - ΔH * D) ΔH = ΔG + TΔS 9. What are the units of ΔG (Gibbs free energy change)? * A) J * B) J K-1 mol-1 * C) KJ mol-1 * D) K 10. For a reaction to be feasible (spontaneous), what must be true about ΔG? * A) ΔG must be positive. * B) ΔG must be zero. * C) ΔG must be negative. * D) ΔG can be any value. 11. If a reaction has a positive ΔH and a positive ΔS, when will it be feasible? * A) At all temperatures * B) At no temperatures * C) At high temperatures * D) At low temperatures 12. If a reaction has a negative ΔH and a negative ΔS, when will it be feasible? * A) high temperatures * B) low temperatures * C) all temperatures * D) no temperatures 13. A reaction has ΔH = -20 kJ mol-1 and ΔS = -50 J K-1 mol-1. At what temperature (in Kelvin) will the reaction become feasible? * A) 400 K * B) 0.4 K * C) 250 K * D) The reaction will not be feasible at any temperature. 14. A reaction is feasible at all temperatures. What must be true about ΔH and ΔS? * A) ΔH is positive, and ΔS is positive. * B) ΔH is negative, and ΔS is negative. * C) ΔH is positive, and ΔS is negative. * D) ΔH is negative, and ΔS is positive. 15. A solid dissolving in a solvent has an endothermic enthalpy. However, it readily dissolves. What must be true about the entropy change. * A) negative, driving dissolution * B) negative, preventing dissolution * C) positive, preventing dissolution * D) positive, driving dissolution Answers and Explanations: 1. B) The disorder or randomness of a system. - This is the de nition of entropy. 2. C) A gas - Gases have the highest entropy due to the greatest disorder. 3. B) J K-1 mol-1 - These are the standard units of entropy. 4. C) It increases. - The second law of thermodynamics states that the total entropy of the universe always increases for spontaneous processes. 5. C) Dissolving a solid in a solvent - This increases disorder (more possible arrangements). 6. A) ΣS°(products) - ΣS°(reactants) - This is how you calculate the standard entropy change of a reaction. 7. C) G, Overall energy change of a reaction - correct de nition. fi fi 8. B) ΔG = ΔH - TΔS - This is the key equation relating Gibbs free energy, enthalpy, and entropy. 9. C) KJ mol-1 - correct units. 10. C) ΔG must be negative. - A negative ΔG indicates a spontaneous or feasible process. 11. C) At high temperatures - At high temperatures, the TΔS term will become larger and positive, making ΔG negative (ΔG = ΔH - TΔS). 12. B) low temperatures If ΔH is negative, and ΔS is negative, then at low temperatures, the –TΔS term in the Gibbs’ equation will become less positive, ensuring that the ΔG value is negative overall. 13. A) 400 K - ΔG = 0 at the point of feasibility. 0 = -20000 J mol-1 - T(-50 J K-1 mol-1); T = 20000/50 = 400 K. Note the conversion of kJ to J. 14. D) ΔH is negative, and ΔS is positive. - If ΔH is negative and ΔS is positive, ΔG will *always* be negative (ΔG = ΔH - TΔS). 15. D) positive, driving dissolution If a reaction is endothermic, it is only spontaneous if the entropy change is positive and the TΔS term is large enough to make ΔG negative.