ACS CP Thermodynamics & Chemical Kinetics 1 — Questions and Answers
Question 1: For a reaction at standard conditions with ΔG° = -20.0 kJ/mol at 298 K, what is the equilibrium constant K? (R = 8.314 J/mol·K)
- K ≈ 2,960 (Correct answer)
- K ≈ 0.000338
- K ≈ 8.07
- K ≈ 0.124
Correct answer: K ≈ 2,960
Using ΔG° = -RT ln K, K = e^(-ΔG°/RT) = e^(20000/2478) ≈ 2,960.
Question 2: Which thermodynamic state function represents the maximum non-expansion work obtainable from a process at constant temperature and pressure?
- Enthalpy (H)
- Helmholtz free energy (A)
- Gibbs free energy (G) (Correct answer)
- Internal energy (U)
Correct answer: Gibbs free energy (G)
Gibbs free energy (G) represents the maximum non-expansion work available from a system at constant T and P.
Question 3: According to Hess's Law, the standard enthalpy change of a reaction is:
- Equal to the activation energy of the forward reaction
- Independent of the pathway taken (Correct answer)
- Always negative for all spontaneous reactions
- Dependent only on the physical state of the products
Correct answer: Independent of the pathway taken
Hess's Law states that enthalpy is a state function, so the total ΔH is independent of the reaction pathway.
Question 4: For an ideal gas expanding isothermally and reversibly, which relationship correctly applies the First Law of Thermodynamics?
- ΔU > 0 and q = 0
- ΔH < 0 and w = 0
- q = -w and ΔU = 0 (Correct answer)
- ΔS = 0 and ΔG = 0
Correct answer: q = -w and ΔU = 0
For an ideal gas at constant temperature, ΔU = 0; applying the first law (ΔU = q + w) gives q = -w.
Question 5: The Clausius-Clapeyron equation is primarily used to relate which two properties during phase transitions?
- Pressure and volume at constant temperature
- Vapor pressure and temperature (Correct answer)
- Entropy change and heat capacity
- Free energy and reaction quotient
Correct answer: Vapor pressure and temperature
The Clausius-Clapeyron equation d(ln P)/dT = ΔHvap/RT² relates vapor pressure to temperature during phase transitions.
Question 6: For an exothermic reaction at equilibrium, Le Chatelier's principle predicts that increasing temperature will:
- Shift equilibrium to the right, producing more products
- Shift equilibrium to the left, increasing reactant concentrations (Correct answer)
- Have no effect on the equilibrium position
- Increase K while decreasing the reaction rate
Correct answer: Shift equilibrium to the left, increasing reactant concentrations
For an exothermic reaction, heat is effectively a product; increasing temperature shifts equilibrium left to consume the added heat.
Question 7: Which of the following correctly expresses the Gibbs-Helmholtz relationship at constant pressure?
- ΔG = ΔH + TΔS
- ΔG = ΔU - TΔS
- ΔG = ΔH - TΔS (Correct answer)
- ΔG = ΔH + PΔV
Correct answer: ΔG = ΔH - TΔS
The Gibbs equation ΔG = ΔH - TΔS combines enthalpy and entropy contributions to determine spontaneity.
ACS CP Thermodynamics & Chemical Kinetics 1
For a reaction at standard conditions with ΔG° = -20.0 kJ/mol at 298 K, what is the equilibrium constant K? (R = 8.314 J/mol·K)