3.1.8.2 Gibbs free-energy change, ∆G, and entropy change, ∆S Quiz: AQA Chemistry, Unit 1

The 20 questions

1. What value must Gibbs free-energy change, ΔG, have for a reaction to be feasible?

   • Zero or negative
   • Strictly positive
   • Strictly zero
   • Zero or positive

2. Which relationship links the Gibbs free-energy change, enthalpy change and entropy change?

   • delta G = delta H / T + delta S
   • delta G = T delta H - delta S
   • delta G = delta H + T delta S
   • delta G = delta H - T delta S

3. For NH3 synthesis, delta H = -92 kJ/mol and delta S = -198 J K^-1 mol^-1. What is delta G at 298 K?

   • About +33 kJ/mol
   • About -59 kJ/mol
   • About -92 kJ/mol
   • About -33 kJ/mol

4. For NH3 synthesis with delta H = -92 kJ/mol and delta S = -198 J K^-1 mol^-1, above what temperature does the reaction become non-feasible?

   • About 298 K
   • About 192 K
   • About 465 K
   • About 930 K

5. Using standard entropy values for CaCO3 = 93, CaO = 40 and CO2 = 214 J K^-1 mol^-1, what is the entropy change for CaCO3 -> CaO + CO2?

   • -161 J K^-1 mol^-1
   • +347 J K^-1 mol^-1
   • +161 J K^-1 mol^-1
   • +54 J K^-1 mol^-1

6. For CaCO3 -> CaO + CO2 with delta H = +178 kJ/mol and delta S = +161 J K^-1 mol^-1, what is the minimum temperature at which the decomposition is feasible?

   • About 161 K
   • About 1106 K
   • About 2 K
   • About 178 K

7. Which physical state change always results in a positive entropy change?

   • Liquid to gas
   • Gas to liquid
   • Gas to solid
   • Liquid to solid

8. Besides enthalpy change and temperature, what property determines the feasibility of a reaction?

   • Rate constant
   • Bond enthalpy
   • Entropy change
   • Activation energy

9. At what temperature condition is a reaction with ΔH = -90 kJ mol⁻¹ and ΔS = -150 J K⁻¹ mol⁻¹ feasible?

   • Above 600 K
   • Below 600 K
   • Above 0.6 K
   • Below 0.6 K

10. For a reaction with delta H = -50 kJ/mol and delta S = -100 J K^-1 mol^-1 at 300 K, what is delta G?

    • -50 kJ/mol
    • -20 kJ/mol
    • +20 kJ/mol
    • -80 kJ/mol

11. Which combination of ΔH and ΔS signs makes a reaction feasible only at high temperatures?

    • +ΔH and +ΔS
    • -ΔH and +ΔS
    • +ΔH and -ΔS
    • -ΔH and -ΔS

12. Which combination of ΔH and ΔS signs makes a reaction feasible only at low temperatures?

    • -ΔH and -ΔS
    • -ΔH and +ΔS
    • +ΔH and +ΔS
    • +ΔH and -ΔS

13. Estimate the entropy change for vaporising water at its boiling point of 373 K, given delta Hvap = +40.7 kJ/mol.

    • About +0.109 J K^-1 mol^-1
    • About +273 J K^-1 mol^-1
    • About +109 J K^-1 mol^-1
    • About +40.7 J K^-1 mol^-1

14. On a graph of delta G against temperature T, what does the gradient of the straight line represent?

    • delta H
    • delta S
    • delta G
    • -delta S

15. A reaction has delta H = -40 kJ/mol and delta S = -100 J K^-1 mol^-1. What is delta G at 400 K?

    • -80 kJ/mol
    • 0 kJ/mol
    • +80 kJ/mol
    • -40 kJ/mol

16. A reaction has delta H = +50 kJ/mol and delta S = +0.1 kJ K^-1 mol^-1. At what temperature does delta G = 0?

    • 500 K
    • 1000 K
    • 50 K
    • 5 K

17. Which expression correctly rearranges delta G = delta H - T delta S to give the entropy change?

    • delta S = T (delta H - delta G)
    • delta S = (delta G - delta H)/T
    • delta S = (delta H - delta G)/T
    • delta S = (delta H + delta G)/T

18. How can a reaction with negative ΔH and negative ΔS be made feasible?

    • Lowering the temperature
    • Adding a catalyst
    • Increasing the pressure
    • Raising the temperature

19. How do you convert ΔS from J K⁻¹ mol⁻¹ to kJ K⁻¹ mol⁻¹?

    • Add 273
    • Divide by 100
    • Divide by 1000
    • Multiply by 1000

20. If ΔH is negative and ΔS is positive, at what temperatures is the reaction feasible?

    • High temperatures only
    • Low temperatures only
    • No temperatures
    • All temperatures

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