7.4 Practice Problems

Attempt these problems as if they were real exam questions in an exam environment.

Only look up information if you get severely stuck. Never look at the solution until you have exhausted all efforts to solve the problem. Having to look up information or the solution should be an indicator that the previous layers (1–5) in the Structured Learning Approach have not been mastered.

  1. Which of following has the largest entropy?

    1. Na(s)
    2. Na(g)
    3. Ne(g)
    4. He(g)

    Solution


  2. Which of the following has the largest entropy?

    1. H2(g)
    2. H2O(s)
    3. H2O(l)
    4. H2O(g)

    Solution


  3. The entropy of a substance increases when undergoing which process?

    1. freezing
    2. vaporization
    3. condensation

    Solution


  4. Predict the entropy change for the following reaction:

    \[\mathrm{C_3H_8}(g) + \mathrm{5O_2}(g) \longrightarrow \mathrm{3CO_2}(g) + \mathrm{4H_2O}(g)\]

    1. ΔS > 0
    2. ΔS < 0
    3. ΔS = 0

    Solution


  5. Predict the entropy change for the following reaction.

    \[3\mathrm{CO_2}(g) + 4\mathrm{H_2O}(g) \longrightarrow \mathrm{C_3H_8}(g) + 5\mathrm{O_2}(g)\]

    1. ΔS > 0
    2. ΔS = 0
    3. ΔS < 0
    4. cannot be predicted

    Solution


  6. What is the temperature (in °C) at which the following reaction is at equilibrium?

    \[\mathrm{CCl_4}(g) \longrightarrow \mathrm{C}(s,\text{graphite}) + 2\mathrm{Cl_2}(g) \\[1.5ex] \Delta H^{\circ} = 95.7~\mathrm{kJ~mol^{-1}} \quad \Delta S^{\circ} = 142.2~\mathrm{J~mol^{-1}~K^{-1}}\]

    1. 53
    2. 239
    3. 400
    4. 673

    Solution


  7. What is the temperature (in °C) at which the following reaction switches between being spontaneous and non-spontaneous according to the following thermodynamic values?

    \[\mathrm{H_2O}(l) \longrightarrow \mathrm{H_2O}(g) \\[1.5ex] \Delta H^{\circ} = 44.01~\mathrm{kJ~mol^{-1}} \quad \Delta S^{\circ} = 118.8~\mathrm{J~mol^{-1}~K^{-1}}\]

    1. 40.2
    2. 97.3
    3. 148.6
    4. 370.5

    Solution


  8. Liquid water is in equilibrium with water vapor with a vapor pressure of 0.0313 atm (at 25 °C). What is ΔGrxn (in kJ) for this process at equilibrium given the following information?

    \[\mathrm{H_2O}(l) \longrightarrow \mathrm{H_2O}(g) \quad \Delta G^{\circ} = 8.59~\mathrm{kJ~mol^{-1}}\]

    1. –12
    2. 0
    3. 54
    4. 134

    Solution


  9. What is ΔS°rxn (in J K–1) for the following reaction at 25 °C?

    \[\mathrm{4NH_3}(g) + 5\mathrm{O_2}(g) \longrightarrow \mathrm{NO}(g) + \mathrm{6H_2O}(g)\]

    Substance S° (J mol–1 K–1)

    H2O(g)

    188.8

    NH3(g)

    192.8

    O2(g)

    205.2
    1. 1.6
    2. 88
    3. 104
    4. 178.8

    Solution


  10. What is the sign for ΔH and ΔS, respectively, for a reaction that is spontaneous at any temperature?

    1. –,+
    2. +,+
    3. +,–
    4. –,–

    Solution


  11. What is ΔGrxn (in kJ mol–1) for the following reaction at 25 °C?

    \[\mathrm{SO_2}(g) + \frac{1}{2}\mathrm{O_2}(g) \longrightarrow \mathrm{SO_3}(g)\]

    Substance ΔH°f (kJ mol–1) S° (J mol–1 K–1)

    O2(g)

    0

    205.2

    SO2(g)

    -296.83

    248.2

    SO3(g)

    -395.72

    256.76
    1. –234.8
    2. –70.9
    3. –40.3
    4. 66.4

    Solution


  12. What is ΔS°rxn (in J) for the following reaction at 25 °C?

    \[\mathrm{CH_3OH}(l) + 3\mathrm{O_2}(g) \longrightarrow \mathrm{2CO_2}(g) + \mathrm{4H_2O}(l)\]

    Substance S° (J mol–1 K–1)

    CH3OH(l)

    126.8

    O2(g)

    205.2

    CO2(g)

    213.8

    H2O(l)

    70
    1. –208.4
    2. –161.6
    3. –125.5
    4. 88.9

    Solution


  13. A reaction is found to have a ΔG° = –123.45 kJ mol–1. Predict K.

    1. K > 0
    2. K ≈ 0
    3. K < 0

    Solution


  14. What is ΔG (in kJ) for the following reaction when PN2 = 0.870 atm, PH2 = 0.250 atm and PNH3 = 12.9 atm (at 25 °C)?

    1. 6.3
    2. 9.68
    3. 34.1
    4. 54.4

    Solution





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