Unit 5 Thermodynamics

5.4 Exercises

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Section 5.4 Exercises

  1. The following sequence of reactions occurs in the commercial production of aqueous nitric acid:
    [latex]\begin{array}{l l} 4 \text{NH}_3(g) + 5\text{O}_2(g) \longrightarrow 4 \text{NO}(g) + 6\text{H}_2 \text{O}(l) & \Delta H = -907 \;\text{kJ} \\[1em] 2 \text{NO}(g) + \text{O}_2(g) \longrightarrow 2\text{NO}_2(g) & \Delta H = -113 \;\text{kJ} \\[1em] 3\text{NO}_2(g) + \text{H}_2 \text{O}(l) \longrightarrow 2\text{HNO}_3(aq) + \text{NO}(g) & \Delta H = -139 \;\text{kJ} \end{array}[/latex]

    Determine the total energy change for the production of one mole of aqueous nitric acid by this process, using ammonia and oxygen as the only reactants.

  2. From the molar heats of formation (Source: OpenStax Chemistry 2e), determine how much heat is required to evaporate one mole of water: [latex]\text{H}_2 \text{O}(l) \longrightarrow \text{H}_2 \text{O}(g)[/latex]
  3. Calculate ΔH°298 for the process [latex]\text{Sb}(s) + \frac{5}{2} \text{Cl}_2(g) \longrightarrow \text{SbCl}_5(g)[/latex]

    from the following information:

    [latex]\begin{array}{l l} \text{Sb}(s) + \frac{3}{2}\text{Cl}_2(g) \longrightarrow \text{SbCl}_3(g) & \Delta H^{\circ}_{298} = -314 \;\text{kJ} \\[1em] \text{SbCl}_3(s) + \text{Cl}_2(g) \longrightarrow \text{SbCl}_5(g) & \Delta H^{\circ}_{298} = -80 \;\text{kJ} \end{array}[/latex]

  4. Calculate ΔH for the process [latex]\text{Hg}_2 \text{Cl}_2(s) \longrightarrow 2\text{Hg}(l) + \text{Cl}_2(g)[/latex]

    from the following information:

    [latex]\begin{array}{l l} \text{Hg}(l) + \text{Cl}_2(g) \longrightarrow \text{HgCl}(s) & \Delta H = -224 \;\text{kJ} \\[1em] \text{Hg}(l) + \text{HgCl}_2(s) \longrightarrow \text{Hg}_2 \text{Cl}_2(s) & \Delta H = -41.2 \;\text{kJ} \end{array}[/latex]

  5. Calculate the standard molar enthalpy of formation of NO(g) from the following data:
    [latex]\begin{array}{l l} \text{N}_2(g) + 2\text{O}_2(g) \longrightarrow 2\text{NO}_2(g) & \Delta H^{\circ}_{298} = 66.4 \;\text{kJ} \\[1em] 2\text{NO}(g) + \text{O}_2(g) \longrightarrow 2\text{NO}_2(g) & \Delta H^{\circ}_{298} = -114.1 \;\text{kJ} \end{array}[/latex]
  6. Using the data (Source: OpenStax Chemistry 2e), calculate the standard molar enthalpy change for each of the following reactions:

    (a) [latex]\text{Si}(s) + 2\text{F}_2(g) \longrightarrow \text{SiF}_4(g)[/latex]

    (b) [latex]2\text{C}(s) + 2\text{H}_2(g) \longrightarrow \text{CH}_3 \text{CO}_2 \text{H}(l)[/latex]

    (c) [latex]\text{CH}_4(g) + \text{N}_2(g) \longrightarrow \text{HCN}(g) + \text{NH}_3(g)[/latex]

    (d) [latex]\text{CS}_2(g) + 3\text{Cl}_2(g) \longrightarrow \text{CCl}_4(g) + \text{S}_2 \text{Cl}_2(g)[/latex]

  7. The decomposition of hydrogen peroxide, H2O2, has been used to provide thrust in the control jets of various space vehicles. Using the data (Source: OpenStax Chemistry 2e), determine how much heat is produced by the decomposition of exactly 1 mole of H2O2 under standard conditions. [latex]2\text{H}_2 \text{O}_2(l) \longrightarrow 2\text{H}_2 \text{O}(g) + \text{O}_2(g)[/latex]
  8. Calculate the enthalpy of combustion of butane, C4H10(g) for the formation of H2O(g) and CO2(g). The enthalpy of formation of butane is −126 kJ/mol.
  9. The white pigment TiO2 is prepared by the reaction of titanium tetrachloride, TiCl4, with water vapour in the gas phase: [latex]\text{TiCl}_4(g) + 2\text{H}_2 \text{O}(g) \longrightarrow \text{TiO}_2(s) + 4\text{HCl}(g)[/latex].

    How much heat is evolved in the production of exactly 1 mole of TiO2(s) under standard state conditions?

  10. In the early days of automobiles, illumination at night was provided by burning acetylene, C2H2. Though no longer used as auto headlamps, acetylene is still used as a source of light by some cave explorers. The acetylene is (was) prepared in the lamp by the reaction of water with calcium carbide, CaC2:[latex]\text{CaC}_2(s) + \text{H}_2 \text{O}(l) \longrightarrow \text{Ca(OH)}_2(s) + \text{C}_2 \text{H}_2(g)[/latex] (unbalanced equation). Calculate the standard enthalpy of the reaction. The ΔH°f of CaC2 is −63.35 kJ/mol.

Solutions

  1. −494 kJ.  There are multiple ways to solve this.  Keep the first reaction unchanged, add 2× the second reaction and (4/3)× the third reaction.
  2. 44.01 kJ
  3. −394 kJ
  4. 265 kJ
  5. 90.3 kJ mol−1 of NO
  6. (a) −1615.0 kJ mol−1; (b) −484.3 kJ mol−1; (c) 164.2 kJ mol−1; (d) −232.1 kJ mol−1
  7. −54.04 kJ
  8. −2.66 × 103 kJ mol−1
  9. 66.3 kJ evolved
  10. −122.8 kJ

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