Chapter 5 Solutions — Thermodynamics

Exercises

1. 1.
   Choose the correct answer. A thermodynamic state function is a quantity

   • (i) used to determine heat changes
   • (ii) whose value is independent of path
   • (iii) used to determine pressure volume work
   • (iv) whose value depends on temperature only.
2. 2.
   For the process to occur under adiabatic conditions, the correct condition is:

   • (i) DeltaT = 0
   • (ii) Deltap = 0
   • (iii) q = 0
   • (iv) w = 0
3. 3.
   The enthalpies of all elements in their standard states are:

   • (i) unity
   • (ii) zero
   • (iii) < 0
   • (iv) different for each element
4. 4.
   DeltaU of combustion of methane is -X kJ mol⁻¹. The value of DeltaH is

   • (i) = DeltaU
   • (ii) > DeltaU
   • (iii) < DeltaU
   • (iv) = 0
5. 5.
   The enthalpy of combustion of methane, graphite and dihydrogen at 298 K are -890.3 kJ mol⁻¹, -393.5 kJ mol⁻¹, and -285.8 kJ mol⁻¹ respectively. Enthalpy of formation of CH₄(g) will be

   • (i) -74.8 kJ mol⁻¹
   • (ii) -52.27 kJ mol⁻¹
   • (iii) +74.8 kJ mol⁻¹
   • (iv) +52.26 kJ mol⁻¹.
6. 6.
   A reaction, A + B -> C + D + q is found to have a positive entropy change. The reaction will be

   • (i) possible at high temperature
   • (ii) possible only at low temperature
   • (iii) not possible at any temperature
   • (v) possible at any temperature
7. 7.
   In a process, 701 J of heat is absorbed by a system and 394 J of work is done by the system. What is the change in internal energy for the process?
8. 8.
   The reaction of cyanamide, NH₂CN(s), with dioxygen was carried out in a bomb calorimeter, and DeltaU was found to be -742.7 kJ mol⁻¹ at 298 K. Calculate enthalpy change for the reaction at 298 K. NH₂CN(g) + 3/2 O₂(g) -> N₂(g) + CO₂(g) + H₂O(l)
9. 9.
   Calculate the number of kJ of heat necessary to raise the temperature of 60.0 g of aluminium from 35°C to 55°C. Molar heat capacity of Al is 24 J mol⁻¹ K⁻¹.
10. 10.
    Calculate the enthalpy change on freezing of 1.0 mol of water at 10.0°C to ice at -10.0°C. DeltafusH = 6.03 kJ mol⁻¹ at 0°C. Cp[H₂O(l)] = 75.3 J mol⁻¹ K⁻¹ Cp[H₂O(s)] = 36.8 J mol⁻¹ K⁻¹
11. 11.
    Enthalpy of combustion of carbon to CO₂ is -393.5 kJ mol⁻¹. Calculate the heat released upon formation of 35.2 g of CO₂ from carbon and dioxygen gas.
12. 12.
    Enthalpies of formation of CO(g), CO₂(g), N₂O(g) and N₂O₄(g) are -110, -393, 81 and 9.7 kJ mol⁻¹ respectively. Find the value of DeltarH for the reaction: N₂O₄(g) + 3CO(g) -> N₂O(g) + 3CO₂(g)
13. 13.
    Given N₂(g) + 3H₂(g) -> 2NH₃(g); DeltarH = -92.4 kJ mol⁻¹ What is the standard enthalpy of formation of NH₃ gas?
14. 14.
    Calculate the standard enthalpy of formation of CH₃OH(l) from the following data: CH₃OH(l) + 3/2 O₂(g) -> CO₂(g) + 2H₂O(l) ; DeltarH = -726 kJ mol⁻¹ C(graphite) + O₂(g) -> CO₂(g) ; DeltacH = -393 kJ mol⁻¹ H₂(g) + 1/2 O₂(g) -> H₂O(l); DeltafH = -286 kJ mol⁻¹.
15. 15.
    Calculate the enthalpy change for the process CCl₄(g) -> C(g) + 4 Cl(g) and calculate bond enthalpy of C-Cl in CCl₄(g). DeltavapH(CCl₄) = 30.5 kJ mol⁻¹ DeltafH(CCl₄) = -135.5 kJ mol⁻¹ DeltaaH(C) = 715.0 kJ mol⁻¹ (enthalpy of atomisation) DeltaaH(Cl₂) = 242 kJ mol⁻¹
16. 16.
    For an isolated system, DeltaU = 0, what will be DeltaS?
17. 17.
    For the reaction at 298 K, 2A + B -> C DeltaH = 400 kJ mol⁻¹ and DeltaS = 0.2 kJ K⁻¹ mol⁻¹ At what temperature will the reaction become spontaneous considering DeltaH and DeltaS to be constant over the temperature range.
18. 18.
    For the reaction, 2 Cl(g) -> Cl₂(g), what are the signs of DeltaH and DeltaS?
19. 19.
    For the reaction 2 A(g) + B(g) -> 2D(g) DeltaU = -10.5 kJ and DeltaS = -44.1 J K⁻¹. Calculate DeltaG for the reaction, and predict whether the reaction may occur spontaneously.
20. 20.
    The equilibrium constant for a reaction is 10. What will be the value of DeltaG? R = 8.314 J K⁻¹ mol⁻¹, T = 300 K.
21. 21.
    Comment on the thermodynamic stability of NO(g), given 1/2 N₂(g) + 1/2 O₂(g) -> NO(g); DeltarH = 90 kJ mol⁻¹ NO(g) + 1/2 O₂(g) -> NO₂(g); DeltarH = -74 kJ mol⁻¹
22. 22.
    Calculate the entropy change in surroundings when 1.00 mol of H₂O(l) is formed under standard conditions. DeltafH = -286 kJ mol⁻¹.