Thermodynamics Practice Problems
1. Calculate H for the reaction of sodium oxide with sulfur dioxide.
Na2O(s) + SO2(g) → Na2SO3(s)
2. Use enthalpies of combustion to calculate H for the oxidation of 1-butanol
to make butanoic acid.
C4H9OH(l) + O2(g) → C3H7COOH(l) + H2O(l)
Combustion of butanol:
C4H9OH(l) + 6O2(g) → 4CO2(g) + 5H2O(l)
Hc = -2675.9 kJ/mol
Combustion of butanoic acid:
C3H7COOH(l) + 5O2(g) → 4CO2(g) + 4H2O(l)
Hc = -2183.6 kJ/mol
3. Determine the free energy change for the reduction of CuO with hydrogen.
Products and reactants are at 25°C.
CuO(s) + H2(g) → Cu(s) + H2O(l)
H =-128.5 kJ/mol
S = -70.1 J/mol K
4. Calculate the enthalpy change at 25°C for the reaction of sodium iodide and chlorine.
Use only the data given.
2NaI(s) + Cl2(g) → 2NaCl(s) + I2(l)
S = -79.9 J/mol K
G = -98.0 kJ/mol
5. Calculate the enthalpy for the combustion of decane. H0f for liquid decane is 300.9 kJ/mol.
C10H22(l) + 31/2O2(g) → 10CO2(g) + 11H2O(l)
6. Find the enthalpy of the reaction of magnesium oxide with hydrogen chloride.
MgO(s) + 2HCl(g) → MgCl2(s) + H2O(l)
Use the following equations and data.
Mg(s) + 2HCl(g) → MgCl2(s) + H2(g) H = -456.9 kJ/mol
Mg(s) + 1/2O2(g) → MgO(s)
H = -601.6 kJ/mol
H2O(l) → H2(g) + 1/2O2(g)
H = 285.8 kJ/mol
7. In locations where natural gas, which is mostly methane, is not available, many people burn propane,
which is delivered by truck and stored in a tank under pressure.
a. Write the chemical equations for the complete combustion of 1 mol of
methane, CH4, and 1 mol of propane, C3H8.
b. Calculate the enthalpy change for each reaction to determine the amount of
energy released by burning 1 mol of each fuel.
c. Using the enthapies of combustion you calculated, determine the energy
output per kilogram of each fuel. Which fuel yields more energy per unit mass?
Answers:
1.
2.
3.
4.
5.
6.
7.
-390. kJ/mol
-492.3 kJ/mol
-107.6 kJ/mol
-121.8 kJ/mol
-7171.4 kJ/mol (-7379.7 kJ/mol)
-141.1 kJ/mol
a) CH4 + 2O2  CO2 + 2H2O
C3H8 + 5O2  3CO2 + 4H2O
b) methane: H = -802.2 kJ/mol
propane: H = -2043 kJ/mol
c) output (methane) = -4.998 x 104 kJ/kg
output (propane) = -4.632 x 104 kJ/kg
so methane yields more energy per unit mass than propane