WORKSHEET 4
Standard Heats of Formation
1. Calculate ∆H° in kilojoules for the following reactions.
a) 2NO(g) + O2(g)  2NO2(g)
b) NaOH(s) + HCl(g)  NaCl(s) + H2O(g)
2. Use a standard enthalpies of formation table to determine the change in enthalpy for each
of these reactions.
a) 2CO(g) + O2(g)  2CO2(g)
b) CH4(g) + 2O2(g)  CO2(g) + 2H2O(l)
c) 2H2S(g) + 3O2(g)  2H2O(l) + 2SO2(g)
3. Calculate the ∆H° for these reactions. In each case, state whether the reaction is
exothermic or endothermic, rewrite the equation as a thermochemical equation to include
the heat term, and indicate whether the products have a greater or smaller enthalpy then
the reactants.
a) SO2(g) + 1/2O2(g)  SO3(g)
b) CaO(s) + H2O(l)  Ca(OH)2(s)
c) N2(g) + 3H2(g)  2NH3(g)
d) C6H6(l) + 11/2O2(g)  6C(s) + 3H2O(l)
e) NH3(g) + HCl(g)  NH4Cl(s)
4. When heated cautiously, ammonium nitrate, NH4NO3, decomposes.......
2 NH4NO3(s)  2 N2O(g) + 4 H2O(g) + N2(g)
a) Calculate ΔH° for the decomposition reaction.
b) Calculate q for the reaction when 1.00 kg of NH4NO3 decomposes.
5. A 2.56 g sample of solid sulfur is burned in excess oxygen in a calorimeter to form
SO2(g). The calorimeter has a heat capacity of 923 JK-1 and contains 815 g of water.
Calculate the temperature change that would occur from this reaction.
6. Palmitic acid, C16H32O2, is typical of the materials available from fats and oils insofar as
chemical energy is concerned.
a) Write the thermochemical equation of the complete combustion of one mole of this
compound, a solid, assuming that gaseous carbon dioxide and liquid water form. The
standard heat of combustion of palmitic acid is -9957.92 kJ/mol.
b) Using the molar heat of combustion of palmitic acid and data from the appropriate
source, estimate the ∆H°f for palmitic acid.
7. Ethane thiol, CH3CH2SH (g), has ΔH°f = - 46.0 kJmol-1. Calculate the molar heat of
combustion, (ie, kJmol -1) for this compound. The products of the combustion reaction
are CO2 (g), H2O (l) and SO2 (g).
8. When iron metal, Fe, reacts with excess oxygen, Fe2O3, is formed. Calculate the
amount of heat energy evolved when 5.58 g of iron is used for this reaction.
Answers:
1a) -114 kJ
b) -65.4 kJ
5 Δt = 5.47 K
8 -41.2 kJ
b) -135.1 kJ
2a) -566 kJ
b) -890.7 kJ
c) -91.8 kJ
d) -906.4 kJ
e) -176.2 kJ
6a) C16H32O2(l) + 23 O2(g)  16CO2(g) + 16 H2O(l) + 9957.92
c) -1124 kJ
4a) -71.9 kJ
b) -910.9 kJ
3a) -98.9 kJ
b) -449 kJ
7 -1.90 x 103 kJ