Heat Problem Set
Name ________________________________________________
Heats of Formation
1. For each of the following compounds, write a balanced thermochemical equation depicting the
formation of one mole of the compound from its elements in their standard states and use the
appendix to obtain the value of ∆Hfº.
[a] NO2 (g)
[b] SO3 (g)
[c] NaBr (s)
[d] Pb(NO3)2 (s)
(Brown/Lemay, 12th edition, 5.69)
2. The ∆Hfº values of the two allotropes of oxygen, O2 and O3, are 0 and 142.2 kJ/mol, respectively, at
25ºC. Which is the more stable form at this temperature?
(Chang, 11th edition, 6.46)
3. Use the values of ∆Hfº in the appendix to calculate ∆Hº for the following reactions:
[a]
[b]
Ca3(PO4)2 (s) + 3 H2SO4 (l)  3 CaSO4 (s) + 2 H3PO4 (l)
[c]
NH3 (g) + HCl (g)  NH4Cl (s)
(Zumdahl, 9th edition, Ch. 6, 79)
4. Using the appendix values, calculate the value of ∆Hfº for each of the following reactions:
[a] 2 SO2 (g) + O2 (g)  2 SO3 (g)
[b] Mg(OH)2 (s)  MgO (s) + H2O (l)
[c] N2O4 (g) + 4 H2 (g)  N2 (g) + 4 H2O (g)
[d] SiCl4 (l) + 2 H2O (l)  SiO2 (s) + 4 HCl (g)
(Brown/Lemay, 12th edition, 5.73)
5. Pentaborane-9, B5H9, is a colorless, highly reactive liquid that will burst into flame when exposed to
oxygen. The reaction is:
2 B5H9 (l) + 12 O2 (g)  5 B2O3 (s) + 9 H2O (l)
∆Hfº = -8890.2 kJ/mol
Calculate the kilojoules of heat released per gram of the compound reacted with oxygen.
(Chang, 11th edition, 6.58)
6. Gasoline is composed primarily of hydrocarbons, including many with eight carbon atoms called
octanes. One of the cleanest-burning octanes is a compound called 2,3,4-trimethylpentane, which has
the structural formula:
The complete combustion of one mole of this compound to CO2 (g) and H2O (g) leads to a ∆Hº = -5064.9
kJ/mol.
[a] Write a balanced chemical equation for the combustion of 1 mol of C8H18.
[b] Write a balanced equation for the formation of C8H18 from its elements.
[c] By using the information in this problem and appendix data, calculate the ∆Hfº for
2,3,4- trimethylpentane.
(Brown/Lemay, 12th edition, 5.77)
7. The Ostwald process for the commercial production of nitric acid from ammonia and oxygen involves
the following steps:
4 NH3 (g) + 5 O2 (g)  4 NO (g) + 6 H2O (g)
2 NO (g) + O2 (g)  2 NO2 (g)
3 NO2 (g) + H2O (l)  2 HNO3 (aq) + NO (g)
[a] Use the values of ∆Hfº to calculate the value of ∆Hº for each of the following three reactions.
[b] The overall reaction is NH3 + 2 O2  HNO3 + H2O. Use Hess’s Law to determine the overall
enthalpy released in the Ostwald process.
(Zumdahl, 9th edition, Ch. 6, 81)