The Foundations of Chemistry

638 CHAPTER 15: Chemical Thermodynamics

PbCl 2 (s); (c) atomic oxygen, O(g); (d) benzoic acid,

C 6 H 5 COOH(s); (e) hydrogen peroxide, H 2 O 2 (
);

(f ) dinitrogen tetroxide, N 2 O 4 (g).

0 *28.We burn 14.4 g of lithium in excess oxygen at constant

atmospheric pressure to form Li 2 O. Then we bring the

reaction mixture back to 25°C. In this process 605 kJ of

heat is given off. What is the standard molar enthalpy of

formation of Li 2 O?

0 *29.We burn 14.4 g of magnesium in excess nitrogen at con-

stant atmospheric pressure to form Mg 3 N 2. Then we

bring the reaction mixture back to 25°C. In this process

136.7 kJ of heat is given off. What is the standard molar

enthalpy of formation of Mg 3 N 2?

*030.From the following enthalpies of reaction,

4HCl(g)O 2 (g)88n2H 2 O(
)2Cl 2 (g)


H202.4 kJ/mol rxn

^12 H 2 (g)^12 F 2 (g) 88nHF(
)


H600.0 kJ/mol rxn

H 2 (g)^12 O 2 (g) 88nH 2 O(
)


H285.8 kJ/mol rxn

find 
Hrxnfor 2HCl(g)F 2 (g)88n2HF(
)Cl 2 (g).

*031.From the following enthalpies of reaction,

CaCO 3 (s)88nCaO(s)CO 2 (g)


H178.1 kJ/mol rxn

CaO(s)H 2 O(
)88nCa(OH) 2 (s)


H65.3 kJ/mol rxn

Ca(OH) 2 (s)88nCa^2 (aq)2OH(aq)


H16.2 kJ/mol rxn

calculate 
Hrxnfor

Ca^2 (aq)2OH(aq)CO 2 (g)88n

CaCO 3 (s)H 2 O(
)

*032.Given that

S(s)O 2 (g)88nSO 2 (g) 
H296.8 kJ/mol

S(s)^32 O 2 (g)88nSO 3 (g) 
H395.6 kJ/mol

determine the enthalpy change for the decomposition

reaction 2SO 3 (g)n2SO 2 (g)O 2 (g).

*033.Aluminum reacts vigorously with many oxidizing agents.

For example,

4Al(s)3O 2 (g)88n2Al 2 O 3 (s)


H3352 kJ/mol

4Al(s)3MnO 2 (s)88n3Mn(s)2Al 2 O 3 (s)


H1792 kJ/mol

Use this information to determine the enthalpy of for-

mation of MnO 2 (s).

*034.Given that

2H 2 (g)O 2 (g)88n2H 2 O(
)


H571.6 kJ/mol

C 3 H 4 (g)4O 2 (g)88n3CO 2 (g)2H 2 O(
)


H1937 kJ/mol.

C 3 H 8 (g)5O 2 (g)88n3CO 2 (g)4H 2 O(
)


H2220 kJ/mol.

determine the heat of the hydrogenation reaction

C 3 H 4 (g)2H 2 (g)88nC 3 H 8 (g)

*035.Determine the heat of formation of liquid hydrogen per-

oxide at 25°C from the following thermochemical

equations.

H 2 (g)^12 O 2 (g)88nH 2 O(g)


H^0  2 241.82 kJ/mol

2H(g)O(g)88nH 2 O(g)


H^0  2 926.92 kJ/mol

2H(g)2O(g)88nH 2 O 2 (g)


H^0 1070.60 kJ/mol

2O(g)88nO 2 (g)


H^0  2 498.34 kJ/mol

H 2 O 2 (
)88nH 2 O 2 (g)


H^0  22 51.46 kJ/mol

*036.Use data in Appendix K to find the enthalpy of reaction

for

(a) NH 4 NO 3 (s)88nN 2 O(g)2H 2 O(g)

(b) 2FeS 2 (s)^121 O 2 (g)88nFe 2 O 3 (s)4SO 2 (g)

(c) SiO 2 (s)3C(s)88nSiC(s)2CO(g)

*037.Repeat Exercise 36 for

(a) CaCO 3 (s)88nCaO(s)CO 2 (g)

(b) 2HI(g)F 2 (g)88n2HF(g)I 2 (s)

(c) SF 6 (g)3H 2 O(
)88n6HF(g)SO 3 (g)

*038.The internal combustion engine uses heat produced dur-

ing the burning of a fuel. Propane, C 3 H 8 (g), is sometimes

used as the fuel. Gasoline is the most commonly used fuel.

Assume that the gasoline is pure octane, C 8 H 18 (
) and

the fuel and oxygen are completely converted into CO 2 (g)

and H 2 O(g). For each of these fuels, determine the heat

released per gram of fuel burned.

*039.Propane, C 3 H 8 (g), is used as the fuel for some modern

internal combustion engines. Methane, CH 4 (g), has been

proposed by the movie industry as the post-Apocalypse

fuel when gasoline and propane are supposedly no longer

available. Assume that the fuel and oxygen are completely

converted into CO 2 (g) and H 2 O(g). For each of these

fuels determine the heat released per gram of fuel burned.

Compare your answers to the answers for Exercise 38.