Thermodynamics WS 6
Hess’s Law Version 1
1.
Calculate the heat of reaction, in kJ/mol, for the reaction
SO2(g) + H2(g) + O2(g)  H2SO4(l)
given the following information:
S(s) + O2(g)  SO2(g)
H2(g) + S(s) + 2 O2(g)  H2SO4(l)
H = -297.1 kJ/mol
H = -811.7 kJ/mol
Is the reaction exothermic or endothermic? (A: -514.6 kJ)
2.
Calculate the heat of reaction, in kJ/mol C3H8, for the reaction
C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g)
given the following information:
3 C(s) + 4 H2(g)  C3H8(g)
C(s) + O2(g)  CO2(g)
H2(g) + ½ O2(g)  H2O(g)
H = -103.8 kJ/mol C3H8
H = -393.3 kJ/mol
H = -241.8 kJ/mol
Is the reaction exothermic or endothermic? (A: -2043.4 kJ)
3.
Calculate the heat of reaction, in kJ/mol NH3 for the reaction
4 NH3(g) + 5 O2(g)  4 NO(g) + 6 H2O(g)
given the following information
½ N2(g) + ½ O2(g)  NO(g)
H = +90.4 kJ/mol
½ N2(g) + 3/2 H2(g)  NH3(g) H = -46.0 kJ/mol
H2(g) + ½ O2(g)  H2O(g)
H = -241.8 kJ/mol
Is the reaction exothermic or endothermic? (A: -905 kJ)
From D. C. Heath & Company
-1-
Thermodynamics WS 6
Hess’s Law Version 2
4. Using the table below, calculate Hrxn for each of the following reactions. Assume a temperature of 25oC.
a. H2O(l)  H2O(g)
(A: 44.012 kJ)
b. Cu2S + S  2 CuS
(A: -26.7 kJ)
c. 2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(l)
Substance
C2H6(g)
CO2(g)
Cu2S(s)
CuS(s)
H2O(l)
H2O(g)
O2(g)
S8(s)
Hfo (kJ/mol)
-84.68
-393.509
-79.5
-53.1
-285.830
-241.818
0
0
-2-
(A: -3119.6 kJ)