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Solutions

1. Calculate the DHorxn from the DHof  for the following reaction:

a)    PbO (s)   +   CO (g)     ------->    Pb (s)     +  CO2 (g)         DHorxn = -65.7 kJ

b)   SiH4 (g)   +  2 O2 (g)    ------->    SiO2 (s)   +  2 H2O (l)     DHorxn = -1516.8 kJ

c) 2 FeS2 (s)  +  5.5 O2 (g)    ------->  Fe2O3 (s)  +  4 SO2 (g)   DHorxn = -1656.4 kJ
 

2. Determine the DHrxn  using Hess' Law

a)  Determine the enthalpy of formation for MnO2 (s)

        Mn (s)  +   O2 (g)   -------->   MnO2 (s)

Given:

4 Al (s)   +  3 O2 (g)  ------->  2 Al2O3 (s)                                  DHrxn  = -3352 kJ/mol
 

4 Al (s)  +   3 MnO2 (s)  ------->    3 Mn (s)   +   2 Al2O3 (s)       DHrxn = -1937 kJ/mol

DHf = -471.7 kJ/mol
 

b)  Determine the heat of hydrogenation for

        C3H4 (g)  +  2 H2 (g)   -------->   C3H8 (g)

Given:

2 H2 (g)   +   O2 (g)  ------->  2 H2O (l)                                      DHrxn  = -571.6 kJ/mol
 

C3H4 (g)  +   4 O2 (g)  ------->    3 CO2 (g)   +   2 H2O (l)          DHrxn = -1937 kJ/mol
 

C3H8 (g)  +   5 O2 (g)  ------->    3 CO2 (g)   +   4 H2O (l)          DHrxn = -2220 kJ/mol

DHrxn = -288.6 kJ/mol