##### Solutions

1.

2H2(g) + O2(g) ® 2 H2O(g) + 2(242 kJ)              DH = -484 kJ/mol

C(s) + O2(g) ® CO2(g)                                        DH = -394 kJ/mol

CH4(g) + 75 kJ® C(s) + 2 H2(g)                          DH = 75 kJ/mol

Sum:

CH4(g) + 2 O2(g) ® CO2(g) + 2 H2O(g)                     DH = -803 kJ/mole with sig figs: -8.0 X 102 kJ/mole

2.         a.                                  ®                        DH= + 74.8 kJ

®                                   DH= -110.5 kJ

Sum:     + ®        + 2             DH = -35.7 kJ

b.         Calculate DH  for

®                      DH = 103.8 kJ

3C(s) + 3 O2(g) ®          3 CO2(g)                       DH = -393.5(3) kJ

4H2(g) + 2 O2(g) -->4 H2O(g)                                        DH = -242(4) kJ

Sum :  ®3 DH = -2045 kJ with sig figs: -2.05 X 103 kJ/mole

3.                     12 NH3(g)  +     15O2(g)            ®        12NO(g)           + 18 H2O(l)      DH = -293(12) kJ

12NO2(g)          + 4H2O(l)          ®        8 HNO3(aq)       +  4 NO(g)        DH = -45(12) kJ

12NO(g)           + 6 O2(g)  + 59 kJ         ®        12 NO2(g)                     DH = + 59(12) kJ

Sum : 12 NH3(g) + 21 O2(g)  ®  8 HNO3(aq)       +          14 H2O(l)          + 4 NO(g)         DH = -3348 kJ with sig figs: -3.3 X 103 kJ/mole

4.                     2NO (g) +          2*93 kJ            ®        N2(g)     +          O2(g)                 DH = 186 kJ

N2(g)       +          2O2(g)               ®        2 NO2 (g)                                              DH = -34(2) kJ

Sum:     2 NO(g)            + O2(g)             ®        2 NO2(g)                      DH = 118kJ with sig figs: 1.2 X 102 kJ/mole

b.         If you had used the data from #3, could you have gotten the same answer in one step? How?

Simply double the 3rd equation from that example: NO(g)           + 0.5 O2(g)       + 59 kJ®        NO2(g)

5.

 Reaction DH  ( kJ )

 2 B(s) + 1.5 O2(g)  ® B2O3(s) -1273 3H2(g) + 1.5 O2(g)  ®    3H2O(l) -286(3) B2O3(s) + 3 H2O(g) ® B2H6(l) + 3 O2(g) +2035 3H2O(l) ) ® 3H2O(g) 44 (3)

Sum:                 2 B(s) + 3H2(g)  ®        B2H6(l)                                   DH  = 36 kJ