1. LAW OF HESS H.W. 10/10/07 C A B B C B A C A ∆H1= +30kJ ∆H2= +60kJ
5.60 C A B
∆H1= +30kJ
∆H2= +60kJ B C
∆H2= +60kJ B A C
∆Hreact= +90kJ
∆Hreact= +90kJ
∆H1= +30kJ
B cross cancels across the arrow.
To do so, the compound and phase must be the same. A

2. MULTIPLY BY 3/2 TO CANCELL THE O2
5.62
2H2
O2(g) 
∆H1= kJ +
2H2O
∆H2= kJ
3 O2 (g)
2O3 
REVERSE THIS TO GET O3 AS A REACTANT, THEN MULTIPLY BY 1/2 TO CANCELL THE O2, RECIPROCATE SIGN OF ∆H WHEN YOU REVERSE A REACTION
MULTIPLY BY 3/2 TO CANCELL THE O2
3H2
3/2 O2(g) + 
3H2O
∆H1=3/2 ( kJ) O3 
3/2 O2 (g)
∆H2=1/2 ( kJ)
3H2 O3  +
3H2O
∆H= ( kJ)

3. MULTIPLY BY 3/2 TO CANCELL THE O2
5.62
2H2
O2(g) 
∆H1= kJ +
2H2O
∆H2= kJ
3 O2 (g)
2O3 
REVERSE THIS TO GET O3 AS A REACTANT, THEN MULTIPLY BY 1/2 TO CANCELL THE O2, RECIPROCATE SIGN OF ∆H WHEN YOU REVERSE A REACTION
MULTIPLY BY 3/2 TO CANCELL THE O2
3H2
3/2 O2(g) + 
3H2O
∆H1=3/2 ( kJ) O3 
3/2 O2 (g)
∆H2=1/2 ( kJ)
3H2 O3  +
3H2O
∆H= ( kJ)