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

MULTIPLY BY 3/2 TO CANCELL THE O2 5.62 2H2 O2(g)  ∆H1=-483.6 kJ + 2H2O ∆H2=+284.6 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 (-483.6 kJ) O3  3/2 O2 (g) ∆H2=1/2 (-284.6 kJ) 3H2 O3  + 3H2O ∆H= (-867.7 kJ)

MULTIPLY BY 3/2 TO CANCELL THE O2 5.62 2H2 O2(g)  ∆H1=-483.6 kJ + 2H2O ∆H2=+284.6 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 (-483.6 kJ) O3  3/2 O2 (g) ∆H2=1/2 (-284.6 kJ) 3H2 O3  + 3H2O ∆H= (-867.7 kJ)