Hess’s Law Equation – Sample Problem
Calculate the heat given off when one mole of B5H9 reacts with excess oxygen
according to the following reaction:
Compound
B5H9(g)
B2O3(g)
O2(g)
H2O(g)
Hf (kJ/mol-K)
73.2
-1272.77
0
-241.82
In the reaction above 2 moles of B5H9 react with 12 moles of O2 to yield five moles of
B2O3 and 9 moles of H2O. We find the Hf by subtracting the sum of the enthalpies
of the reactant from the sum of the enthalpies of the products:
o
NOTE: The heat of formation of O2 is zero because this is the form of
the oxygen in its most thermodynamically stable state.