Heat of Formation Worked Example Problem

1. Heat of Formation Problem

Calculate ΔH for the following reaction:

8 Al(s) + 3 Fe₃O₄(s) --> 4 Al₂O₃(s) + 9 Fe(s)

Heat of Formation Solution

ΔH for a reaction is equal to the sum of the heats of formation of the product compounds minus the sum of the heats of formation of the reactant compounds:

ΔH = Σ ΔH_f products - Σ ΔH_f reactants

Omitting terms for the elements, the equation becomes:

ΔH = 4 ΔH_f Al₂O₃(s) - 3 ΔH_f Fe₃O₄(s)

The values for ΔH_f may be found in the Heats of Formation of Compounds table. Plugging in these numbers:

ΔH = 4(-1669.8 kJ) - 3(-1120.9 kJ)

ΔH = -3316.5 kJ

Answer

ΔH = -3316.5 kJ

2. Heat of Formation Problem

Calculate ΔH for the ionization of hydrogen bromide:

HBr(g) --> H⁺(aq) + Br^-(aq)

Heat of Formation Solution

ΔH for a reaction is equal to the sum of the heats of formation of the product compounds minus the sum of the heats of formation of the reactant compounds:

ΔH = Σ ΔH_f products - Σ ΔH_f reactants

Remember, the heat of formation of H⁺ is zero. The equation becomes:

ΔH = ΔH_f Br^-(aq) - ΔH_f HBr(g)

The values for ΔH_f may be found in the Heats of Formation of Compounds of Ions table. Plugging in these numbers:

ΔH = -120.9 kJ - (-36.2 kJ)

ΔH = -84.7 kJ

Answer

ΔH = -84.7 kJ