Heat of Formation Worked Problem

Heat of Formation Review

You may wish to review the Laws of Thermochemistry and Endothermic and Exothermic Reactions before you begin. Tables are available for heats of formation of common compounds and ions in aqueous solution.

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

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