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# Heat of Formation Worked Problem

## Example Problems for Heat of Formation

Heat of formation is a measure of energy of a reaction.

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These are worked example problems calculating the heat of formation.

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 ΔHf products - Σ ΔHf reactants

Omitting terms for the elements, the equation becomes:

ΔH = 4 ΔHf Al2O3(s) - 3 ΔHf Fe3O4(s)

The values for ΔHf 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

ΔH = -3316.5 kJ

Anne Marie Helmenstine, Ph.D.