Heat of Formation Worked Problem

Learn How To Calculate Heat of Formation

Heat of formation is a measure of energy of a reaction.
Heat of formation is a measure of energy of a reaction. PM Images, Getty Images

Heat of formation is the enthalpy change that occurs when a pure substance forms from its elements under conditions of constant pressure. 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.

Remember, heat of formation will tell you whether heat was absorbed or released and the quantity of heat.

Heat of Formation Problem #1

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

Answer

ΔH = -3316.5 kJ

Heat of Formation Problem #2

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 ΔHf products - Σ ΔHf reactants

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

ΔH = ΔHf Br-(aq) - ΔHf HBr(g)

The values for ΔHf 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 = -120.9 kJ + 36.2 kJ

ΔH = -84.7 kJ

Answer

ΔH = -84.7 kJ