Heat of Formation Worked Example Problem

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. Review The symbol for the standard heat of formation (also known as the standard enthalpy of formation) is ΔHf or ΔHf ° where: Δ indicates a change H indicates enthalpy, which is only measured as a change, not as an instantaneous value  ° indicates thermal energy (heat or temperature) f means formed or that a compound is being formed from its component elements 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. Problem 1 Calculate ΔH for the following reaction: 8 Al(s) 3 Fe3O4(s) → 4 Al2O3(s) 9 Fe(s) 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 ÃŽ £ ΔHf products - ÃŽ £ 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 Problem 2 Calculate ΔH for the ionization of hydrogen bromide: HBr(g) → H(aq) Br-(aq) 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 ÃŽ £ ΔHf  products - ÃŽ £ 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