The standard heat (enthalpy) of formation of a compound is defined as the heat associated with the formation of one mole of the compound from its elements in their standard states (25oC and 1 atm). Example:

H2(g) + ½O2(g) → H2O(l)       ΔH = -286 kJ

All elements in their standard states (nitrogen gas, silver metal, solid carbon in the form of graphite, etc.) have a standard enthalpy of formation of zero, as there is no change involved in their formation.

ΔH for a reaction is equal to the sum of the heats of formation of the products compounds minus the sum of the heats of formation of the reactant compounds.

ΔHrxn = ∑ ΔHf products - ∑ ΔHf reactants

Example: Calculate the enthalpy of the reaction:

CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)

 Substance Hf (kJ/mol) CH4(g) -75 O2(g) 0 CO2(g) -394 H2O(l) -286

ΔHrxn = [HCO2 + 2(HH2O)] - HCH4

= [-394 + 2(-286)] - [-75]

= -891 kJ

Your turn: Calculate the ΔH for the reaction

Cu2S(s) + S(s) → 2CuS(s)

 Substance Hf (kJ/mol) Cu2S -79.5 CuS -53.1
Interactive popup. Assistance may be required.

ΔHrxn = 2(HCuS) – [HCu2S + 2(HS)]
= 2(-53.1) – [-79.5 + 2(0)]
= -26.7 kJ