Example Question - 3

Use the standard enthalpies of formation below to calculate the enthalpy change of combustion of propane.C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l)Δ_fH values: C₃H₈(g) = -104 kJ mol^-1, CO₂(g) = -394 kJ mol^-1, H₂O(l) = -286 kJ mol^-1.Using Hess's law, Δ_rH = ΣΔ_fH(products) - ΣΔ_fH(reactants). The enthalpy of formation of O₂ is zero, because it is an element in its standard state.Products: 3(-394) + 4(-286) = -1182 + (-1144) = -2326 kJ mol^-1. Reactants: -104 + 5(0) = -104 kJ mol^-1.Δ_cH = -2326 - (-104) = -2222 kJ mol^-1. Always multiply each value by the number of moles in the balanced equation, and take care with the signs when subtracting.