Question

Calculate the approximate enthalpy change, ΔHrxn, for the combustion of methane:

Answer 1

We subtract the enthalpies of the products by the enthalpies of the reactants, taking into account their stoichiometric coefficients:
2ΔH(H2O) + ΔH(CO2) - ΔH(CH4) - ΔH(2O2)= 2(928) + 1598 - 1656 - 2(498)= 802 kJ / 1 mol methane

Answer 2

The heat of combustion is obtained by getting the difference between the summation of enthalpies of the products and the enthalpies of the reactants. Hence the equation is enthalpy change= 1598 kJ+2*928 kJ-1656 kJ-498 kJ. The answer is 1300 kJ.