The standard enthalpy of formation is defined as the change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions.
The enthalpy of combustion of a substance is defined as the heat energy given out when one mole of a substance burns completely in oxygen.
The formation reaction of methane is
C(graphite)+2H2(g)→CH4(g)
The heat of combustion data for C(graphite),H2(g)andCH4(g)
ΔHfo=∑ΔHCombustion(reactants)−∑ΔHCombustion(products)ΔHfo(CH4)=ΔHCo(C(graphite))+2×ΔHCo(H2(g))−ΔHCo(CH4(g))
ΔHfo(CH4)=−393.5−2×285.8+890.3
ΔfHo of CH4(g)=−74.8kJ/mol