The standard enthalpy change of combustion, ΔHc o, is the change in enthalpy when one mole of a substance, in its most stable form, is burnt in excess oxygen under standard conditions.
Combustion reactions are always exothermic, e.g.
The standard enthalpy change of combustion of iron is also the standard enthalpy change of formation of iron (III) oxide and the standard enthalpy change of reaction between iron and oxygen to give iron (III) oxide.