What is the difference between standard enthalpy of combustion and standard enthalpy of formation?
Standard enthalpy change of combustion, or
Standard enthalpy of formation, or
Combustion is always an exothermic process. Because of this, enthalpy change of combustion must always be positive; on the other hand, enthalpy change of formation can be either positive or negative, since a reaction to form 1 mole of a substance can be either exothermic or endothermic.