What is the difference between standard enthalpy of combustion and standard enthalpy of formation?

1 Answer
Dec 30, 2015

Standard enthalpy change of combustion, or #DeltaH_c^theta#, is the energy released as heat when a substance undergoes complete combustion at standard temperature and pressure . It is typically measured in #"kJ"*"mol"^(-1)#, in which case it may further be regarded as the energy released by 1 mole of the substance.

Standard enthalpy of formation, or #DeltaH_f^theta#, is the change of enthalpy (so it may be +ive or -ive) when exactly 1 mole of a compound is formed, provided the reactants and products are at standard temperature and pressure.


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.