Question

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

Answer 1

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.

Explanation:

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.