Enthalpy of Combustion and Formation

These are 2 very important terms that describe the enthalpy change when a substance is formed (formation) and the enthalpy change when a substance is burnt (combustion)

In layman’s terms, the enthalpy of combustion is the energy change when some stuff is burned. The enthalpy of formation the heat content change when some stuff is formed. Unfortunately life is not that simple!

These two terms are more correctly defined as follows. These  definitions should be learnt like a parrot!

•  The standard enthalpy of combustion DH_c^q is the heat content change which occurs when one mole of a substance is completely burned in oxygen, under standard conditions.

• The standard enthalpy of formation DH_f^q is the heat content change when one mole of a compound is formed from its elements in their standard states, under standard conditions.