Heat of combustion is also called the enthalpy of combustion because the energy evolved from the combustion reaction results from the change in the overall enthalpy of the starting substance as it reacts completely with oxygen. The terms heat of combustion and enthalpy of combustion are used interchangeably due to the First law of thermodynamics and the relationships between heat at constant pressure ("q"), the change in internal energy (ΔU), and the change in enthalpy (ΔH). 