Standard enthalpy change of combustion: the definition and the sign
Dr Lavan Ganeshkumar, PhD · updated 4 October 2026
The standard enthalpy change of combustion is the enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, with all reactants and products in their standard states.
Words that must appear
- One mole of a substance
- Completely burned
- In oxygen
The sign
Always exothermic, so negative.
Do not confuse with
Enthalpy of formation, which builds a compound from its elements.
Common questions
What is the standard enthalpy change of combustion?
The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions, with all reactants and products in their standard states.
Is the enthalpy of combustion positive or negative?
Negative. Combustion is exothermic.
Why is an experimental enthalpy of combustion usually less exothermic than the data-book value?
Heat is lost to the surroundings, combustion is incomplete, and the fuel or water evaporates.
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