It is the heat energy given out (enthalpy change) when one mole of a substance burns completely in oxygen under standard conditions. Enthalpy of combustion = Enthalpy of products - Enthalpy of reactants. As this is an exothermic reaction, the enthalpy of combustion will be negative. Hope this helps 👍
The enthalpy change when one mole of a compound is burned completely in oxygen with all products and reactants in standard states under standard conditions.
That’s the definition That will get you the 3 marks in the exam