What is the standard enthalpy change of combustion defined as?

• A. The enthalpy change when one mole of water is produced.
• B. The enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions.
• C. The enthalpy change when a solution forms.
• D. The enthalpy change when one mole of a compound is formed from elements.

Answer: (B)

The standard enthalpy change of combustion is the energy change that occurs when one mole of a substance burns completely in excess oxygen under standard conditions (298 K and 1 atm). This definition ensures the reaction goes to full oxidation and is measured under consistent conditions so values can be compared. Because combustion releases heat, the enthalpy change is typically negative. For fuels, the products are usually carbon dioxide and water, reflecting complete combustion. Other descriptions describe enthalpy changes for completely different processes: forming water, forming a solution, or forming a compound from elements—these are enthalpy changes of formation or solution, not combustion.