The standard enthalpy of combustion is the enthalpy change when
- A. one mole of a substance is burnt completely in excess oxygen under standard conditions
- B. one gram of a substance is burnt
- C. one mole of a substance is formed from its elements
- D. one mole of a substance is dissolved in water
Explanation
The definition specifies one mole, complete combustion and excess oxygen, and combustion values are always negative because burning releases energy. Incomplete combustion would give a different and smaller value, which is why excess oxygen is stipulated. Formation from elements is a separate quantity that may be positive or negative.