What is the standard enthalpy change of formation defined as?

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

Answer: (A)

The standard enthalpy change of formation is about making exactly one mole of a compound from its elements in their standard states, at standard conditions (usually 298 K and 1 atm). This quantity, ΔHf°, tells you the heat absorbed or released when you assemble the compound from its most stable elemental forms at those defined conditions. It’s the reference point used to calculate the enthalpy changes in many reactions via Hess’s law. The idea isn’t about burning in oxygen or about dissolving or any other process; it specifically describes formation from elements in their standard states, which is why this option is the best choice.