What is the standard enthalpy change ΔH° defined as?

• A. The enthalpy change when all reactants and products are in their standard states.
• B. The enthalpy change when one mole of water is formed.
• C. The enthalpy change when a solution is formed.
• D. The enthalpy change when a gas is formed.

Answer: (A)

Standard enthalpy change ΔH° is the enthalpy change for a reaction measured under standard conditions, with all reactants and products in their standard states (the most stable form at 1 bar and 298 K). Because enthalpy is a state function, this value depends only on the initial and final states, not on the path taken. So it describes the energy change when the balanced reaction, with substances in their standard states, proceeds at 298 K and 1 bar. It’s a general property of the reaction as written, not tied to forming a specific substance, a solution, or a gas.