Which statement defines standard enthalpy of formation?

• A. ΔHf° is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states.
• B. ΔHf° is the enthalpy change when 1 mole of elements is formed from a compound in their standard state.
• C. ΔHf° is the enthalpy change when 1 mole of a compound is decomposed into elements in standard states.
• D. ΔHf° is the enthalpy change for any reaction at standard state.

Answer: (A)

Standard enthalpy of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states (the elements in their most stable form at 1 atm and 25°C). This convention sets the enthalpy of formation for elements in their standard states to zero, and the values for compounds can be positive or negative depending on whether energy is absorbed or released to form the compound. The described option matches this precise formation process. The other statements describe decomposition, formation of elements from a compound, or apply to any reaction, which are not what standard enthalpy of formation specifies.