Heat of formation refers to the enthalpy change when a pure substance is formed from its elements under constant pressure?

• A. At constant volume
• B. At high temperature
• C. At standard state
• D. At constant pressure

Answer: (D)

Heat of formation is an enthalpy change. Enthalpy, H, is the heat content of a system at constant pressure, and at constant pressure the heat added to or released from the system equals the change in enthalpy (q_p = ΔH). The heat of formation specifically refers to forming one mole of a compound from its elements in their standard states, and that enthalpy change is measured under those standard conditions at constant pressure. So the defining feature here is the constant-pressure aspect tied to enthalpy changes. The other ideas don’t capture this relationship: constant volume would reflect internal energy change (not enthalpy), high temperature isn’t part of the definition, and while standard states are part of the broader definition, the statement’s emphasis on constant pressure is what identifies the heat of formation concept.