At constant pressure, the change in enthalpy observed when reactants are converted into products is called which term?

• A. Heat of Reaction
• B. Enthalpy Change
• C. Reaction Enthalpy
• D. Gibbs Free Energy Change

Answer: (C)

At constant pressure, the heat exchanged with the surroundings equals the enthalpy change of the reaction. The energy change associated with turning reactants into products is called the reaction enthalpy, written as ΔHrxn. It is defined as the enthalpy of the products minus the enthalpy of the reactants. Since the pressure is held constant, qp = ΔH, so the observed heat flow directly reflects the enthalpy change of the reaction. This term is the most specific way to describe the energy change tied to the chemical transformation. Terms like heat of reaction describe the same quantity in heat-transfer terms, but the standard label for the energy change of the reaction itself is reaction enthalpy. Enthalpy change is a broader phrase for any process, and Gibbs free energy change would refer to a different thermodynamic potential related to spontaneity, not the enthalpy change.