Heat absorbed or released at constant pressure is associated with which thermodynamic quantity?

• A. Enthalpy change (ΔH)
• B. Internal energy change (ΔU)
• C. Gibbs free energy change (ΔG)
• D. Entropy change (ΔS)

Answer: (A)

Heat transferred at constant pressure is given by the enthalpy change. Enthalpy is defined as H = U + PV, so for a process at constant pressure (ΔP = 0),

ΔH = ΔU + Δ(PV) = ΔU + PΔV.

From the first law, q = ΔU + w, and for a PV work process w = PΔV at constant pressure. Substituting gives q_p = ΔU + PΔV = ΔH. So the heat absorbed or released when the pressure is held constant equals the enthalpy change.

The internal energy change, ΔU, is the heat at constant volume (no PV work) or the energy change overall, but not the heat at constant pressure when PV work is occurring. Gibbs free energy, ΔG, relates to the maximum useful work under constant temperature and pressure, via ΔG = ΔH − TΔS, not the direct heat transfer. Entropy change, ΔS, ties to heat for a reversible path through q_rev = TΔS, but it is not the amount of heat transferred itself. Therefore, the quantity that directly corresponds to heat absorbed or released at constant pressure is the enthalpy change.