The change in enthalpy is equal to the heat exchanged between a system and its surroundings at constant pressure.

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The change in enthalpy is equal to the heat exchanged between a system and its surroundings at constant pressure.

Enthalpy reflects heat flow when pressure is constant. Since enthalpy H equals U + PV, the change in enthalpy is ΔH = ΔU + Δ(PV) = ΔU + PΔV + VΔP. At constant pressure, ΔP = 0, so ΔH = ΔU + PΔV. From the first law, ΔU = q_p − w, and for only pressure-volume work, w = PΔV. So ΔU = q_p − PΔV. Plugging in gives ΔH = (q_p − PΔV) + PΔV = q_p. Thus the heat exchanged with the surroundings at constant pressure equals the change in enthalpy. At constant volume, no PV work occurs, so heat added changes internal energy, not enthalpy. For an ideal gas, ΔH also equals nCpΔT, but the general statement refers specifically to heat at constant pressure.

Enthalpy reflects heat flow when pressure is constant. Since enthalpy H equals U + PV, the change in enthalpy is ΔH = ΔU + Δ(PV) = ΔU + PΔV + VΔP. At constant pressure, ΔP = 0, so ΔH = ΔU + PΔV. From the first law, ΔU = q_p − w, and for only pressure-volume work, w = PΔV. So ΔU = q_p − PΔV. Plugging in gives ΔH = (q_p − PΔV) + PΔV = q_p. Thus the heat exchanged with the surroundings at constant pressure equals the change in enthalpy.

At constant volume, no PV work occurs, so heat added changes internal energy, not enthalpy. For an ideal gas, ΔH also equals nCpΔT, but the general statement refers specifically to heat at constant pressure.

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