What is the equation for enthalpy?

• A. H = PV + U
• B. H = PV − U
• C. H = U − PV
• D. H = P + V + U

Answer: (A)

The concept being tested is how enthalpy combines internal energy with the energy needed to make room for the system in its surroundings. Enthalpy is defined as H = U + PV, where U is the internal energy and PV is the pressure–volume work required to create space for the system. This PV term represents flow work or the energy associated with displacing the environment as the system occupies volume.

This definition also connects to practical heat transfer: for processes at constant pressure, the heat added equals the enthalpy change, ΔH = ΔU + Δ(PV) = δQ_p. The combination U and PV into a single state function H makes it convenient to analyze processes where pressure is constant.

The other forms would mix energy terms incorrectly or imply incompatible units. Subtracting PV or subtracting U would not reflect how energy is stored in and around the system, and simply adding P, V, and U together ignores that PV is already an energy term with the correct units.