Which statement best defines enthalpy?

Which statement best defines enthalpy?

• A. Total heat/energy of a system
• B. Rate of heat transfer in a system
• C. Entropy change of a system
• D. Pressure–volume work

Answer: (A)

Enthalpy is a measure of the heat content of a system—the energy that can be exchanged as heat under constant pressure. It’s defined as H = U + PV, so it combines the internal energy with the energy tied up in the system’s pressure and volume. This is why, when heat is added at constant pressure, the amount of heat that flows in equals the change in enthalpy (dQ at constant pressure = dH). That makes the statement “total heat/energy of a system” the best fit for what enthalpy represents. The other ideas point to different concepts: rate of heat transfer describes how fast heat moves, entropy change concerns disorder, and pressure–volume work refers to the work done during expansion or compression (which is related to, but not itself, enthalpy).

Enthalpy is a measure of the heat content of a system—the energy that can be exchanged as heat under constant pressure. It’s defined as H = U + PV, so it combines the internal energy with the energy tied up in the system’s pressure and volume. This is why, when heat is added at constant pressure, the amount of heat that flows in equals the change in enthalpy (dQ at constant pressure = dH). That makes the statement “total heat/energy of a system” the best fit for what enthalpy represents. The other ideas point to different concepts: rate of heat transfer describes how fast heat moves, entropy change concerns disorder, and pressure–volume work refers to the work done during expansion or compression (which is related to, but not itself, enthalpy).