Enthalpy is defined as?

• A. The sum of kinetic energy and potential energy of a system.
• B. The energy content per unit mass.
• C. The energy required to raise the temperature of a system by one degree.
• D. Total heat content, H = E + pv.

Answer: (D)

Enthalpy represents the energy content of a system including the energy needed to make room for it in the surroundings. It combines the internal energy with the pressure-volume term, defined as H = E + pV (often written U + pV). This formulation is especially useful for processes at constant pressure, where the heat added equals the change in enthalpy because the system not only increases its internal energy but also does work to displace its surroundings.

The best answer reflects this definition by describing enthalpy as the total heat content, H = E + pV. The other ideas describe different concepts: kinetic plus potential energy is mechanical energy, not enthalpy; energy per unit mass is specific energy; and energy required to raise temperature by one degree is related to specific heat.