Question

Internal Energy (U) of a substance is defined as the sum of all categories of energy, i.e., Kinetic (temperature), potential, bonding, etc. Then, Enthalpy is defined as: H=U+PV. But, it seems to me thatPV (pressure x volume) is a form of potential energy, even thoughUUpresumably already includes potential energy. Why does it look as though potential energy is being included twice? But, it seems to me that PV (pressure x volume) is a form of potential energy, even though U presumably already includes potential energy. Why does it look as though potential energy is being included twice?

Answer 1

Enthalpy includes internal energy, which consists of the kinetic and potential energies within a system, and an additional term PV, which represents the work related to volume change against external pressure.

Step-by-step explanation:

The question about enthalpy and internal energy arises from a common confusion in thermodynamics. The internal energy (U) of a system does comprise the kinetic and potential energies of its atoms and molecules, including energy due to intermolecular forces.

However, the term PV in the enthalpy equation H = U + PV refers to the work done against the external pressure when a system expands or the work that must be done to compress the surroundings when the system is compressed at a constant external pressure. Therefore, PV is a separate term representing the work related to a volume change in a system, not the potential energy within the substance itself.