How is enthalpy defined in thermodynamics?

Enthalpy is fundamentally defined as the internal energy of a system plus the product of its pressure and volume. This definition captures not only the energy contained within a substance but also accounts for the work done by the system when it expands or contracts against external pressure.

In thermodynamics, enthalpy is a crucial property, especially in processes occurring at constant pressure, such as in heating or phase changes. When considering reactions or processes in open systems, where pressure is often constant, the change in enthalpy provides valuable insight into the heat transfer associated with these processes.

Internal energy represents the total energy of the molecule's motion and interactions, but it does not account for volume work done when the substance changes its state. The addition of the product of pressure and volume extends the concept of energy to include the work done during the expansion or compression of the substance. Thus, when we combine these two components, we arrive at the comprehensive measure of enthalpy.

This understanding is essential for various applications in engineering and physical sciences, where energy balances and efficiency evaluations are critical for designing and operating thermal systems.