What is the correct definition of enthalpy?

Enthalpy is defined as the sum of a system's internal energy and the product of its pressure and volume. This definition is significant in thermodynamics because it provides a useful parameter for measuring heat transfer at constant pressure, which is a common condition in many chemical reactions and physical processes. In equations, enthalpy (H) is often expressed as H = U + PV, where U is the internal energy, P is pressure, and V is volume.

This concept is essential for understanding how energy changes occur in chemical reactions, as it allows scientists to quantify the amount of heat absorbed or released during these reactions. This understanding plays a crucial role in fields such as chemistry and engineering, particularly when designing systems that involve heat exchange.

In contrast, the other options describe different physical concepts: boiling point refers specifically to temperature, the measure of disorder corresponds to entropy, and the energy required to change a phase relates to latent heat. These concepts are important but do not encapsulate the full definition of enthalpy.