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Enthalpy is a thermodynamic property of a system. It is defined as the sum of the internal energy of the system plus the product of its pressure and volume. Mathematically, enthalpy (H) is expressed as H = U + PV, where U is internal energy, P is pressure, and V is volume.

Enthalpy is a state function, meaning its value depends only on the current state of the system, not on the path taken to reach that state. In many chemical and physical processes, it is more convenient to work with enthalpy changes rather than internal energy changes. The change in enthalpy (ΔH) for a process at constant pressure is equal to the heat absorbed or released by the system. This is often referred to as the heat of reaction.

If ΔH is negative, the process is exothermic, meaning it releases heat into the surroundings. If ΔH is positive, the process is endothermic, meaning it absorbs heat from the surroundings. Enthalpy is commonly measured in units of joules (J) or kilojoules (kJ). It plays a crucial role in understanding energy transformations in various fields, including chemistry, physics, and engineering, particularly in the study of chemical reactions, phase transitions, and heat transfer.