Enthalpy is a thermodynamic state function defined as internal energy plus the product of pressure and volume. It is conventionally represented by the symbol H. The quantity is especially useful for processes occurring at constant pressure because under suitable conditions the change in enthalpy corresponds to the heat transferred to or from a closed system when only pressure volume work is involved. Enthalpy is an extensive property, so its numerical value depends on the amount of substance present, while specific and molar enthalpies provide corresponding intensive forms. It is widely used in chemistry, engineering and physical science because many practical processes occur approximately at atmospheric or controlled pressure.
Enthalpy combines internal energy with a pressure volume term because the energy balance for systems undergoing expansion or compression at constant external pressure naturally involves both quantities. Internal energy represents the energy contained within the system, while the pressure volume term accounts for the energy associated with maintaining space against surrounding pressure. For a closed system undergoing only pressure volume work at constant pressure, the first law leads to a relationship in which the enthalpy change equals heat transfer at constant pressure. This makes enthalpy particularly convenient for chemical reactions, phase changes and many laboratory processes.
Enthalpy changes are used extensively in chemical thermodynamics. Standard enthalpy of formation represents the enthalpy change associated with forming one mole of a substance from its constituent elements in their standard states. Enthalpy of combustion describes heat released when a substance undergoes complete combustion under specified conditions. Enthalpy of fusion, vaporisation and sublimation describe enthalpy changes associated with phase transitions. These quantities can be tabulated and combined using Hess law because enthalpy is a state function. Consequently the enthalpy change for a reaction can be calculated from other reactions whose enthalpy changes are known.
Enthalpy is also central to phase equilibrium and thermodynamic property calculations. During melting, boiling or sublimation, energy is required to alter intermolecular arrangements and this energy appears as a change in enthalpy while temperature can remain constant during an equilibrium phase transition at fixed pressure. In engineering, tables and equations of state provide enthalpy values for water, steam, refrigerants and other working fluids. Engineers use enthalpy differences to calculate energy transfers in turbines, compressors, pumps, boilers, condensers and heat exchangers.
Flow systems make extensive use of Enthalpy because the steady flow energy equation naturally incorporates the combination of internal energy and flow work represented by enthalpy. When a fluid enters or leaves a control volume it must displace surrounding material, and the pressure volume contribution is therefore conveniently included in the enthalpy term. This simplifies energy balances for turbines, compressors, nozzles and heat exchangers. For many steady flow devices kinetic and potential energy changes can also be included, allowing enthalpy differences to be related directly to heat transfer and shaft work under specified assumptions.
Enthalpy depends on the chosen reference state because absolute values are often defined relative to an arbitrary reference, whereas physically meaningful calculations generally depend on differences in enthalpy. Thermodynamic property tables therefore specify reference conventions. For ideal gases, enthalpy is primarily a function of temperature because internal energy depends only on temperature under the ideal gas model. For real gases and condensed phases, pressure and other variables can also affect enthalpy. Heat capacity at constant pressure provides the relationship between enthalpy changes and temperature changes under appropriate conditions.
Enthalpy is often incorrectly described as simply the amount of heat contained in a substance. This is not correct because heat is energy transferred across a system boundary due to a temperature difference or other thermodynamic driving condition, whereas enthalpy is a state property. A system can possess a defined enthalpy without currently transferring heat. The relationship between heat and enthalpy change depends on process conditions, especially pressure and the types of work involved. Maintaining this distinction is important when interpreting chemical reaction data and engineering energy balances.
Enthalpy has strong connections with Entropy and Free Energy. Enthalpy describes an energy accounting quantity, while entropy describes a fundamental aspect of energy distribution and process direction. Free energy combines enthalpy and entropy with temperature to determine the maximum useful work or spontaneity conditions for particular thermodynamic settings. These relationships allow scientists to distinguish energy conservation from energy availability. A reaction can release substantial enthalpy while still requiring consideration of entropy and temperature before its equilibrium behaviour or useful work potential can be determined.
