          # Enthalpy

• enthalpy i/ ( listen), a property of a thermodynamic system, is equal to the system's internal energy plus the product of its pressure and volume. in a system enclosed so as to prevent mass transfer, for processes at constant pressure, the heat absorbed or released equals the change in enthalpy.

the unit of measurement for enthalpy in the international system of units (si) is the joule. other historical conventional units still in use include the british thermal unit (btu) and the calorie.

enthalpy comprises a system's internal energy, which is the energy required to create the system, plus the amount of work required to make room for it by displacing its environment and establishing its volume and pressure.

enthalpy is a state function that depends only on the prevailing equilibrium state identified by the system's internal energy, pressure, and volume. it is an extensive quantity.

change in enthalpy (Δh) is the preferred expression of system energy change in many chemical, biological, and physical measurements at constant pressure, because it simplifies the description of energy transfer. in a system enclosed so as to prevent matter transfer, at constant pressure, the enthalpy change equals the energy transferred from the environment through heat transfer or work other than expansion work.

the total enthalpy, h, of a system cannot be measured directly. the same situation exists in classical mechanics: only a change or difference in energy carries physical meaning. enthalpy itself is a thermodynamic potential, so in order to measure the enthalpy of a system, we must refer to a defined reference point; therefore what we measure is the change in enthalpy, Δh. the Δh is a positive change in endothermic reactions, and negative in heat-releasing exothermic processes.

for processes under constant pressure, Δh is equal to the change in the internal energy of the system, plus the pressure-volume work p Δv done by the system on its surroundings (which is positive for an expansion and negative for a contraction). this means that the change in enthalpy under such conditions is the heat absorbed or released by the system through a chemical reaction or by external heat transfer. enthalpies for chemical substances at constant pressure usually refer to standard state: most commonly 1 bar (100 kpa) pressure. standard state does not, strictly speaking, specify a temperature (see standard state), but expressions for enthalpy generally reference the standard heat of formation at 25 °c (298 k).

the enthalpy of an ideal gas is a function of temperature only, so does not depend on pressure. real materials at common temperatures and pressures usually closely approximate this behavior, which greatly simplifies enthalpy calculation and use in practical designs and analyses.

• history
• formal definition
• other expressions
• physical interpretation
• relationship to heat
• applications
• diagrams
## Not to be confused with Entropy. "Heat content" redirects here. For the term in oceanography, see Ocean heat content. Thermodynamics The classical Carnot heat engine Branches Classical Statistical Chemical Quantum thermodynamics Equilibrium / Non-equilibrium Laws Zeroth First Second Third Systems State Equation of state Ideal gas Real gas State of matter Equilibrium Control volume Instruments Processes Isobaric Isochoric Isothermal Adiabatic Isentropic Isenthalpic Quasistatic Polytropic Free expansion Reversibility Irreversibility Endoreversibility Cycles Heat engines Heat pumps Thermal efficiency System propertiesNote: Conjugate variables in italics Property diagrams Intensive and extensive properties Process functions Work Heat Functions of state Temperature / Entropy (introduction) Pressure / Volume Chemical potential / Particle number Vapor quality Reduced properties Material properties Property databases Specific heat capacity  $c=$ $T$ $\partial S$ $N$ $\partial T$ Compressibility  $\beta =-$ $1$ $\partial V$ $V$ $\partial p$ Thermal expansion  $\alpha =$ $1$ $\partial V$ $V$ $\partial T$ Equations Carnot's theorem Clausius theorem Fundamental relation Ideal gas law Maxwell relations Onsager reciprocal relations Bridgman's equations Table of thermodynamic equations Potentials Free energy Free entropy Internal energy$U(S,V)$ Enthalpy$H(S,p)=U+pV$ Helmholtz free energy$A(T,V)=U-TS$ Gibbs free energy$G(T,p)=H-TS$ HistoryCulture History General Entropy Gas laws "Perpetual motion" machines Philosophy Entropy and time Entropy and life Brownian ratchet Maxwell's demon Heat death paradox Loschmidt's paradox Synergetics Theories Caloric theory Theory of heat ("living force") Mechanical equivalent of heat Motive power Key publications "An Experimental EnquiryConcerning ... 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Enthalpy comprises a system's internal energy, which is the energy required to create the system, plus the amount of work required to make room for it by displacing its environment and establishing its volume and pressure. Enthalpy is a state function that depends only on the prevailing equilibrium state identified by the system's internal energy, pressure, and volume. It is an extensive quantity. Change in enthalpy (ΔH) is the preferred expression of system energy change in many chemical, biological, and physical measurements at constant pressure, because it simplifies the description of energy transfer. In a system enclosed so as to prevent matter transfer, at constant pressure, the enthalpy change equals the energy transferred from the environment through heat transfer or work other than expansion work. The total enthalpy, H, of a system cannot be measured directly. The same situation exists in classical mechanics: only a change or difference in energy carries physical meaning. Enthalpy itself is a thermodynamic potential, so in order to measure the enthalpy of a system, we must refer to a defined reference point; therefore what we measure is the change in enthalpy, ΔH. The ΔH is a positive change in endothermic reactions, and negative in heat-releasing exothermic processes. For processes under constant pressure, ΔH is equal to the change in the internal energy of the system, plus the pressure-volume work p ΔV done by the system on its surroundings (which is positive for an expansion and negative for a contraction). This means that the change in enthalpy under such conditions is the heat absorbed or released by the system through a chemical reaction or by external heat transfer. Enthalpies for chemical substances at constant pressure usually refer to standard state: most commonly 1 bar (100 kPa) pressure. Standard state does not, strictly speaking, specify a temperature (see standard state), but expressions for enthalpy generally reference the standard heat of formation at 25 °C (298 K). The enthalpy of an ideal gas is a function of temperature only, so does not depend on pressure. Real materials at common temperatures and pressures usually closely approximate this behavior, which greatly simplifies enthalpy calculation and use in practical designs and analyses. Contents 1 History 2 Formal definition 3 Other expressions 3.1 Cardinal functions 4 Physical interpretation 5 Relationship to heat 6 Applications 6.1 Heat of reaction 6.2 Specific enthalpy 6.3 Enthalpy changes 6.4 Open systems 7 Diagrams 7.1 Some basic applications 7.2 Throttling 7.3 Compressors 8 See also 9 Notes 10 References 11 Bibliography 12 External links  