Editing Enthalpy (section)

{{Short description|Measure of energy in a thermodynamic system}}
{{Distinguish|Entropy}}
{{Infobox physical quantity
| name = Enthalpy
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| unit =joule
| otherunits =
| symbols = ''H''
| baseunits = kg⋅m<sup>2</sup>⋅s<sup>−2</sup>
| dimension =
| extensive = 
| intensive =
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{{Thermodynamics |potentials}}

'''Enthalpy''' ({{IPAc-en|ˈ|ɛ|n|θ|əl|p|i|audio=en-US-enthalpy.ogg}}) is the sum of a [[thermodynamic system]]'s [[internal energy]] and the product of its [[pressure]] and [[volume]].<ref name=GoldBk-enth>{{GoldBookRef |title=enthalpy |file=E02141 }}</ref> It is a [[state function]] in [[thermodynamics]] used in many measurements in chemical, biological, and physical systems at a constant external pressure, which is conveniently provided by the large ambient atmosphere. The pressure–volume term expresses the [[work (physics)|work]] <math>W</math> that was done against constant external pressure <math>P_\text{ext}</math> to establish the system's physical dimensions from <math>V_\text{system, initial}=0</math> to some final volume <math>V_\text{system, final}</math> (as <math>W=P_\text{ext}\Delta V</math>), i.e. to make room for it by displacing its surroundings.<ref>
{{cite book
 |first=Mark W. |last=Zemansky
 |year=1968
 |title=Heat and Thermodynamics |edition=5th
 |at=chapter&nbsp;11 [https://archive.org/details/heatthermodynami0000zema/page/275 p&nbsp;275]
 |publisher=McGraw-Hill
 |location=New York, NY
 |url=https://archive.org/details/heatthermodynami0000zema
 |url-access=registration
}}
</ref><ref>
{{cite book
 |first1=G.J. |last1=van&nbsp;Wylen
 |first2=R.E. |last2=Sonntag
 |year=1985
 |title=Fundamentals of Classical Thermodynamics |edition=3rd
 |at=section&nbsp;5.5
 |publisher=John Wiley & Sons
 |place=New York, NY
 |isbn=978-0-471-82933-1
}}
</ref>
The pressure-volume term is very small for solids and liquids at common conditions, and fairly small for gases. Therefore, enthalpy is a stand-in for [[energy]] in chemical systems; [[Bond energy|bond]], [[Lattice energy|lattice]], [[solvation]], and other chemical "energies" are actually enthalpy differences. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, not on the path taken to achieve it.

In the [[International System of Units]] (SI), the unit of measurement for enthalpy is the [[joule]]. Other historical conventional units still in use include the [[calorie]] and the [[British thermal unit]] (BTU).

The total enthalpy of a system cannot be measured directly because the internal energy contains components that are unknown, not easily accessible, or are not of interest for the thermodynamic problem at hand. In practice, a change in enthalpy is the preferred expression for measurements at constant pressure, because it simplifies the description of [[energy transfer]]. When transfer of matter into or out of the system is also prevented and no electrical or mechanical (stirring shaft or lift pumping) work is done, at constant pressure the enthalpy change equals the energy exchanged with the environment by [[heat]].

In chemistry, the standard [[enthalpy of reaction]] is the enthalpy change when reactants in their [[standard state]]s ({{nobr|{{math|''p'' {{=}} 1}} [[bar (unit)|bar]];}} usually {{nobr|{{math|''T'' {{=}} 298}} [[Kelvin (unit)|K]]}}) change to products in their standard states.<ref>
{{cite book
 |last1=Atkins        |first1=Peter
 |last2=de&nbsp;Paula |first2=Julio
 |year=2006
 |title=Atkins' Physical Chemistry |edition=8th
 |publisher=W.H.Freeman
 |isbn=0-7167-8759-8
 |page=51
}}
</ref>
This quantity is the [[Standard enthalpy of reaction|standard heat of reaction]] at constant pressure and temperature, but it can be measured by [[Calorimeter|calorimetric]] methods even if the temperature does vary during the measurement, provided that the initial and final pressure and temperature correspond to the standard state. The value does not depend on the path from initial to final state because enthalpy is a [[state function]].

Enthalpies of chemical substances are usually listed for {{convert|1|bar|kPa}} pressure as a standard state. Enthalpies and enthalpy changes for reactions vary as a function of temperature,<ref>
{{cite book
 |first1=Keith J. |last1=Laidler
 |first2=John H.  |last2=Meiser
 |year=1999
 |title=Physical Chemistry |edition=3rd
 |place=Boston, MA
 |publisher=Houghton Mifflin
 |isbn=0-395-91848-0
 |page=66
}}
</ref>
but tables generally list the standard heats of formation of substances at {{cvt|25|°C|K}}. For [[endothermic]] (heat-absorbing) processes, the change {{math|Δ''H''}} is a positive value;  for [[exothermic]] (heat-releasing) processes it is negative.

The enthalpy of an [[ideal gas]] is independent of its pressure or volume, and depends only on its temperature, which correlates to its thermal energy. Real gases at common temperatures and pressures often closely approximate this behavior, which simplifies practical thermodynamic design and analysis.

The word "enthalpy" is derived from the Greek word ''enthalpein'', which means "to heat".<ref>{{Cite book |last=Çengel |first=Yunus A. |title=Thermodynamics: an engineering approach |last2=Boles |first2=Michael A. |last3=Kanoglu |first3=Mehmet |date=2019 |publisher=McGraw-Hill Education |isbn=978-1-259-82267-4 |edition=Ninth |location=New York, NY |page=123}}</ref><ref>{{LSJ|e)nqa/lpw|ἐνθάλπειν}}, {{LSJ|e)n|ἐν}}, {{LSJ|qa/lpw|θάλπειν|ref}}.</ref>