Editing Enthalpy (section)

==Physical interpretation==
The {{mvar|U}} term is the energy of the system, and the {{mvar|pV}} term can be interpreted as the [[work (thermodynamics)|work]] that would be required to "make room" for the system if the pressure of the environment remained constant. When a system, for example, {{mvar|n}} [[Mole (unit)|moles]] of a gas of [[Volume (thermodynamics)|volume]] {{mvar|V}} at [[pressure]] {{mvar|p}} and [[temperature]] {{mvar|T}}, is created or brought to its present state from [[absolute zero]], energy must be supplied equal to its internal energy {{mvar|U}} plus {{mvar|pV}}, where {{mvar|pV}} is the [[Work (physics)|work]] done in pushing against the ambient (atmospheric) pressure.

In [[physics]] and [[statistical mechanics]] it may be more interesting to study the internal properties of a constant-volume system and therefore the internal energy is used.<ref>
{{cite book
 |first=F. |last=Reif
 |year=1967
 |title=Statistical Physics
 |publisher=McGraw-Hill
 |place=London, UK
}}
</ref><ref>
{{cite book
 |first1=C. |last1=Kittel
 |first2=H. |last2=Kroemer
 |year=1980
 |title=Thermal Physics
 |publisher=Freeman
 |location=London, UK
}}
</ref>
In [[chemistry]], experiments are often conducted at constant [[atmospheric pressure]], and the pressure–volume work represents a small, well-defined energy exchange with the atmosphere, so that {{math|Δ''H''}} is the appropriate expression for the [[heat of reaction]]. For a [[heat engine]], the change in its enthalpy after a full cycle is equal to zero, since the final and initial state are equal.