# Isenthalpe change of state

In thermodynamics , a change of state is referred to as isenthalpic where the enthalpy does not change: ${\ displaystyle H}$ {\ displaystyle {\ begin {aligned} H & = {\ text {const.}} \\\ Leftrightarrow \ mathrm {d} H & = 0 \\\ Leftrightarrow \ delta U + V \ cdot \ mathrm {d} p & = 0 \\\ Leftrightarrow \ delta U & = - V \ cdot \ mathrm {d} p \ end {aligned}}} With

• ${\ displaystyle U}$ : internal energy
• ${\ displaystyle \ delta}$ : incomplete differential
• ${\ displaystyle V}$ : Volume
• ${\ displaystyle p}$ : Pressure
• ${\ displaystyle V \ cdot \ mathrm {d} p}$ : Main part of the technical work .

An example of an isenthalpic process is the expansion of a gas through a throttle valve , see Joule-Thomson effect .

According to the equation of state of an ideal gas, the following applies

{\ displaystyle {\ begin {aligned} p \ cdot V & = {\ text {const.}} \\\ Leftrightarrow T & = {\ text {const.}} \ end {aligned}}} For ideal gases, the isenthalphic processes are precisely the isotherms .

An isothermal reversible process is always isenthalpic, but the reverse is not true.

The lines of equal enthalpy in the state diagram are also called isenthalps (or throttle curves).