### Enthalpy

Posted:

**Sat Jan 20, 2018 8:28 pm**For the enthalpy equation: H = U + PV, if change in enthalpy measures the change in heat without work, why is PV added to the internal energy U?

Thank you!

Thank you!

Created by Dr. Laurence Lavelle

https://lavelle.chem.ucla.edu/forum/

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=75&t=25940

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Posted: **Sat Jan 20, 2018 8:28 pm**

For the enthalpy equation: H = U + PV, if change in enthalpy measures the change in heat without work, why is PV added to the internal energy U?

Thank you!

Thank you!

Posted: **Sat Jan 20, 2018 9:31 pm**

The change in enthalpy isn't the change in heat without work, it refers to the transfer of heat at a constant pressure, which implies that work is being done. Enthalpy is the total energy content of a system so the change in enthalpy is equal to the change in internal energy due to transfer of heat plus work (PV) because work is being done on the system so that it stays at the constant pressure. Remember that U = H - PV and PV is negative because work is being done by the system. That equation is also just another form of your equation.

Posted: **Sat Jan 20, 2018 9:56 pm**

PV is also negative because as work is being done by the system, it is losing energy.