### Enthalpy equation

Posted:

**Sun Jan 21, 2018 4:26 pm**In the equation H= U + PV, where does the PV at the end come from?

Created by Dr. Laurence Lavelle

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

https://lavelle.chem.ucla.edu/forum/viewtopic.php?f=76&t=26000

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Posted: **Sun Jan 21, 2018 4:26 pm**

In the equation H= U + PV, where does the PV at the end come from?

Posted: **Sun Jan 21, 2018 4:39 pm**

It comes from the equation for calculating internal heat when changing the energy of a closed system at a constant pressure. That equation is U = H - PV, so when rearranged you can get H = U + PV

hope this helps!

hope this helps!

Posted: **Sun Jan 21, 2018 4:51 pm**

Work, at constant pressure, is equal to -PdeltaV.

The change in internal energy of a system is equal to q + w. Therefore, deltaU is equal to q + (-PdeltaV).

At constant pressure, q is equal to deltaH.

So, deltaU = deltaH + (-PdeltaV).

Hope that helps a little.

The change in internal energy of a system is equal to q + w. Therefore, deltaU is equal to q + (-PdeltaV).

At constant pressure, q is equal to deltaH.

So, deltaU = deltaH + (-PdeltaV).

Hope that helps a little.