## Difference in equation for volume and pressure

BiancaYugar_1D
Posts: 10
Joined: Fri Sep 25, 2015 3:00 am

### Difference in equation for volume and pressure

How does the equation for internal energy change when it is at a constant volume vs. when it is at a constant pressure?

Chem_Mod
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Joined: Thu Aug 04, 2011 1:53 pm
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### Re: Difference in equation for volume and pressure

It does not change. The two expressions for internal energy are:

deltaU = q+w
deltaU = Cv*deltaT

DevinSandoval_1D
Posts: 24
Joined: Fri Sep 25, 2015 3:00 am

### Re: Difference in equation for volume and pressure

Well regardless of whether volume or pressure are constant, the equation for internal energy (∆u=q + w) still applies.

If there is no change in volume, ∆V=0. w=-P x ∆V, so if ∆V=0, the w=0. This means that ∆u=q+0 or ∆u=q.

If there is no change in pressure, ∆P=0. Since enthalpy is defined the amount of heat released or absorbed at a constant pressure, we can substitute the variable ∆H for q. In this case then, ∆u=∆H-w or ∆u=∆H-P∆V. From here you would fill in the values of ∆H, P, and ∆V, which should be given.