## constant external pressure

$w=-P\Delta V$
and
$w=-\int_{V_{1}}^{V_{2}}PdV=-nRTln\frac{V_{2}}{V_{1}}$

Karyn Nguyen 1K
Posts: 72
Joined: Fri Apr 06, 2018 11:04 am

### constant external pressure

How do we know if the external pressure is constant in order to use $w=P_{ex}\Delta v$?

Shally Li 2C
Posts: 30
Joined: Fri Sep 28, 2018 12:22 am

### Re: constant external pressure

It should either be given or you could be given enthalpy. The definition of enthalpy is heat at constant pressure, so if you're given enthalpy, you know pressure is constant.

Fayez Kanj
Posts: 62
Joined: Fri Sep 28, 2018 12:18 am
Been upvoted: 1 time

### Re: constant external pressure

Hello!

In most cases, this is explicitly stated in the problem ("against a constant pressure" of ...). However, if it is not, then you could try to infer. If the volume changes, then this is most likely going to be a reaction that involves a change in pressure (since we use w=nRTlnV2/V1). Isobaric also means constant pressure.

Hope this helps :)