## Isothermal, Const. P, Const V

Alex Tchekanov Dis 2k
Posts: 118
Joined: Sat Aug 24, 2019 12:16 am

### Isothermal, Const. P, Const V

Could someone please organize all the equations that are specific to a system that has constant Pressure, a system that has constant volume and an isothermal (constant temperate) system. I feel like there are so many equations that get mixed up and I'm not sure when to use which one.

Brian_Ho_2B
Posts: 221
Joined: Fri Aug 09, 2019 12:16 am

### Re: Isothermal, Const. P, Const V

For constant pressure, qp = enthalpy, and qp doesn't necessarily equal delta U. For constant volume, qv = delta U = delta H IF there is no nonexpansion work involved. For isothermal, reversible reactions where delta T is zero, q = -w, delta S = q/T, and
w = -nRTln(V2/V1).

Paul Hage 2G
Posts: 105
Joined: Thu Jul 25, 2019 12:17 am

### Re: Isothermal, Const. P, Const V

System with Constant External Pressure: w=-P(external)ΔV
System with Changing Pressure (Reversible Expansion): w=-nRT(ln(V2/V1))
Change in internal energy with ΔP=0 (constant external pressure): ΔU=ΔH-PΔV because q=ΔH and w=-PΔV at constant pressure.