Constant pressure/volume

Moderators: Chem_Mod, Chem_Admin

Guadalupe T 1E
Posts: 58
Joined: Fri Sep 29, 2017 7:04 am

Constant pressure/volume

Postby Guadalupe T 1E » Thu Jan 18, 2018 6:03 pm

What exactly does Cp=5/2*R and Cv= 3/2*R mean? When would we use it?

Vincent Tse 1K
Posts: 30
Joined: Fri Sep 29, 2017 7:05 am
Been upvoted: 1 time

Re: Constant pressure/volume

Postby Vincent Tse 1K » Thu Jan 18, 2018 6:45 pm

Cv = 3/2 * R is the relationship that you derive for monoatomic gases. Cv is the heat capacity at a constant volume.

The derivation involves using ΔE = (3/2)nRΔT (for monoatomic gases) and the first law of thermodynamics, Q = ΔE + W.
W is 0 at constant volume, so you plug in ΔE on the right, the substitute the Q on the left with Q=nCvΔT. The n's and ΔT's cancel, leaving you with that relationship.

Cp = 5/2 * R is the same idea but at constant pressure instead of volume, so W is not zero (it's equal to PΔV, which in turn is equal to nRΔT).

You use these when dealing with monoatomic ideal gases...diatomic/polyatomic ones have different properties!

Wayland Leung
Posts: 51
Joined: Sat Jul 22, 2017 3:00 am
Been upvoted: 1 time

Re: Constant pressure/volume

Postby Wayland Leung » Thu Jan 18, 2018 10:47 pm

When would we need to use these values of 5/2R and 3/2R? Could you give some context?

Return to “Heat Capacities, Calorimeters & Calorimetry Calculations”

Who is online

Users browsing this forum: No registered users and 1 guest