1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thermodynamics problem, Enthalpy zero for an ideal gas

  1. Feb 3, 2014 #1
    1. The problem statement, all variables and given/known data

    Show that

    [itex]\left(\frac{\partial H}{\partial T}\right)_{T} = 0 [/itex]

    for an ideal gas

    2. Relevant equations

    The question required me to first solve

    [itex]\left(\frac{\partial U}{\partial T}\right)_{P}[/itex] [itex] = C_{P}[/itex] - [itex]P\left(\frac{\partial V}{\partial T}\right)_{P}[/itex]

    but I am unsure if I would use this for the rest of the question

    3. The attempt at a solution

    I have already shown that [itex]\left(\frac{\partial C_{V}}{\partial V}\right)_{T} = 0 [/itex] for an ideal gas but I am struggling to manage this one. I can show it is zero when I have this equation to begin with
    [itex]dH = \left(\frac{\partial H}{\partial T}\right)_{V}dT[/itex] + [itex]\left(\frac{\partial H}{\partial T}\right)_{T}dV[/itex]
    But I am unsure how to get to this point in the first place, so any help here would be excellent.
    Last edited: Feb 3, 2014
  2. jcsd
  3. Feb 3, 2014 #2
    Hi Gibbsboson. Welcome to Physics Forums.

    I think that the equation you are trying to show is written incorrectly. It should read the partial with respect to P.

    Have you learned the general equation for dH in terms of dT and dP for a pure species? If so, what is it?

  4. Feb 4, 2014 #3
    Hi Chestermiller

    Sorry it should be [itex]\left(\frac{\partial H}{\partial V}\right)_{T} = 0 [/itex]

    I don't think I have learnt that equation. The only other equation involving enthalpy is H = U + PV. Or I also have dH = TdS + VdP, but I don't think I can use entropy here.

    A point in the right direction would be brilliant because I have been struggling with this for a while now

    Thanks in advance,

  5. Feb 4, 2014 #4
    Start out with dH = TdS + VdP, and take the partial of this equation with respect to V at constant T. This will give you a term involving the partial of S with respect to V at constant T. The Maxwell relationship you need to evaluate this derives from the equation for dA.

    Give it a shot.

  6. Feb 4, 2014 #5
    Sorry for being a little slow. I end up with

    [itex]\left(\frac{\partial H}{\partial V}\right)_{T} = T\left(\frac{\partial P}{\partial T}\right)_{V} + V\left(\frac{\partial P}{\partial V}\right)_{T}[/itex]

    and I see no way of getting rid of this. When I substitute the ideal gas formula this doesn't cancel. Where should I go from here?


  7. Feb 4, 2014 #6
    That's funny. When I substitute the ideal gas formula into this equation, it cancels for me. Please check your "arithmetic."

  8. Feb 4, 2014 #7
    Apologies, I was being stupid. Got it now.

    Thanks for your help

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted