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!

Homework Help: Gas is isothermally and reversibly condensed

  1. Nov 29, 2008 #1
    1. The problem statement, all variables and given/known data
    gas is isothermally and reversibly condensed
    find w, q, U, H

    2. Relevant equations

    3. The attempt at a solution
    w=-n(g)RT n(g)=-2 w=5.6kJ
    So U=H-w U Should be -70.6-5.6=-76.2, but the book gives -65, Where am I wrong?
    And why is H=q in this process?
  2. jcsd
  3. Nov 29, 2008 #2


    User Avatar
    Science Advisor
    2017 Award

    Re: Thermodynamics

    In the condensation from a gas to a liquid, what is the sign of ΔV? What then, should be the sign on w? Is work being done on the gas by the surroundings or is work being done on the surroundings by the gas?

    Also, to answer your second question, ΔH = q for any reversible process done at constant pressure.
    proof: From the definition of enthalpy, we know that
    ΔH = ΔU + Δ(PV)
    Next, we combine this expression with the first law of thermodynamics, ΔU = q + w, to get:
    ΔH = q + w + Δ(PV)
    At constant pressure, w = -integral(PdV) = -PΔV, and Δ(PV) = PΔV. Plugging these two expressions into the expression above gives the desired result:
    ΔH = q
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook