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: Vaporization and change in internal energy

  1. Jan 14, 2010 #1
    1. The problem statement, all variables and given/known data
    Suppose 1 g of water vaporizes isobarically at atmosphere pressure (1.013 x 105 Pa). Its volume in the liquid state is Vi = 1 cm3, and its volume in the vapor state is Vf = 1671 cm3. Find the change in internal energy


    2. Relevant equations
    [tex]\Delta U = \Delta Q - \Delta W[/tex]

    [tex]\Delta W = p*\Delta V[/tex]

    3. The attempt at a solution
    I want to ask about [tex]\Delta V[/tex].
    [tex]\Delta V = V_2-V_1[/tex]

    V2 = Vf, and what is V1 ? Is it Vi or is it zero because volume of vapor is zero initially?

    Thanks
     
  2. jcsd
  3. Jan 15, 2010 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    Hi songoku! :smile:

    (have a del: ∆ :wink:)
    It's Vi. :smile:
     
  4. Jan 15, 2010 #3
    Re: thermodynamics

    Hi tiny-tim! :biggrin:

    Why is it not zero? I think we have to consider the vapor state, excluding the liquid state. Does the first law of thermodynamics only apply for gas?

    Thanks
     
  5. Jan 15, 2010 #4

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: thermodynamics

    The first law of thermodynamics applies to everything, not just gasses.
     
  6. Jan 15, 2010 #5
    Re: thermodynamics

    I encountered a very similar question.

    Consider 100 g (100 cm3) of a liquid evaporating at constant pressure of 100 kPa to vapor of volume 0.167 m3. Assuming that the latent heat of vaporization of the liquid is 2.26 MJ kg-1 K-1 and the vapor behaves like an ideal gas, find the change internal energy.

    On the manual, it is written :

    note that the volume of gas is zero initially, so ∆V = 0.167 - 0

    Is the manual wrong or am I missing something?

    Thanks
     
  7. Jan 16, 2010 #6

    Redbelly98

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Re: thermodynamics

    That's weird.

    It makes very little difference in the result, but I was pretty sure you should account for the original volume of the liquid -- if the accuracy of the given numbers warrants it.
     
  8. Jan 16, 2010 #7
    Re: thermodynamics

    ok

    Thanks for your help, tiny-tim and Redbelly98 !!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook