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: Total heat removed from phase change

  1. Feb 7, 2010 #1
    Ethyl alcohol has a boiling point of 78.0°C, a freezing point of -114°C, a heat of vaporization of 879 kJ/kg, a heat of fusion of 109 kJ/kg, and specific heat of 2.43 kJ/(kg · K). How much energy must be removed from 0.584 kg of ethyl alcohol that is initially a gas at 78.0°C so that it becomes a solid at -144°C?

    I know start out by finding the heat removed at the phase change.

    Q1 = Lvm
    = 879(0.584)
    = 513.336 kJ

    Then the heat involved to cool the liquid to −114°C...

    Q2 = cmΔT
    = 2.43(0.584)(78− - 114)
    = 272.471 kJ

    Then the phase change at −114°C...

    Q3 = Lfm
    = 109(0.584)
    = 63.656 kJ

    Adding them all together to get the total heat...

    Q1 + Q2 + Q3 = 513.336 + 272.471 + 63.656 = 849.463 kJ

    But webassign won't take my answer. What did I do wrong?

    Please help. Thanks.

    Edit: I figured out what I did wrong. Didn't pay attention to the final temperature. Got it right this time. :)
    Last edited: Feb 7, 2010
  2. jcsd
  3. Dec 9, 2014 #2
    Hi, not sure if you'll be able to see this since this thread was posted 4 years ago but what did you mean by u didn't pay attention to the final temperature? What did you do right?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted