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Homework Help: Very simple thermochemistry question

  1. Dec 5, 2006 #1
    I know this is a physics forum, but this problem is mainly physics, and exceedingly simple:

    1. The problem statement, all variables and given/known data

    A certain reaction releases about 90 kJ of heat when stoichiometric amounts of reactants react. If the reaction goes to completion in 100 g of water whose initial temperature is 20 deg C, what is the final temperature of the water?

    2. Relevant equations

    q = mc(delta T), I believe, so delta T = q / (mc)

    3. The attempt at a solution

    I don't know what I'm doing wrong, but if I plug the 400 kJ = 400000 J into q and the 100 g mass into m and 4.184 J / (mol deg C) into c, I get an insane change in temperature around 215 degrees. I am obviously forgetting some step or misunderstanding something... I've always had trouble with thermodynamics stuff like this. Any help would be greatly appreciated.. thank you!
  2. jcsd
  3. Dec 5, 2006 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Firstly should q not be 90kJ. Secondly the units of c that you provide are J/(g K). Other than that your calculations seem fine.
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