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Homework Help: Calculate ΔHo for the chemical reaction:

  1. Nov 30, 2016 #1
    1. The problem statement, all variables and given/known data
    2. Relevant equations

    3. The attempt at a solution
    2(255) - [(1)(159) + (1)(243)]
    = +108KJ

    The answer key says its b) -108KJ. Am i doing something wrong or is the book wrong?
    Last edited: Nov 30, 2016
  2. jcsd
  3. Dec 1, 2016 #2
    When using bond enthalpies, the way you calculate the heat of reaction is
    [tex]\Delta H° = \sum \Delta H_{\textrm{Broken}} - \sum \Delta H_{\textrm{Formed}}[/tex]
    As in, "the sum of enthalpy changes by broken bonds minus the sum of enthalpy changes by formed bonds." So, the correct answer is -108 kJ.
  4. Dec 1, 2016 #3


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    Staff: Mentor

    ΔH° is not the same thing as a bond enthalpy. Quite the opposite - positive bond enthalpy means negative ΔH° (think why).
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