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Derive a rate law from a mechanism (rates of reactions)

  1. Feb 11, 2007 #1
    1. The problem statement, all variables and given/known data

    The reaction: CO + Cl <-> COCl2
    has the rate law: rate = k[CO][Cl2]^(3/2)

    Show that this rate law is consistent with the mechanism:

    Cl2 <-> 2Cl (fast)
    Cl + CO <-> COCl (fast)
    Cl2 + COCl -> COCl2 + Cl (slow)

    (i dont know how to do superscript and subscript on here. in Cl2, the 2 should be subscsript, and ^(3/2) means that the reaction is 3/2 order with respect to Cl2)

    3. The attempt at a solution

    when you derive a rate law from a proposed mechanism, you usually start with the slow reaction...

    rate = k[Cl2][COCl]

    if i didn't know beforehand that the reaction is 3/2 order with respect to Cl2, i'd say k[COCl]=k-1[CO] => rate = k[Cl2][CO] but apparently this is incorrent. i am not sure how to proceed with the problem
  2. jcsd
  3. Mar 11, 2010 #2


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    Gold Member

    While the last reaction is proceeding slowly, the species reacting fast are in equilibrium with each other.
  4. Dec 22, 2010 #3
    Your mechanism looks a bit confusing to me, because the elementary steps don't seem to add up to the overall equation.
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