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Homework Help: Reaction Rate Question

  1. Feb 11, 2007 #1
    The rate of appearance of I2 in aqueous solution, in the reaction of the I- ion with hydrogen peroxide, was found to change from 3.7*10^-5 mol/dm^3 to 7.9*10^-5 mol/dm^3 over a time interval of 1.0 to 3.0s.
    2H+(aq) + 2I-(aq) + H2O2(aq) --> I2(aq) + 2H2O(l)
    a)What is the rate of appearance of H2 during this period?
    b) At what rate should the concentration of the H+ ion be changing during this same time period?
    c) Is this mechanism likely to occur in one step?

    a) rate(H2) = rate(I2)
    rate(H2)=delta[H2]/delta t
    rate(H2)=(7.9*10^-5 mol/dm^3-3.7*10^-5 mol/dm^3)/(3.0s-1.0s)
    rate(H2)=2.1*10^-5 mol/(dm^3*s)
    b)mol(H+)/mol(H2)=2/1 = 2
    Therefore rate(H+)=2*(-2.1*10^-5 mol/(dm^3*s))
    =-4.2*10^-5 mol/(dm^3*s)
    c) No, it is not likely to occur in one step, as the overall reaction is not elementary: it has a molecularity of 5, and a molecularity of 3 or less must exist to ensure a one-step (elementary) mechanism.

    I am unsure about the whole process. Please tell me if I did this correctly.
  2. jcsd
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