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Homework Help: Rxn Mechanisms and Rate Laws

  1. Mar 30, 2010 #1
    1. The problem statement, all variables and given/known data
    The reaction S2O82- + 3 I-1 -> 2 SO42- + I3-1 is first order in both reactants. The following mechanism is proposed:

    I-1 + S2O82- -> IS2O83-
    IS2O83- -> 2 SO42- + I+
    I+ + I-1 -> I2
    I2 +I-1 -> I3-1

    a) Which step is the rate determining step?
    b) Show that this mechanism is stoichiometrically equal to the overall equation.
    c) Why would you expect the slow step in part (a) to be the slow step?
    d) Why would you expect the third step to be the fastest step?

    2. Relevant equations

    1) How do I determine if a step is slow or fast?
    2) In part (b), when it asks to show that the mechanism is stoichiometrically equal to the overall equation, does that mean adding up the steps? If so, I don't get it because wouldn't I then just add up all the other steps anyway? Or, do I balance the equation to make it look like the overall equation?

    3. The attempt at a solution

    Correct me if I'm wrong, but is the rate for this reaction:
    Rate = k[S2O82-][I-1]

    Other than that, I've been trying to google and YouTube many things to assist me.. I've received no luck. Any help would be appreciated!
  2. jcsd
  3. Apr 1, 2010 #2
    1)If you look at each of the steps, consider the attractive and repulsive forces acting on reactants and products. (This isn't the whole picture, but it's a significant part of it--the more ions/molecules are involved in a given step, the longer you have to wait for them to randomly come together just the right way to react.)

    In the first step, you have to bring together two negatively charged ions which repel one another.
    In the second step, the ion separates into two (-2) ions and one (+1) ion. If the positive ion is exactly in the middle between the other two, the repusive and attractive electrostatic forces will cancel out--but even the slightest deviation from that configuration will result in net repulsive forces which prevent the product ions fom lingering close to one another or recombining.
    The third step has two opposite charges pulling towards one another before they combine.
    The last step has a negative ion combining with a neutral molecule.

    2) If you notice, each step that requires more than one reactant combines reactants in a 1:1 ratio, and each reactant that was a product of a previous step was produced in a 1:1 ratio with the reactants of that step, so there's no balancing to be done.
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