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mechanisms

# mechanisms - Kinetics Consistent Mechanism and...

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Unformatted text preview: Kinetics: Consistent Mechanism and Intermediates Consider the following reaction: H2(g) + 2ICl(g) 2HCl(g) + I2(g) The rate law for this reaction was determined to be R = k [H2][ICl]. Is the following proposed mechanism consistent with the above data? H2(g) + ICl(g) HI(g) + HCl(g) (fast) HI(g) + ICl(g) HCl(g) + I2(g) (slow) What is the intermediate in the above mechanism? For a mechanism to be consistent: 1. The steps ( in this case two) must add up the overall reaction( at the top ). If species appear on the right of one equation and the left of another, it may be canceled out. In the example above, HI can be canceled out. After canceling out HI and combining the left side of both and the right side of both, the two two equations add up to the top equation. The rate determining step ( slow step ) must give the correct rate law. Since mechanistic steps are elementary reactions, we simply select the slow step ( the rate determining step ) and use the coefficients as powers. In the example above the second reaction is the rate determining step. Therefore, the rate law is: R = k [HI][ICl]. Since this is not the rate law given at the top of the problem, this mechanism is not consistent. In order to be consistent, the slow step would have to give a rate law of: R = k[H2][ICl]. The intermediate is HI. It is produced in one step of the mechanism and consumed in another step. It does not appear in the overall reaction. On the other hand, a catalyst, which also does not appear in the overall reaction, will be a reactant in the first reaction and then be consumed in a later step. 2. ...
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