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During lecture today, Dr. Lavelle went over a problem that involved identifying the correct mechanism according to the experimentally determined rate law. I understand everything about the problem, but I'm just curious about what happens if the tested step matches the observed rate. Like during lecture today, Dr. Lavelle would ask us if student A's step 1 matched the observed rate, if student A's step 2 matched the observed rate, if student B's step 1 matched the observed rate, and if student B's step 2 matched the observed rate. In the end, none of them matched the observed rate, but they did have intermediates which allowed the step to be manipulated to match the observed rate. I understand what happens when they don't match, as was demonstrated in lecture today, but what happens if the step DOES match. Like what if student A's step 1 matched? Is the problem done there? Or if his step 1 didn't match but his step 2 matched? I'm pretty confused, can somebody help me out? Thanks!
I'm pretty sure that questions on the final won't be that simple, but technically yes. If, let's say, step 1 matched exactly the given rate law, you would be done with the problem there as step 1 would be the RDS, aka slow step. The only reason he manipulated the rate law in the second step was because having your second step be slower than the first causes the first to become a bottleneck, where you can then use the equilibrium constant to replace parts of the equation.
If your first step had matched, then you would be finished with the problem, as that step would match the mechanism and would not experience bottlenecking like the second step. But because our first step did not match, we then had to go ahead and go through the procedure of converting our second step into a rate law using the intermediates.
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