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To my understanding, it does not matter whether the slow step is the first step of the mechanism or the second step or the last step. The slow step always determines the rate law of the mechanism regardless of whether it takes place in the beginning of the mechanism or at the end.
the slow step itself makes up the rate law because it is rate determining step. In other words you could only go as fast as your slowest step. If the first step is your slow step then that is the rate law. But if the slow step is your second or third step in the reaction you have to include the rate knows from the steps before in your overall rate law.
The slow step always determines the rate of the reaction. The problem is that when the slow step is not the first step in the rxn, the experimentally observed rate law doesn't always seem to match the rate law obtained by the slow step. This is because the slow step rate might include the concentration of an intermediate, which should not be in the rate law. In this case, you use the pre-equilibrium approach, because the slow step causes the previous step to accumulate and reach a pre-equilibrium state. You simply substitute the intermediate concentration in terms of the reactants in the preceding step, and you should get the experimentally observed rate law. If they don't match, your mechanism is incorrect.
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