Given here is a mechanism that has two steps.

We see them listed there as step 1 and step 2.

To determine which of these two steps is the rate determining step we write

the rate law for the steps.

And when you write a rate law for an elementary step you get to use

the coefficients of the balanced elementary step.

And we have one for A and one for B as the orders of these substances.

So it's first power in A and first power in B, but

we don't write first powers so let me go ahead and

erase those two 1s right there just so we know.

But they don't get written in if they're to the first power.

For the second step,

we know that the rate law would be the rate constant times B times Q.

Now we see here that this rate law

matches the experimentally determined rate law of overall reaction.

That means that step 1 is determining the rate of the reaction.

And we call that the rate determining step,

sometimes called the rate limiting step.