Question

Consider the following mechanism

STEP 1: A <=> B + C (equilibrium)

STEP 2: C + D => E (slow)

OVER ALL: A + C => B + E

Determine the rate law for the overall reaction (where the overall rate constant is represented as k).

RATE= k [ ? ] [ ? ]

Answer 1

slow step determine the rate of reaction.so slow step is also called rate determination step

The rate law for the slow step is:

rate=k [C] [D]
Since [C] is an intermediate, we need to substitute it using equilibrium step.

We know equilibrium means rate of forward reaction = rate of reverse reaction.
so, k1[A] = k2[B] [C] <------- shows equilibrium.

Then solve for [C] to get: k1[A]/k2[B]= [C].
Now, you simply substitute in the initial rate equation you got from the slow step to get:

rate= k(slow) {k1[A]/k2[B]} x [D]

rate=k x k1/k2 [A][D]/[B]

rate= K{ [A] [D] } / [B].   K=k x k1/k2