Question

Question 4 of 10 For a reaction that proceeds by the following two-step mechanism A2 =2A (fast) A+B-C (slow) Which of the following statements is/are correct? 1) The reaction is first order in the concentration of Az. 2) B is a reaction intermediate. 3) The rate-determining step is bimolecular. 4) The units of the specific rate constant are mol1/2_-1/2-1 O A. 1 & 3 only B. 2 only C. 3 only OD. 1 & 4 only O E.2 & 3 only Reset Selection

Answer 1

A is an intermediate because it is produced in step 1 and used in step 2.
Option 2 is wrong

2nd step is rate determining step
It has one molecule of A and one molecule of B in reactant.
So, molecularity is 2
Option 3 is correct

Lets derive rate law.
rate depends on the slowest step
Here 2nd step is slowest.
So, rate law is:
rate = k2[A][B]

here A is an intermediate. It should be removed from rate law.
Using eqn 1:
kc = [A]^2/[A2]
[A] = Kc^1/2 [A2]^1/2

Put this value in rate law expression:
rate = k2[A][B]
rate = k2Kc^1/2 [A2]^1/2[B]
Let k2kc^1/2 be k
rate = k [A2]^1/2[B]

So, the reaction is 1/2 order in A2 and 1st order in B
Option 1 is wrong.

rate = k [A2]^1/2[B]
M.s-1 = k M1/2 M
M.s-1 = k M3/2
K = M-1/2s-1
K = L1/2 mol-1/2 s-1

Answer: C