Question

Consider the following reaction.

4 A + B + 2 C →→ 4 D + E

The following data was collected on the reaction.

Trial	[A]0	[B]0	[C]0	Rate (M/s)
1	0.22	0.75	0.22	9.506
2	0.44	0.75	0.22	19.012
3	0.22	1.5	0.22	19.012
4	0.22	0.75	0.44	9.506

Two mechanisms are proposed:

Mechanism 1:

A + B	→→	D + Y
Y + C	→→	X
X + A + C	→→	D + Z
Z + A	→→	E

Mechanism 2:

A + C	→→	D + Y
Y + B + C	→→	X + D
X + A + C	→→	E

Which mechanism best fits the data? Select an answer ( Mechanism 1, Mechanism 2 )

Which is most likely the rate-determining step? Select an answer( Step 1 ,Step 2, Step 3, Step 4 )

Answer 1

rate law is

rate = K[A]^x[B]^y[C]^z

from trial 1

rate = 9.506 = K *(0.22)^x (0.75)^y (0.22)^z (1)

from trial 2

rate = 19.012 = K* (0.44)^x *(0.75)^x(0.22)^z (2)

trial 3

rate = 19.012 = K *(0.22)^x ( 1.5)^y (0.22)^z (3)

trial 4

rate = 9.506 = K*(0.22)^x(0.75)^y(0.44)^z (4)

now, trial 1/ trial 4

(0.22)/(0.44)^z = (9.506/9.506) = 1

or, Z = 0.

trial 1/trial 2

(0.22)^x/(0.44)^x = (9.506/19.012) = (1/ 2)

or, X = 1

trial 1/ trial 3

(0.75/0.15)^y = (9.506/19.012) = (1/2)

or, Y = 1

then rate law is

rate = K*[ A] [B]

overall order = (1+1) = 2

so, rate of the reaction is independent on concentration of C.

In mechanism 2, in all the steps C is present. But in mechanism 1 , C is not present in all the steps.

Hence, mechanism 1 , best fits the data.

rate of the reaction depends on the slowest step.

By rate law the rate of the reaction depends only on the concentration of A and B, so rate-determining step is step 1.