(2)Pre-Equilibria Conditions in Reaction Mechanisms CO(g) + Cl2(g) --> COCl2(g)
ID: 593653 • Letter: #
Question
(2)Pre-Equilibria Conditions in Reaction Mechanisms
CO(g) + Cl2(g) --> COCl2(g)
The above reaction obeys the mechanism:
Cl2 = 2Cl Fast equilibrium
Cl + CO = COCl Fast equilibrium
COCl + Cl2 --> COCl2 + Cl Slow
2Cl --> Cl2 Fast
(a) Consider each of the following expressions and select "Yes" or "No" to indicate which represent a correct statement of the rate law that is consistent with the given mechanism.
d[COCl2]/dt = k[CO][Cl]^3/2
-d[CO]/dt = k[Cl2]^3/2[CO]
d[COCl2]/dt = k[Cl2]^2/3[CO]
d[COCl2]/dt = k[Cl2]
d[COCl2]/dt = k[CO][Cl2]^3
d[Cl2]/dt = k[Cl]^2
(again say yes or no to the above expressions)
(b) Suppose that:
k1/k-1 = 1×10-2 mol L-1,
k2/k-2 = 1×106 L mol-1,
k3 = 1×10-4 L mol-1 s-1,
and k4 = 1×103 L mol-1 s-1.
What would be the value of the overall rate constant, k, including the correct units?
Explanation / Answer
The first reaction
forward reaction
Cl2 = 2Cl
rate constant = k1
rate = (k1)[Cl2]
reverse reaction
2Cl = Cl2
rate constant = k-1
rate = (k-1)[Cl]^2
At equilibrium,
(k1)[Cl2] = (k-1)[Cl]^2
The second reaction
forward reaction
Cl + CO = COCl
rate constant = k2
rate = (k2)[Cl][CO]
reverse reaction
COCl = Cl + CO
rate constant = k-2
rate = (k-2)[COCl]
At equilibrium
(k2)[Cl][CO] = (k-2)[COCl]
The third reaction
forward reaction
COCl + Cl2 = COCl2 + Cl
rate constant = k3
rate = (k3)[COCl][Cl2]
This a slow step, hence it is rate determining step
From the first reaction rate
(k1)[Cl2] = (k-1)[Cl]2
[Cl] = {(k1/k-1)[Cl2]}1/2
From the second reaction rate
(k2)[Cl][CO] = (k-2)[COCl]
[COCl] = (k2/k-2)[Cl][CO]
From the third reaction rate and put the values of [COCl] and [Cl]
(k3)[COCl][Cl2] = {k3*(k2/k-2)*(k1/k-1)1/2}[CO][Cl2]3/2
= {k}[CO][Cl2]3/2
Now let's check out the correct statements
d[COCl2]/dt = k[Cl2]3/2 [CO] = Yes
it shows the appearance of COCl2 and from the third reaction rate it is a correct statement.
-d[CO]/dt = k[CO][Cl2]3/2 = yes
It is the reaction rate of disappearance of CO
It is same as the appearance of COCl2.
d[COCl2]/dt = k[CO][Cl2]2/3 = No
it is the rate of reaction of appearance of COCl2 which is not the correct representation as in for the third reaction rate
d[COCl2]/dt = k[Cl2]3 [CO] = No
It depends on the power 3/2 not just 3
d[Cl2]/dt = k[Cl]2 = No
From the first and third reaction rate
The appearance rate of Cl2
d[Cl2]/dt = k1[Cl]2 - k-1[Cl2] - k[Cl2]3/2[CO]
overall rate constant k = k3*(k2/k-2)*(k1/k-1)1/2
k = 1×10-4 (L mol-1 s-1) *(1×106 L mol-1)*(1×10-2 mol L-1)1/2
k = 10 s-1
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.