For the reaction: 2NO(g) + Cl2(g) <--> 2NOCl(g) a mechanism involving the follow

ID: 1075757 • Letter: F

Question

For the reaction: 2NO(g) + Cl2(g) <--> 2NOCl(g) a mechanism involving the following steps has been proposed:

(1)

NO(g) + Cl2(g) --> NOCl2(g)

(2)

NOCl2(g) + NO(g) --> 2NOCl(g)

and its rate law has been determined to be: rate = k[NO][Cl2].

Based on the information given above, which step is the rate-limiting step?

a. Step (1)

b. Step (2)

c. Both Steps (1) & (2)

d. Either Steps (1) or (2)

e. Cannot be determined.

(1)

NO(g) + Cl2(g) --> NOCl2(g)

(2)

NOCl2(g) + NO(g) --> 2NOCl(g)

Lets assume first step is rate determining:

Now if the first elementary step 1 in the mechanism were rate-determining, then rate law will be

rate = k[NO][Cl2] (based upon the molecularity of first step NO(g) + Cl2(g) --> NOCl2(g))

Clearly we see that it matches with the determined rate law rate = k[NO][Cl2]. Hence, step (1) is the rate determining step.

Note: I have solved a similar question previously in which the determined rate law in the question was given to be

rate = k[NO]2[Cl2] and not rate = k[NO][Cl2]. In that case, the first step won't be the rate determining one.

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