Chemical Kinetics Test

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Chemical Kinetics Test - 19

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Question 1
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Direction (Q. Nos. 1-14) This section contains 14 multiple choice questions. Each question has four choices (a), (b), (c) and (d), out of which ONLY ONE is correct.
A chemical reaction proceeds through the following steps :
Step I, 2A ⇌ X fast
Step II, X+B ⇌ Y slow
Step III, Y+B ⇌ Product fast
Q. The rate law for the overall reaction is

A
K'[A]²

B
K'[B]²

C
K'[A][B]

D
K'[A]²[B]

Solution

By slow step II,

By reversible fast step

∴
∴
Question 2
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The following reaction obey rate law
It can follow mechanism
I. NO₂ + O₃ NO₃ + O₂ ,
NO₃+NO₂ N₂O₅
II. NO₂ + O NO₃
NO₃+NO₂ N₂O₅
Select the correct mechanism.

A
Only I

B
Only II

C
Both I and II

D
None of these

Solution

In I, slow step is

Thus,

∴
In fast reversible step,

∴

∴
Question 3
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A chemical reaction is said to take place through the various stages with ΔG° values indicated by the graph:
Stages I and II represent respectively

A
a

B
b

C
c

D
d

Solution

I is reaction intermediate while II is transition complex.
Question 4
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Following reaction takes place by mechanism:
Step I
Step II
Hence for the given reaction is

A
K₁[NO₂]² - K₂[NO₃][CO]

B
K₁[NO₂]² + K₂[NO₃][CO]

C
K₁[NO₂]²

D
K₁[NO₂] + K₂[CO]

Solution

Slow step I is the rate-determining step.
Question 5
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For the reaction,
Steps are

A
Rate law is in agreement with net stoichiometry of the reaction

B
Rate law is in agreement with slow step

C
Rate law is in agreement with net stoichiometry of the reaction as well as with slow step

D
None of the above

Solution

By slow step II,

By fast step l,

∴

Net reaction is

Thus, rate law is based on the slow step.
Question 6
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For the reaction,
A + 2B → C+ D
The rate law is k [A] [B] .
Select the correct statement.

A
Following mechanism is consistent with the stoichiometry of the net reaction and with the rate law

B
Following mechanism is consistent with the stoichiometry of the net reaction but not with the rate law

C
Both (a) and (b)

D
None of the above

Solution

(a) Slow step is the rate-determining step, thus by
rate law = k[A][B]
Net reaction,

Thus, this mechanism is not consistent with the stoichiometry, but is consistent with rate law.
(b) By slow step,
Rate law = k₃[A][B₂]
By fast step,

∴
∴
Thus, this mechanism is consistent with the stoichiometry of the net reaction but not with the rate law.
Thus, (b) is correct.
Question 7
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For the reaction,
Mechanism is given by
Rate law is true, if

A
step I is the slow step

B
step II is the slow step

C
step III is the slow step

D
step I and step II are the slow steps

Solution

If step II is fast and reversible, then

∴
∴
Question 8
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For the following S_N reaction,
Rate-determining step is

A

B

C

D

Solution

H₂O is a weak nucleophile and cannot attack tertiary carbon of maximum electron density directly. Thus, rate-determining slow step is
step is

and rate law is
and [H₂O] is not involved.
It is S_N1 reaction.
Question 9
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Reaction between nitric oxide and hydrogen takes place by the following mechanism:

Q.
Rate of the overall reaction in terms of the concentration of the reactants is

A

B

C

D

Solution

Slow step II is the rate-determining step.
...(1)
By step I in equilibrium,

∴
Put the value of [N₂O₂] in Eq. (i), we get
∴
Question 10
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Consider the following reaction,
The rate equation for this reaction is, rate = k[CI₂] [H₂S]
Which of these mechanisms is/are consistent with this rate equation?

[AIEEE2010]

A
Only II

B
Only I

C
Both (a) and (b)

D
Neither (a) nor (b)

Solution

Slow step is the rate-determining step.

and

∴
which is not the rate law.
Question 11
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The following mechanism has been proposed for the reaction,
It involves the following steps:
Q.
The order of the reaction w.r.t. NO(g) is

A
1

B
2

C
3

D
zero

Solution

From slow step,

From equilibrium step,

∴
Question 12
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Consider the three following proposed mechanism for the overall equation.

Q.
Which mechanism predicts a third order reaction?

A
Only I

B
I and II

C
II and III

D
I, II and III

Solution

It is second order in A and first order in B thus, total order = 3.
II.
∴

Thus, it is second order in A and first order in B.
Total order = 3

Thus, it is second order in A and first order in B.
Thus total order = 3
Question 13
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Consider the following proposed mechanism :

A

B

C

D

Solution
Question 14
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Based on the following steps:
Rate law is

A

B

C

D

Solution

Slow step (II) is the rate-determining step.

By fast step I in equilibrium.
∴
∴