Reaction Mechanisms in Organic Chemistry (CHMS4104)
BSc, Chemistry - CHMS
Semester: First Semester
Level: 400
Year: 2018
CHMS 4104: Organic reaction Mechanism
Test 1: 1hr 30min
Part A: MCQs
Circle the correct answer.
1 The series of steps that most reactions undergo, from initial reactants to final products, is called the:
A. catalytic conversion B. entropy C. activation energy D. reaction mechanism
2. The rate determining step for a complex reaction is the one which is:
A. fastest B. Last in the sequence C. slowest D. first in the sequence.
3. Activation energy is the amount of energy required to:
A. break the bonds between the reacting molecules B. convert the reactants into the activated complex. C. make
the reacting particles collide D. form the bonds between the product molecules
4. the activation energy for the following reaction is 40.8 Kcal
H
2
(g) + I
2
(g) → 2 HI(g) + 3.0 kcal
The activation energy for the reverse reaction is
A. 3.8kcal B. 40.8 kcal C. 3.0 kcal D. 37.8 kcal
5. The activation energy of the forward reaction is best represented by what number shown on the graph
A. 2 B. 4 C. 5 D. 3
6. For the second-order reaction below, the rate constant of the reaction is 3.9 x 10-5 M
-1
s
-1
. How long (in
seconds) is required to decrease the concentration of A from 1.07 M to 0.24 M?
2A→ B
A. 5.8 x 10
3
s B. 6.5xl0
3
s
C
X.3
X
I0
4
S
ml 3.8
X
10"
S
.
7.
A possible mechanism for the reaction OH + NO
2
→ HNO
3
in the presence of N
2
is
OH + NO ⇌ (HO .. NO
2
)
HO .. NO
2
)
+ N
2
→
HNO
3
+ N
2
Apply the steady state approximation to the mechanism. Within that approximation, which one of the following
rate laws is correct?
(a) rate =
(b) rate =
(c) rate =
(d) rate = [
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9. In which reaction are reacting molecules trransformed directly into products?
A. stepwise reaction C. elementary reaction
C. radical reaction D. chain reaction
10. Select the molecule whose bond would easily be broken homolytically?
A. HC1 B. HF C. H
2
SO
4
D. PhCOO(CH
3
)
3
Short Answers
Write the answers in the spaces provided only.
11. The following mechanism has been proposed for the reaction between chloroform, CHCl
3
, and chlorine
a. Determine the overall equation for this reaction
Step 1: Cl
2
(g) →2 Cl (g) fast
Step 2: Cl (g) + CHC1
3
(g) → HCl(g) + CC1
3
(g) slow
Step 3: Cl (g) + CC1
3
(g) → CC1
4
(g) fast
OVERALL:
b. Identify the reaction intermediates for the reaction
c. Which step is the rate determining step?
12. The possible mechanism for the reaction 2NO(g) + 0
2
(g) → 2NO
2(g)
is
a. NO + 0
2
⇌ NO
3
(slow)
b. NO
3
+ NO → NO
2
+ NO
2
(fast)
What is the predicted rate law and order of the reaction?
13. For the elementary step of a chemical reaction : A + H
2
O ⟶B, rate [A]
1
.
What is the (i) Molecularity and (ii) Order of the reaction
Molecularity __________ order of reaction __________
14. Distinguish between a “light” and a “loose” transition metal.
15. Derive the rate equation for the formation of F in terms of the concentrations of A, B and D in the following
mechanism assuming that A, B and C are in equilibrium and E is a highly reactive intermediate. (10 points).
A + B ⇌ C
C + D → E
E + E→ F
16. For the reaction A + 2B ⟶ C + D, the rate =K[A][B]
2
. Propose a plausible mechanism in which the rate-
determining step is unimolecular. (10 points)
17. Draw the mechanism for the following transformation using the curved-arrow notation based on the data you
are given. There maybe more than one step. (10 points)
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18. The hammett equation is given by log(k
a
/K
b
) = p. (6 points)
(a) what is called?
(b) what do you infer from the sign and magnitude of ?
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