Chemical Kinetics(12th Grade > Chemistry ) Questions and answers for exam Preparation

12th Grade > Chemistry

CHEMICAL KINETICS MCQs

Total Questions : 30 | Page 1 of 3 pages

Question 1. Chemical reactions with very high

E_{a}

values are generally:

Very fast
Very slow
Moderately fast
Spontaneous

Discuss Question

Answer: Option B. -> Very slow
:
B
A reaction with very high

E_{a}

will have very small fraction of effective collision of molecules.

Question 2. The half-life for the reaction,

N_{2} O_{5} \Rightarrow 2 N O_{2} + \frac{1}{2} O_{2}

is 12 minutes at

30^{\circ} C

. Starting with 100 g of

N_{2} O_{5}

how many gram of

N_{2} O_{5}

will remain after a period of 1 hour 36 minutes.

1.25 g
0.39 g
1.77 g
0.5 g

Discuss Question

Answer: Option B. -> 0.39 g
:
B
One hour 36 minutes, i.e., 96 minutes is 8 half lives
Hence the amount left over is

\frac{100}{2^{8}} = \frac{100}{256} = 0.39 g

Question 3. A first order reaction is half completed in 45 minutes. How long does it need for 99.9% of the reaction to be completed?

20 hours
10 hours
7.5 hours
5 hours

Discuss Question

Answer: Option C. -> 7.5 hours
:
C
Each half life would reduce the concentration to half of its original value. So, 99.9% completion takes place in 10 half-lives. Remember

2^{10} = 1024

and

1 / 1024 \approx 0.001

Hence time taken = 45

\times

10 minutes = 450 minutes = 7.5 hours.

Question 4. The rate at which a substance reacts depends upon:

Atomic weight
Atomic number
Molecular weight
Active mass

Discuss Question

Answer: Option D. -> Active mass
:
D
The rate of a reaction depends on the active mass of the reactants

Question 5. Certain bimolecular reactions which follow first order kinetics are called

Bimolecular reaction s
Unimolecular reactions
First order reaction
Pseudo unimolecular reactions

Discuss Question

Answer: Option D. -> Pseudo unimolecular reactions
:
D
Pseudo unimolecular reactions

Question 6. The conversion of A to B follows second order kinetics. Doubling the concentration of A increases the rate of formation of B by a factor of

Discuss Question

Answer: Option B. -> 4
:
B
For a second order kinetics of the reaction A

\to

B we have rate =

k [A]^{2}

. When conc. of A is doubled the rate of the reaction increases by a factor of

2^{2} (= 4)

. Since rate of conversion of A = rate of formation of B. Hence rate of formation of B also increases 4 times.

Question 7. The rate equation for the reaction 2A + B

\to

C is found to be a : r = k[A][B]. The correct statement in relation of this reaction is that the

Rate of formation of C is twice the rate of disappearance of A
t12 is a constant
Unit of k is s−1
Value of k is independent of the initial concentration of A and B

Discuss Question

Answer: Option D. -> Value of k is independent of the initial concentration of A and B
:
D
The order of the reaction is two. Option (b) and (c) apply to first order reactions. Options (a) is wrong since the rate of formation of C is half the rate of disappearance of A. Option (d) is always true since value of k for any order is independent of the initial concentration of the reactants.

Question 8. The activation energy for a reaction at the temperature TK was found to be 2.303

R T J m o l^{- 1}

. The ratio of the rate constant to Arrhenius factor is ___

10−2
10−1
2×10−2
2×10−3

Discuss Question

Answer: Option B. -> 10−1
:
B

k = A e^{\frac{- E_{a}}{R T}}

E_{a} = 2.303 R T

∴ \frac{k}{A} = e^{- (\frac{2.303 R T}{R T})}; \frac{k}{A} = e^{- 2.303}

l o g_{e} (\frac{k}{A}) = l o g_{e} e^{- 2.303}

l o g_{e} (\frac{k}{A}) = - 2.303

2.303 l o g_{10} (\frac{k}{A}) = - 2.303

l o g (\frac{k}{A}) = - 1; l o g (\frac{A}{K}) = 1

∴ \frac{A}{k} = a n t i l o g 1 = 10

\frac{k}{A} = \frac{1}{10} = 10^{- 1}

Question 9. The half-life of a first order reaction is 24 hrs. If we start with 10 M initial concentration of the reactant then conc. after 96 hrs will be