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

12th Grade > Chemistry

CHEMICAL KINETICS MCQs

Total Questions : 30 | Page 3 of 3 pages

Question 21. Velocity constant of a reaction at 290 K was found to

3.2 \times 10^{- 3} s e c^{- 1}

. When the temperature is raised to 310 K, it will be about:

6.4×10−3
3.2×10−4
9.6×10−3
1.28×10−2

Discuss Question

Answer: Option D. -> 1.28×10−2
:
D
From the Arhenius eqn, we know that the rate of a reaction doubles for every 10 K rise in temperature.

\frac{k_{2}}{k_{1}} = 2^{2} \Rightarrow k_{2} = 3.2 \times 10^{- 3} \times 4 = 1.28 \times 10^{- 2} s e c^{- 1}

Question 22. The concentration of a reactant changes from .03M to .02M in 25 minutes. The average rate of reaction, using time in seconds is

0.4×10−3mol/l/s
0.024mol/l/s
0.24mol/l/s
6.67×10−6mol/l/s

Discuss Question

Answer: Option D. -> 6.67×10−6mol/l/s
:
D

R a t e = \frac{Δ [R]}{Δ t} = \frac{- (0.02 - 0.03)}{25 \times 60} = 6.67 \times 10^{- 6} m o l / l / s

Question 23. Which among the following is a false statement?

For a first order reaction t12=0.693k
Rate of zero order reaction is independent of initial concentration of reactant
Half-life of a third order reaction is inversely proportional to the square of initial concentration of the reactant
Molecularity of a reaction may be zero or fraction

Discuss Question

Answer: Option D. -> Molecularity of a reaction may be zero or fraction
:
D
Molecularity can never be zero or fractional.

Question 24. The rate of a reaction doubles when its temperature changes from 300 K to 310 K. Activation energy of such a reaction will be:

(R = 8.314 J K^{- 1} m o l^{- 1} a n d l o g 2 = 0.301)

(IIT-JEE-2013)

48.6kJmol−1
58.5kJmol−1
60.5kJmol−1
53.6kJmol−1

Discuss Question

Answer: Option D. -> 53.6kJmol−1
:
D
As per Arrhenius equation:

l o g \frac{K_{2}}{K_{1}} = \frac{E_{a}}{2.3 R} [\frac{T_{2} - T_{1}}{T_{2} T_{1}}]

2.3 l o g 2 = \frac{E_{a}}{8.314} [\frac{10}{300 \times 310}]

∴ E_{a} = 53.6 k J m o l^{- 1}

Question 25. Plots showing the variation of the rate constant (k) with temperature (T) are given below. The plot that follows the Arrhenius equation is

Discuss Question

Answer: Option D. -> 53.6kJmol−1
:
A
Arrhenius equation

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

l o g k = l o g A - \frac{E_{a}}{2.303 R T}

Plots Showing The Variation Of The Rate Constant (k) With Te...

Question 26. Half-life of a reaction is found to be inversely proportional to the cube of initial concentration. The order reaction is

Discuss Question

Answer: Option D. -> 4
:
D

t_{0} .5 \propto \frac{1}{a^{n - 1}}

Given

t_{0} .5 \propto \frac{1}{a^{3}}

Hence,

n - 1 = 3

∴ n = 4

Question 27. For a reaction X

\to

Y, the rate of the reaction has been found to be third order with respect to X. What happens when the concentration of X is doubled.

rate become double
rate becomes three times
rate become six times
rate becomes eight times

Discuss Question

Answer: Option D. -> rate becomes eight times
:
D
Rate =

k [X]^{3}

When X becomes double, rate become

2^{3}

= 8 times

Question 28. In the reaction, P + Q ⟶ R + S
the time taken for 75% reaction of P is twice the time taken for 50% reaction of P. The concentration of Q varies with reaction time as shown in the figure. The overall order of the reaction is

Discuss Question

Answer: Option D. -> 1
:
D
Overall order of reaction can be decided by the data given

t_{75 ﹪} = 2 t_{50 ﹪}

∴ It is a first order reaction with respect to P.
From graph [Q] is linearly decreasing with time, i.e., order of reaction w.r.t. Q is zero and the rate expression is

r = [P]^{1} [Q]^{0} .

Therefore 1 is the right answer.

Question 29. The half life of a first order reaction is

Independent of the initial concentration of the reactant
Directly proportional to the initial concentration of the reactant
Inversely proportional to the initial concentration of the reactant
Directly proportional to the square of the initial concentration of the reactant

Discuss Question

Answer: Option A. -> Independent of the initial concentration of the reactant
:
A
Statements in options (b) (c) and (d) correspond to orders 0, 2 and 3 respectively. Hence the answer for the given question is found in the option (a).

Question 30. The rate law expression for four reactions are give below. In which case, the percentage increase in the rate of reaction would be maximum when the concentration of A is doubled?

r=k[A]
r=k[A]32
r=k[A]∘
r=k[A]2

Discuss Question

Answer: Option D. -> r=k[A]2
:
D
Larger the order of reaction more the percent increase in the rate of reaction for a given increase in concentration.