Chemical Kinetics(11th And 12th > Chemistry ) Questions and answers for exam Preparation

11th And 12th > Chemistry

CHEMICAL KINETICS MCQs

Total Questions : 30 | Page 1 of 3 pages

If in the fermentation of sugar in an enzymatic solution which is initially 0.12 M the concentration of sugar is reduced to 0.06 M in 10 h and to 0.03 M in 20 h, hence order of the reaction is

Discuss Question

Answer: Option B. -> 1
:
B

t_{1 / 2}

is constant. Hence order = 1

Question 2.

Milk turns sour at $40^{\circ}$ C three times as faster as $0^{\circ}$ C. Hence, $E_{a}$ in the process of turning of milk sour is

$\frac{2.303 \times 2 \times 313 \times 273}{40} l o g 3$
$\frac{2.303 \times 2 \times 313 \times 273}{40} l o g (1 / 3) c a l$
$\frac{2.303 \times 2 \times 40}{273 \times 313} l o g 3 c a l$
$N o n e o f t h e s e$

Discuss Question

Answer: Option A. ->

\frac{2.303 \times 2 \times 313 \times 273}{40} l o g 3

:
A

K_{40^{\circ} C} = A e^{- E a / R \times 313} K_{0^{\circ} C} = A e^{- E a / R \times 273} 3 = e^{- E a / R \times (1 / 313 - 1 / 273)}

On taking logs on both sides, you get the required answer

Question 3.

The data for the reaction is A + B $\to$ C is
$\begin{matrix} E x p . & [A]_{0} & [B]_{0} & I n i t i a l r a t e 1 & 0.012 & 0.035 & 0.10 2 & 0.024 & 0.035 & 0.80 3 & 0.012 & 0.070 & 0.10 4 & 0.024 & 0.070 & 0.80 \end{matrix}$

$r = k [B]^{3}$
$r = k [A]^{3}$
$r = k [A] [B]^{4}$
$r = k [A]^{2} [B]^{2}$

Discuss Question

Answer: Option B. ->

r = k [A]^{3}

:
B
A quick way to solve this would be to look at rows one and two. Straight away you can tell that rate does not depend on [B]

Question 4.

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

6.25 M
1.25 M
0.125 M
0.625 M

Discuss Question

Answer: Option D. -> 0.625 M
:
D

10 24 h r s - -- \to 5 24 h r s - -- \to 2.5 24 h r s - -- \to 1.25 24 h r s - -- \to 0.625

Question 5.

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 6.

Which of these does not influence the rate of reaction?

Nature of the reactants
Concentration of the reactants
Temperature
Order of a reaction

Discuss Question

Answer: Option D. -> Order of a reaction
:
D
Order of a reaction is equal to the sum of the stoichiometric coefficients expressed in a rate law.

Question 7.

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 8.

For the reaction A + B $\to$ C, it is found that doubling the concentration of A increases the rate by 4 times and doubling the concentration of B doubles the reaction rate. What is the overall order of the reaction?

4
$\frac{3}{2}$
3
1

Discuss Question

Answer: Option C. -> 3
:
C
rate =

k [A]^{x} [B]^{y}

\begin{matrix} 4 = k [2]^{x} & 2 = k [2]^{y} x = 2 & y = 1 \end{matrix}

Question 9.

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 10.

Which one of the following is a second order reaction?

$H_{2} + B r_{2} \to 2 H B r$
$N H_{4} N O_{3} \to N_{2} + 3 H_{2} O$
$H_{2} + C l_{2} \to 2 H C l$
$C H_{3} C O O C H_{3} + N a O H \to C H_{3} C O O N a + C H_{3} O H$

Discuss Question

Answer: Option D. ->

C H_{3} C O O C H_{3} + N a O H \to C H_{3} C O O N a + C H_{3} O H

:
D
Choices (a) and (c) are photochemical reaction of zero order. Choice (b) is first order reaction. In choice (d), rate of reaction depends on 1st power of