# Case Study Questions for Class 12 Chemistry Chapter 4 Chemical Kinetics

There is Case Study Questions in class 12 Chemistry in session 2020-21. For the first time, the board has introduced the case study questions in the board exam. The first two questions in the board exam question paper will be based on Case Study and Assertion & Reason. The first question will have 5 MCQs out of which students will have to attempt any 4 questions. The second question will carry 5 Assertion & Reason type questions with the choice to attempt any four. Here are the questions based on case study.

Case Study Question 1:

The half-life of a reaction is the time required for the concentration of reactant to decrease by half, i.e.,

[A]t = [A]/2

For first order reaction,

t1/2 = 0.693/k

this means t1/2 is independent of initial concentration. Figure shows that typical variation of concentration of reactant exhibiting first order kinetics. It may be noted that though the major portion of the first order kinetics may be over in a finite time, but the reaction will never cease as the concentration of reactant will be zero only at infinite time.

The following questions are multiple choice question. Choose the most appropriate answer:

(i) A first order reaction has a rate constant k=3.01 x 10-3 /s. How long it will take to decompose half of the reactant?

(a) 2.303 s             (b) 23.03 s             (c) 230.3 s             (d) 2303 s

(ii) The rate constant for a first order reaction is 7.0 x 10-4 s-1. If initial concentration of reactant is 0.080 M, what is the half-life of reaction?

• 990 s
• 79.2 s
• 12375 s
• 10.10 x 10-4 s

(iii) For the half-life period of a first order reaction, which one of the following statements is generally false?

• It is independent of initial concentration.
• It is independent of temperature.
• It decreases with the introduction of a catalyst.
• None of these.

(iv) The rate of a first order reaction is 0.04 mol L-1 s-1 at 10 minutes and 0.03 mol L-1 s-1 at 20 minutes after initiation. The half life of the reaction is

• 4.408 min
• 44.086 min
• 24.086 min
• 2.408 min

OR

The plot of t1/2 vs initial concentration [A]0 for a first order reaction is given by

(ii) a
(iii) b
(iv) c
or
b