Chemical Kinetics MCQs for NEET — Chemistry Questions with Answers

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Rate equation for a second order reaction is:

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Explanation

(c) dx/dt = K(a-x)2 is differential form of II order. Integrate it to get (c).

For the reaction 2NO2 + F2 → 2NO2F, following

mechanism has been provided,

 NO2 + F2   slow  NO2F+F

NO2 + F   fast NO2F

Thus, rate expression of the above

reaction can be written as:

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Explanation

(b) Slowest step of mechanism decides the rate expression, thus, rate = K[NO2][F2]

Rate constant of reaction can be expressed by Arrhenius equation as,

                             K=Ae-EaRT   

In this equation, Ea represents:

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Explanation

(b) Ea is activation energy. If Ea + iR = Ethreshhold energy, then only molecules may react.

Note: It is not necessary for all molecules having E>Ea to react. They may react.

For a first order reaction A Product, the initial concentration of A is 0.1 M and after 40 minute it becomes 0.025 M. Calculate the rate of reaction at reactant concentration of 0.01M:

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Explanation

(a) K = 2.303/40 log(0.1/0.025)

K = 0.03466 min-1 

rate = Kx0.01 = 0.03466x0.01= 3.47x10-4 M min-1  

Select the intermediate in the following reaction mechanism:

O3(g)  O2(g) +O(g)

O(g) +O3(g)  2O2(g)

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Explanation

(b) The intermediate species is one which is formed and used up during the course of reaction.

A reactant with initial concentration 1.386 mol litre-1 showing first order change takes 40 minute to become half. If it shows zero order change taking 20 minute to becomes half under the similar conditions, the ratio, K1/K0 for first order and zero order kinetics will be:

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Explanation

(a) For I order : (t1/t2)1 = 0.693/K1

For zero order : (t1/2)0 = a/2K0

K1/K0 x 1.386/2x0.693 = (t1/2)0/(t1/2)1

K1/K0 =(20x2x0.693) / 40x1.386 = 0.5mol-1litre

In a first order reaction, the concentration of the reactant is decreased from 1.0 M to 0.25M in 20 minute. The rate constant of the reaction would be:

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Explanation

(d) 

t=2.303Kloga(a-x)   K=2.30320log10.25=0.06931min-1

The following mechanism has been proposed for the reaction of NO with Br2 to form NOBr:

NO(g) + Br2(g)  NOBr2(g)

NOBr2(g) + NO(g) 2NOBr(g)

If the second step is the rate determining step, the order of the reaction with respect to NO(g) is:

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Explanation

(d) r = K[NO][NOBr2]

Kc = [NOBr2]2 /[NO][Br2]

r = K[NO]2[Br2]xKc = K'[NO]2[Br2]

The rate constant of a first order reaction is 4x10-3 sec-1. At a reactant concentration of 0.02 M, the rate of reaction would be:

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Explanation

(a) r = K[A] = 4x10-3x0.02 = 8x10-5 Msec-1

If concentration of reactants is increased by 'X', the rate constant K becomes:

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Explanation

(c) Rate constant is a characteristic constant for a given reaction.

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