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Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 1 www.embibe.com CBSE NCERT Solutions for Class 12 Chemistry Chapter 4

Back of Chapter Questions

both in minutes and seconds.

Solution:

Average

Negative sign indicate the decrease the concentration Average rate of reaction in seconds ൌସൈଵ଴షర

Concept Insight:

Average rate depends upon the change in concentration of reactants or products and time taken for that change to occur. Rate of disappearance of ൌ Decrease in concentration of ȀTime taken Rate of appearance of ൌ Increase in concentration of ȀTime taken

Solution:

Average rate ൌଵ

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 2 www.embibe.com ൌെͳ

Concept Insight:

For expression the rate of reaction where stoichiometric coefficients of reactants or products are not equal to one, rate of disappearance of any of the reactants or rate of appearance of products is divided by their respective stoichiometric coefficients. the order of the reaction?

Solution:

Sum of power of the concentration of the reactants in the rate law expression is called the order of that chemical reaction.

Hence the order of the reaction ൌଵ

Concept Insight: Sum of power of the concentration of the actants in the rate law expression is called the order of the chemical reaction.

4. The conversion of molecules to follows second order kinetics. If

concentration of is increased to three times how will it affect the rate of formation of ?

Solution:

The reaction ՜ follows second order kinetics.

Therefore, the rate equation for this reaction will be: then equation (1) can be written as:

Now, the rate equation will be:

Class- XII-CBSE-Chemistry Chemical Kinetics

Hence, the rate of formation will increase by nine times. reactant take to reduce to ͵ǫ

Solution:

Given:

Formula:

ଷsecond the decomposition is a first order reaction, calculate the rate constant of the reaction.

Solution:

͸Ͳ minutes

Class- XII-CBSE-Chemistry Chemical Kinetics

7. What will be the effect of temperature on rate constant?

Solution:

The rate constant of reaction is doubled or triple with an ͳͲ or rise in temperature. However, the exact d rate of a chemical reaction on temperature is given by

Arrhenius equation,

Where,

ൌ rate constant ൌ Arrhenius factor or the frequency factor ൌ temperature ൌ gas constant ୟൌ activation energy Concept Insight: The effect of temperature on rate constant is given by Arrhenius equation.

8. The rate of the chemical reaction doubles for an increase of ͳͲ in absolute

Solution:

Given that

We know that the rate of the reaction doubles when temperature is increased by

Formula: ୩మ

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 5 www.embibe.com We get: Concept insight: The rate of reaction doubles with ͳͲ rise of temperature. or greater than activation energy?

Solution:

Given that

According to Arrhenius equation

In this formula term Ȃ୉౗Ȁୖ୘ represent the number of molecules which have energy equal or more than activation energy Number of molecules ൌȂ୉౗Ȁୖ୘ put the values we get number of molecules

Class- XII-CBSE-Chemistry Chemical Kinetics

Back of chapter questions

1. From the rate expression for the following reactions, determine their order of

reaction and the dimensions of the rate constants.

Solution:

Formula used for Dimension of ൌୖୟ୲ୣ

Rate ൌ mole/litre/time

Therefore, order of the reaction ൌʹ

Dimension of ൌୖୟ୲ୣ

Therefore, order of the reaction ൌʹ

Dimension of ൌୖୟ୲ୣ

Therefore, order of reaction ൌଷ

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 7 www.embibe.com Dimension of ൌୖୟ୲ୣ

2. For the reaction:

Solution:

We know

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 8 www.embibe.com 3. The decomposition of ଷ on platinum surface is zero order reaction. What are

Solution:

Given:

The decomposition of ଷ on a platinum surface zero order reaction and showing below.

We know, Rate of reaction ൌെଵ

rate of reaction ൌ

Therefore,

Rate of reaction ൌെଵ

and the reaction rate is given If the pressure is measured in bar and time in minutes, then what are the units of rate and rate constants ?

Solution:

Given:

Mentioned in question the pressure is in bar and time in minutes We know , Rate ൌ [change in pressure ]/ timeൌ [bar]/ time units of rate ൌ bar ିଵ

Class- XII-CBSE-Chemistry Chemical Kinetics

of diethyl ether

5. Mention the factors that affect the rate of a chemical reaction.

Solution:

These are the factors that affect the rate of a chemical reaction

6. A reaction is second order with respect to a reactant. How is the rate of reaction

affected if the concentration of the reactant is (i) doubled (ii) reduced to half ?

Solution:

՜ product

It is mentioned in question reaction is second order with respect to a reactant so Therefor the rate become four time increase as concentration become double

Class- XII-CBSE-Chemistry Chemical Kinetics

Therefor the rate become decrease by ଵ

ସ୲୦ time as concentration become half

7. What is the effect of temperature on the rate constant of a reaction? How can this

temperature effect on rate constant be represented quantitatively?

Solution:

for a chemical reaction. by the Arrhenius equation.

Where ൌ the rate constant,

ൌ Arrhenius factor or the frequency factor, ൌ gas constant, ൌ Temperature and ୟൌ The energy of activation for the reaction

8. In a pseudo first hydrolysis of ester in water, the following results were obtained:

[Ester] (i) Calculate the average rate of reaction between the time interval ͵Ͳ to ͸Ͳ seconds. (ii) Calculated the pseudo first order rate constant for the hydrolysis of ester.

Solution:

(i) At time ͵Ͳ second to ͸Ͳ-second Average rate of reaction between the time interval,

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 11 www.embibe.com (ii) The formula used for pseudo-first-order reaction,

(i) Write the differential rate equation (ii) How is the rate affected on increasing the concentration of three times? (iii) How is the rate affected when the concentrations of both and are doubled?

Solution:

(i) The differential rate equation will be (ii) According to question the concentration of is increased three times, then rate. (iii) According to the question, the concentrations of both and are doubled,

Class- XII-CBSE-Chemistry Chemical Kinetics

different initial concentration of A and B as given below: What is the order of the reaction with respect to A and B?

Solution:

Dividing equation (i) by (ii), we obtain

Dividing equation (iii) by (ii), we obtain

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 13 www.embibe.com From the above calculation, we get the order of the reaction with respect to ൌଷ

and with respect to ൌ zero.

11. The following results have been obtained during the kinetic studies of the

reaction: formation of D/ mol ିଵିଵ Determine the rate law and the rate constant for the reaction.

Solution:

Therefore, rate of the reaction is given by,

According to the question,

Dividing equation (iv) by (i), we obtain

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 14 www.embibe.com Dividing equation (iii) by (ii), we obtain

Therefore, the rate law is

From experiment (I), we get

From experiment (II), we get

From experiment (III), we get

From experiment (IV) we get

12. The reaction between and is first order with respect to and zero order with

Class- XII-CBSE-Chemistry Chemical Kinetics

Solution:

According to question the given reaction is of the first order with respect to and

Therefore, the rate of the reaction is given by,

From experiment II, we get

From experiment II, we get

From experiment III, we get

From experiment IV, we get

Concept Insight: First calculate the Rate constant and then putting each the value in different experiment we can calculate different values.

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 16 www.embibe.com 13. Calculate the half-life of a first order reaction from their rate constants given

below: (i) ʹͲͲௗିଵ (ii) ʹௗିଵ

Solution:

According to question the unit of rate constant is ିଵ (i) Half life ൬భ (ii) Half life, ൬భ (iii) Half life ൬భ

14. The half-life for radioactive decay of ଵସ is 5730 years. An archaeological

artifact containing wood had only 80% of the ଵସ found in a living tree. Estimate the age of the sample.

Solution:

half-life for radioactive decay of ଵସൌͷ͹͵Ͳ years We know radioactive decay reaction is the first order

Here, ൌ଴଺ଽଷ

It is known that,

Class- XII-CBSE-Chemistry Chemical Kinetics

In gas phase at ͵ͳͺ are given below:

t(s) 0 400 800 1200 1600 2000 2400 2800 3200 (ii) Find the half-life period for the reaction. (iv) What is the rate law? (v) Calculate the rate constant. (vi) Calculate the half-life period from and compare it with (ii).

Solution:

(i) (ii) half life: time required for a substance to reduce to half of its initial concentration

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 18 www.embibe.com (iii) straight line. Therefore, the rate law of the reaction is (vi) Half-lift is given by,

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 19 www.embibe.com ଵ graph. to reduce the initial concentration of the reactant to its ଵ ଵ଺୲୦ value?

Solution:

ଵ଺ǡ reduce the initial concentration of the reactant to its

Formula ,

Concept Insight: If the initial concentration is one than the final concentration is If ͳɊ of ଽ଴ was absorbed in the bones of a newly born baby instead of calcium, how much of it will remain after ͳͲ years and ͸Ͳ years and ͸Ͳ years if it is not lost metabolically.

Solution:

ଽ଴ was absorbed in the bones ൌͳɊ

Class- XII-CBSE-Chemistry Chemical Kinetics

Formula,

18. For a first order reaction, show that time required for ͻͻΨ completion is twice the

time required for the completion of ͻͲΨ of reaction.

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 21 www.embibe.com Solution:

Given: order reaction ൌ first

Reaction complete ͻͻΨ then

୩ equation ͳ

Reaction Complete ͻͲΨ then

୩ equation ʹ Hence, first order reaction the time required for ͻͻΨ completion of a is twice the Concept Insight: Let initial concentration be than the final concentration is

Solution:

Given: order of reaction ൌ first

Class- XII-CBSE-Chemistry Chemical Kinetics

Formula for భ

following data are obtained.

Calculate the rate constant.

Solution:

order reaction which is represented by the following equation.

At ൌͲ ଴ Ͳ Ͳ

At ൌ ଴െ

Class- XII-CBSE-Chemistry Chemical Kinetics

Hence, the average value of rate constant is

Note: There is a slight variation in this answer and the one given in the NCERT text book.

21. The following data were obtained during the first order thermal decomposition of

Experiment Time/ିଵ Total pressure/atm

Solution:

following equation.

At ൌͲ ଴ Ͳ Ͳ

At ൌ ଴െ

Class- XII-CBSE-Chemistry Chemical Kinetics

below:

Ȁ୭ 0 20 40 60 80

ͳͲହൈȀିଵ 0.0787 1.70 25.7 178 2140

Solution:

From the given data, we obtain

Class- XII-CBSE-Chemistry Chemical Kinetics

Practice more on Chemical Kinetics Page - 25 www.embibe.com ͳͲହൈ

Slope of the line,

According to Arrhenius equation,

Slope ൌെ୉౗

Again,

In ൌ In െ୉౗

In ൌ In ൅୉౗

When ൌʹ͹͵ǡ

Class- XII-CBSE-Chemistry Chemical Kinetics

When ൌ͵Ͳ൅ʹ͹͵ൌ͵Ͳ͵ǡ

Then, at ଵ

Again, when ൌͷͲ൅ʹ͹͵ൌ͵ʹ͵ǡ

Then, at ଵ

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