chemistry

CHEMICAL KINETICS NCERT QUESTIONS
Exercises

4.1 From the rate expression for the following reactions, determine their

order of reaction and the dimensions of the rate constants.

(i) 3NO(g) → N2O (g) Rate = k[NO]2

(ii) H2O2 (aq) + 3I– (aq) + 2H+ → 2H2O (l) + 3 I− Rate = k[H2O2][I-]

(iii) CH3CHO (g) → CH4 (g) + CO(g) Rate = k [CH3CHO]3/2

(iv) C2H5Cl (g) → C2H4 (g) + HCl (g) Rate = k [C2H5Cl]

4.2 For the reaction:

2A + B → A2B

the rate = k[A][B]2 with k = 2.0 × 10–6 mol–2 L2 s–1. Calculate the initial

rate of the reaction when [A] = 0.1 mol L–1, [B] = 0.2 mol L–1. Calculate

the rate of reaction after [A] is reduced to 0.06 mol L–1.

4.3 The decomposition of NH3 on platinum surface is zero order reaction. What

are the rates of production of N2 and H2 if k = 2.5 × 10–4 mol–1 L s–1?

4.4 The decomposition of dimethyl ether leads to the formation of CH4, H2

and CO and the reaction rate is given by

Rate = k [CH3OCH3]3/2

4.6 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 ?

4.7 What is the effect of temperature on the rate constant of a reaction? How can

this temperature effect on rate constant be represented quantitatively?

4.8 In a pseudo first order hydrolysis of ester in water, the following results were

obtained:

t/s 0 30 60 90

[Ester]/mol L–1 0.55 0.31 0.17 0.085

(i) Calculate the average rate of reaction between the time interval 30 to

60 seconds.

(ii) Calculate the pseudo first order rate constant for the hydrolysis of ester.

4.9 A reaction is first order in A and second order in B.

(i) Write the differential rate equation.

(ii) How is the rate affected on increasing the concentration of B three times?

(iii) How is the rate affected when the concentrations of both A and B are

doubled?

4.10 In a reaction between A and B, the initial rate of reaction (r0) was measured

for different initial concentrations of A and B as given below:

A/ mol L–1 0.20 0.20 0.40

B/ mol L–1 0.30 0.10 0.05

r0/mol L–1s–1 5.07 × 10–5 5.07 × 10–5 1.43 × 10–4

What is the order of the reaction with respect to A and B?

4.11 The following results have been obtained during the kinetic studies of the reaction:

2A + B → C + D

Experiment [A]/mol L–1 [B]/mol L–1 Initial rate of formation

of D/mol L–1 min–1

I 0.1 0.1 6.0 × 10–3

II 0.3 0.2 7.2 × 10–2

III 0.3 0.4 2.88 × 10–1

IV 0.4 0.1 2.40 × 10–2

Determine the rate law and the rate constant for the reaction.

4.12 The reaction between A and B is first order with respect to A and zero order

with respect to B. Fill in the blanks in the following table:

Experiment [A]/ mol L–1 [B]/ mol L–1 Initial rate/

mol L–1 min–1

I 0.1 0.1 2.0 × 10–2

II – 0.2 4.0 × 10–2

III 0.4 0.4 –

IV – 0.2 2.0 × 10–2

4.13 Calculate the half-life of a first order reaction from their rate constants

given below:

(i) 200 s–1 (ii) 2 min–1 (iii) 4 years–1

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

artifact containing wood had only 80% of the 14C found in a living tree.

Estimate the age of the sample.

4.15 The experimental data for decomposition of N2O5

[2N2O5 → 4NO2 + O2]

in gas phase at 318K are given below:

t/s 0 400 800 1200 1600 2000 2400 2800 3200

102 × [N2O5]/ 1.63 1.36 1.14 0.93 0.78 0.64 0.53 0.43 0.35

mol L–1

(i) Plot [N2O5] against t.

(ii) Find the half-life period for the reaction.

(iii) Draw a graph between log[N2O5] and t.

(iv) What is the rate law ?

(v) Calculate the rate constant.

(vi) Calculate the half-life period from k and compare it with (ii).

4.16 The rate constant for a first order reaction is 60 s–1. How much time will

it take to reduce the initial concentration of the reactant to its 1/16th

value?

4.17 During nuclear explosion, one of the products is 90Sr with half-life of

28.1 years. If 1μg of 90Sr was absorbed in the bones of a newly born

baby instead of calcium, how much of it will remain after 10 years and

60 years if it is not lost metabolically.

4.18 For a first order reaction, show that time required for 99% completion

is twice the time required for the completion of 90% of reaction.

4.19 A first order reaction takes 40 min for 30% decomposition. Calculate t1/2.

4.20 For the decomposition of azoisopropane to hexane and nitrogen at 543

K, the following data are obtained.

t (sec) P(mm of Hg)

0 35.0

360 54.0

720 63.0

Calculate the rate constant.

4.21 The following data were obtained during the first order thermal

decomposition of SO2Cl2 at a constant volume.

SO2Cl2 (g) → SO2 (g) + Cl2 (g)

Experiment Time/s–1 Total pressure/atm

1 0 0.5

2 100 0.6

Calculate the rate of the reaction when total pressure is 0.65 atm.

4.22 The rate constant for the decomposition of N2O5 at various temperatures

is given below:

T/°C 0 20 40 60 80

105 × k/s-1 0.0787 1.70 25.7 178 2140

Draw a graph between ln k and 1/T and calculate the values of A and Ea.

Predict the rate constant at 30° and 50°C.

4.23 The rate constant for the decomposition of hydrocarbons is 2.418 × 10–5s–1

at 546 K. If the energy of activation is 179.9 kJ/mol, what will be the value

of pre-exponential factor.

4.24 Consider a certain reaction A → Products with k = 2.0 × 10–2s–1. Calculate

the concentration of A remaining after 100 s if the initial concentration

of A is 1.0 mol L–1.

4.25 Sucrose decomposes in acid solution into glucose and fructose according

to the first order rate law, with t1/2 = 3.00 hours. What fraction of sample

of sucrose remains after 8 hours ?

4.26 The decomposition of hydrocarbon follows the equation

k = (4.5 × 1011s–1) e-28000K/T

Calculate Ea.

4.27 The rate constant for the first order decomposition of H2O2 is given by the

following equation:

log k = 14.34 – 1.25 × 104K/T

Calculate Ea for this reaction and at what temperature will its half-period

be 256 minutes?

4.28 The decomposition of A into product has value of k as 4.5 × 103 s–1 at 10°C

and energy of activation 60 kJ mol–1. At what temperature would k be

1.5 × 104s–1?

4.29 The time required for 10% completion of a first order reaction at 298K is

equal to that required for its 25% completion at 308K. If the value of A is

4 × 1010s–1. Calculate k at 318K and Ea.

4.30 The rate of a reaction quadruples when the temperature changes from

293 K to 313 K. Calculate the energy of activation of the reaction assuming

that it does not change with temperature.