# NCERT Solutions for Class 12 Chemistry Chemistry Part-1 Chapter 4

## Chemical Kinetics Class 12

### In text : Solutions of Questions on Page Number : 98

Q1 :

For the reaction R P, the concentration of a reactant changes from 0.03 M to 0.02 M in 25 minutes. Calculate the average rate of reaction using units of time both in minutes and seconds.

Average rate of reaction  = 6.67 × 10 - 6M s - 1

Q2 :

In a reaction, 2A Products, the concentration of A decreases from 0.5 mol L-1to 0.4 mol L-1in 10 minutes. Calculate the rate during this interval?

Average rate  = 0.005 mol L - 1 min - 1

= 5 ×10 - 3M min - 1

Q3 :

For a reaction, A + B Ã¢â€ ’Product; the rate law is given by, . What is the order of the reaction?

Q4 :

The conversion of molecules X to Y follows second order kinetics. If concentration of X is increased to three times how will it affect the rate of formation of Y?

Q5 :

A first order reaction has a rate constant 1.15 10-3s-1. How long will 5 g of this reactant take to reduce to 3 g?

Q6 :

Time required to decompose SO2Cl2to half of its initial amount is 60 minutes. If the decomposition is a first order reaction, calculate the rate constant of the reaction.

Q7 :

What will be the effect of temperature on rate constant?

Q8 :

The rate of the chemical reaction doubles for an increase of 10 K in absolute temperature from 298 K. Calculate Ea.

Q9 :

The activation energy for the reaction

2HI(g) H2 + I2(g)

is 209.5 kJ mol-1 at 581K. Calculate the fraction of molecules of reactants having energy equal to or greater than activation energy?

### Exercise : Solutions of Questions on Page Number : 117

Q1 :

From the rate expression for the following reactions, determine their order of reaction and the dimensions of the rate constants.

(i) 3 NO(g) Ã¢â€ ’ N2O(g) Rate = k[NO]2

(ii) H2O2 (aq) + 3 I - (aq) + 2 H+Ã¢â€ ’ 2 H2O (l) + Rate = k[H2O2][I - ]

(iii) CH3CHO(g) Ã¢â€ ’ CH4(g) + CO(g) Rate = k [CH3CHO]3/2

(iv) C2H5Cl(g) Ã¢â€ ’ C2H4(g) + HCl(g) Rate = k [C2H5Cl]

Q2 :

For the reaction:

2A + B → A2B

the rate = k[A][B]2with k= 2.0 x 10-6mol-2L2s-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.

Q3 :

The decomposition of NH3on platinum surface is zero order reaction. What are the rates of production of N2and H2if k = 2.5 x 10-4mol-1L s-1?

Q4 :

The decomposition of dimethyl ether leads to the formation of CH4, H2and CO and the reaction rate is given by

Rate = k [CH3OCH3]3/2

The rate of reaction is followed by increase in pressure in a closed vessel, so the rate can also be expressed in terms of the partial pressure of dimethyl ether, i.e., If the pressure is measured in bar andtime in minutes, then what are the units of rate and rate constants?

Q5 :

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

Q6 :

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?

Q7 :

What is the effect of temperature on the rate constant of a reaction? How can this temperature effect on rate constant be represented quantitatively?

Q8 :

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.

Q9 :

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?

Q10 :

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 - 1 s - 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?

Q11 :

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

2A + B Ã¢â€ ’ 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.

Q12 :

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

Q13 :

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

Q14 :

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.

Q15 :

The experimental data for decomposition of N2O5 in gas phase at 318K are given below:

 t(s) 0 400 800 1200 1600 2000 2400 2800 3200 1.63 1.36 1.14 0.93 0.78 0.64 0.53 0.43 0.35

(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)

Q16 :

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 1/16thvalue?

Q17 :

During nuclear explosion, one of the products is 90Sr with half-life of 28.1 years. If 1ÃŽÂ¼g 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.

Q18 :

For a first order reaction, show that time required for 99% completion is twice the time required for the completion of 90% of reaction.

Q19 :

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

Q20 :

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.

Q21 :

The following data were obtained during the first order thermal decomposition of SO2Cl2at a constant volume. 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.

Q22 :

The rate constant for the decomposition of N2O5 at various temperatures is given below:

 T/°C 0 20 40 60 80 0.0787 1.7 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 º and 50 ºC.

Q23 :

The rate constant for the decomposition of hydrocarbons is 2.418 x 10-5 s-1at 546 K. If the energy of activation is 179.9 kJ/mol, what will be the value of pre-exponential factor.

Q24 :

Consider a certain reaction A → Products with k = 2.0 x 10-2 s-1. Calculate the concentration of A remaining after 100 s if the initial concentration of A is 1.0 mol L-1.

Q25 :

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?

Q26 :

The decomposition of hydrocarbon follows the equation

k = (4.5 x 1011 s-1) e-28000 K/T

Calculate Ea.

Q27 :

The rate constant for the first order decomposition of H2O2is given by the following equation:

log k = 14.34 - 1.25 x 104 K/T

Calculate Eafor this reaction and at what temperature will its half-period be 256 minutes?

Q28 :

The decomposition of A into product has value of k as 4.5 x 103 s-1 at 10°C and energy of activation 60 kJ mol-1. At what temperature would k be 1.5 x 104 s-1?

Q29 :

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

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

4 x 1010 s-1. Calculate k at 318 K and Ea.

Q30 :

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.