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Chapter: 12th Chemistry : Chemical Kinetics

Chemical Kinetics: Important Questions

Chemistry: Chemical Kinetics: Important Questions with Answers, Solution and Explanation

Answer the following questions:

1.        Define average rate and instantaneous rate.

2.        Define rate law and rate constant.

3.        Derive integrated rate law for a zero order reaction A → product .

4.        Define half life of a reaction. Show that for a first order reaction half life is independent of initial concentration.

5.        What is an elementary reaction? Give the differences between order and molecularity of a reaction.

6.        Explain the rate determining step with an example.

7.        Describe the graphical representation of first order reaction.

8.        Write the rate law for the following reactions.

a.        A reaction that is 3/2 order in x and zero order in y.

b.        A reaction that is second order in NO and first order in Br2.

9.        Explain the effect of catalyst on reaction rate with an example.


10.   The rate law for a reaction of A, B and C hasbeenfoundtobe rate = k [ A]2 [B][L]3/2

How would the rate of reaction change when


(i) Concentration of [L] is quadrupled

Solution



(ii) Concentration of both [A] and [B] are doubled

Solution



(iii) Concentration of [A] is halved

Solution



 (iv) Concentration of [A] is reduced to(1/3) and concentration of [L] is quadrupled.

Solution




11.   The rate of formation of a dimer in a second order reaction is 7.5 × 103 mol L1s1 at 0.05 mol L1 monomer concentration. Calculate the rate constant.

Solution

Let us consider the dimerisation of a monomer M

2M → (M)2

Rate= k [M]n

Given that n=2 and [M] = 0.05 mol L-1

Rate = 7.5 X 10-3 mol L-1s-1

k = Rate/[M]n

k = 7.5x10-3/(0.05)2 = 3mol-1Ls-1

 

12.   For a reaction x + y + z products the rate law is given by rate = k [ x ]3/2 [ y ]1/2 what is the overall order of the reaction and what is the order of the reaction with respect to z.

Solution


i.e., second order reaction.

Since the rate expression does not contain the concentration of z , the reaction is zero order with respect to z.

 

13.   Explain briefly the collision theory of bimolecular reactions.

14.   Write Arrhenius equation and explains the terms involved.


15.   The decomposition of Cl2O7 at 500K in the gas phase to Cl2 and O2 is a first order reaction. After 1 minute at 500K, the pressure of Cl2O7 falls from 0.08 to 0.04 atm. Calculate the rate constant in s-1.

Solution


 

16.   Hydrolysis of methyl acetate in aqueous solution has been studied by titrating the liberated acetic acid against sodium hydroxide. The concentration of an ester at different temperatures is given below.

17.   Explain pseudo first order reaction with an example.

18.   Identify the order for the following reactions

(i) Rusting of Iron

(ii) Radioactive disintegration of 92 U238

(iii) 2 A + 3B products ; rate = k [ A]1/2 [B ]2

19.   A gas phase reaction has energy of activation 200 kJ mol-1. If the frequency factor of the reaction is 1.6 × 1013 s1 Calculate the rate constant at 600 K. ( e 40 .09 = 3.8 × 1018 )

Solution


 

20.   For the reaction 2x + y L find the rate law from the following data.


Solution


 

21.   How do concentrations of the reactant influence the rate of reaction?

22.   How do nature of the reactant influence rate of reaction.

23.   The rate constant for a first order reaction is 1.54 × 10-3 s-1. Calculate its half life time.

Solution

We know that, t1/2 = 0.693/ k

t1/2 = 0.693/1.54 x 10-3 s-1 = 450 s

 

24.   The half life of the homogeneous gaseous reaction SO2Cl2 → SO2 + Cl2 which obeys first order kinetics is 8.0 minutes. How long will it take for the concentration of SO2Cl2 to be reduced to 1% of the initial value?

Solution

We know that, k = 0.693/ t1/2

k = 0.693/ 8.0 minutes= 0.087 minutes-1

For a first order reaction,


 

25.   The time for half change in a first order decomposition of a substance A is 60 seconds. Calculate the rate constant. How much of A will be left after 180 seconds?

Solution


 

26.   A zero order reaction is 20% complete in 20 minutes. Calculate the value of the rate constant. In what time will the reaction be 80% complete?

Solution

i) Let A = 100M, [A0]–[A] = 20M,

For the zero order reaction

k=([A0]-[A] / t)

k=(20M / 20min) = 1 Mmin-1

Rate constant for a reaction = 1Mmin-1

ii) To calculate the time for 80% of completion

k = 1Mmin-1, [A0] = 100M, [A0]-[A] = 80M, t = ?

Therefore

t=([A0]-[A] / k) = (80M / 1Mmin-1) = 80min

 

27.   The activation energy of a reaction is 225 k Cal mol-1 and the value of rate constant at 40°C is 1.8 ×105 s1 . Calculate the frequency factor, A.

Solution

Here, we are given that

Ea = 22.5 kcal mol-1 = 22500 cal mol-1

T = 40°C = 40 + 273 = 313 K

k = 1.8 × 10-5 sec-1

Substituting the values in the equation


log A = log (1.8) −5 + (15.7089)

log A = 10.9642

A = antilog (10.9642)

A = 9.208 × 1010 collisions s−1

 

28.   Benzene diazonium chloride in aqueous solution decomposes according to the equation C6H5N2Cl → C6H5Cl + N2 . Starting with an initial concentration of 10 g L1 , the volume of N2 gas obtained at 50 °C at different intervals of time was found to be as under:


Show that the above reaction follows the first order kinetics. What is the value of the rate constant?

Solution

For a first order reaction


In the present case, V∞ = 58.3 ml.

The value of k at different time can be calculated as follows:


Since the value of k comes out to be nearly constant, the given reaction is of the first order. The mean value of k = 0.0676 min-1

 

29.   From the following data, show that the decomposition of hydrogen peroxide is a reaction of the first order:


Where t is the time in minutes and V is the volume of standard KMnO4 solution required for titrating the same volume of the reaction mixture.

Solution


In the present case, Vo = 46.1 ml.

The value of k at each instant can be calculated as follows:


Thus, the value of k comes out to be nearly constant. Hence it is a reaction of the first order.

 

30.   Where t is the time in minutes and V is the volume of standard KMnO4 solution required for titrating the same volume of the reaction mixture.

Solution

i) For the first order reaction k = 2.303/t  log [A0]/[A]

Assume, [A0] = 100 %, t = 50 minutes

Therefore, [A] = 100 – 40 = 60

k = (2.303 / 50) log (100 / 60)

k = 0.010216 min-1

Hence the value of the rate constant is 0.010216 min-1

ii) t = ?, when the reaction is 80% completed,

[A] = 100 – 80 = 20%

From above, k = 0.010216 min-1

 t = (2.303 / 0.010216) log (100 / 20)

t = 157.58 min

The time at which the reaction will be 80% complete is 157.58 min.

 

31.   A first order reaction is 40% complete in 50 minutes. Calculate the value of the rate constant. In what time will the reaction be 80% complete?

 

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