Important Questions for CBSE Class 12 Chemistry – Kinetics

  

Important Questions for CBSE Class 12 Chemistry – Kinetics

PREVIOUS YEARS’ QUESTIONS

2015

 Short Answer Type Questions   [I] [2 Marks]

Question 1:

Write two differences between order of a reaction and molecuiarity of a reaction.

Answer :

 

Question 2:

Define rate constant (k). Write the unit of rate constant for the following:
(i) First order reaction
(ii) Second order reaction

Answer :

Rate constant is equal to the rate of reaction when molar concentration of reactants is equal to unity.
(i)s-1is the unit of first order rate constant.
(ii) L mol-1 s-1is the unit of second order rate constant.

 

 Short Answer Type Questions   [II] [3 Marks]

Question 3:

A first order reaction takes 10 minutes for 25% decomposition. Calculate t1/2 for the reaction. (Given: log 2 = 0.3010, log 3 = 0.4771, log 4 = 0.6021).

Answer :

 

Question 4:

Therateconstantofafirstorderreactionincreasesfrom 2 X 10-2 to 8 X 10-2 when the temperature changes from 300 K to 320 K. Calculate the energy of activation (Ea). (log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021)

Answer :

 

Question 5:

The rate constant of a first order reaction increases from 2 X 10-2 to 4×10-2  when the temperature change from 300Kto310K. Calculate the energy of activation (Ea). (log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021)log 3 = 0.4771, log 4 = 0.6021)

Answer :

 

Long Answer Type Questions    [5 Marks]

Question 6:

For the hydrolysis of methyl acetate in aqueous solution, the following result were obtained.

(i) Show that it follows pseudo first order reaction, as’the concentration of water remains constant.
(ii) Calculate the average rate of reaction between the time interval 30 to 60 seconds. (Given: log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021)

Answer :

 

Question 7:

(a) For a reaction A + B —> P, the rate is given by Rate = k[A]2 [B]
(i) How is the rate of reaction affected if the concentration of A is doubled?
(ii) What is the overall order of reaction if B is present in large excess?
(b) A first order reaction takes 23.1 minutes for 50% completion. Calculate the time required for 75% completion of this reaction.
(Given: log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021)
Answer :

Question 8:

(a) Define the following terms:
(i) Collision frequency
(ii) Rate constant (k)
(b) The rate constant of a first order reaction increases from 4 X 10-2 to 24 X 10-2 when the temperature changes from 300 K to 350 K. Calculate the energy of activation (JEa). (log 2 = 0.301, log 3 = 0.4771, log 4 = 0.6021, log 6 = 0.7782)

Answer :

(a) (i) Collision frequency: It is defined as the number of collision taking place per second per unit volume.
(ii) Rate Constant: It is equal to the rate of reaction when molar

 

Question 9:

For the hydrolysis of methyl acetate in aqueous solution, the following results were obtained:

Show that it follows pseudo first order reaction, as the concentration of water remains constant.
Calculate the average rate of reaction between the time interval 10 to 20 seconds
(Given: log 2 = 0.3010, log 4 = 0.6021)

Answer :

Refer Ans. to Q.6.

Question 10:
(a) For a reaction A + B » P, the rate is given by Rate = k[A][B]2 
(i) How is the rate of reaction affected if the concentration of B is doubled?

(ii) What is the overall order of reaction if A is present in large excess?
(b) A first order reaction takes 30 minutes for 50% completion. Calculate the time required for 90% completion of this reaction.
(log 2 = 0.3010).

Answer :

(a) (i) If the concentration of ‘B’ is doubled, the rate of reaction becomes four times because it is second order w.r.t ‘B’.
(ii) It ‘A’ is present in large excess, overall order of reaction will be equal to 2 because rate of reaction will become independent of cone, of

Question 11:
For the hydrolysis of methyl acetate in aqueous solution, the following results were obtained:

(a)Show that it follows pseudo first order reaction, as the concentration of water remains constant.
(b)Calculate the average rate of reaction between the time interval 10 to 20 seconds. ‘
(Given: log 2 = 0.3010, log 4 = 0.6021)
Answer :

 

Question 12:
(a) For a reaction A + B —» P, the rate is given by Rate = k [A][B]2 
(i) How is the rate of reaction affected if the concentration of B is doubled?
(ii) What is the overall order of reaction if A is present in large excess?
(b) A first order reaction takes 30 minutes for 50% completion. Calculate the time required for 90% completion of this reaction
Answer :
Refer Ans to Q.10.

Question 13:
(a)Define the following terms:
(i)Activation energy
(ii) Rate constant
(b)A first order reaction takes 10 minutes for 25% decomposition. Calculate t1/2 for the reaction.
(Given: log 2 = 0.3010, log 3 = 0.4771, log 4 = 0.6021)
Answer:
(a) (i) Activation energy is the extra energy which must be supplied to
the reactants so that they can undergo effective collision to form products.
(ii) Refer Ans. to Q.8 (a) (ii)
(b) Refer Ans. to Q.3.

Question 14:
(a) For a chemical reaction R —» P, the variation in the concentration, ln[R] vs. time (s) plot given as
(i) Predict the order of the reaction.
(ii) What is the slope of the line?
(iii) Write the unit of rate constant for this reaction.
(b) Show that the time required for 99% completion is double of the time required for the completion of 90% reaction.

Answer :

 

2014
Short Answer Type Questions [I] [2 Marks]

Question 15:
Define the following terms.
(i) Rate constant (k) (ii) Activation energy ( Ea)
Answer :
(i) Refer Ans. to Q.8 (a) (ii)
(ii) Refer Ans. to Q.13 (a) (i)

 

Question 16:
How does change in temperature affect the rate of reaction? How can this effect the rate constant of the reaction represented quantitatively
Answer :
Rate of reaction increases with increase in temperature. The rate of reaction nearly doubles with 10° rise in temperature as shown in following diagram

Distribution curve showing temperature dependece of a rate of reaction.

 

Question 17:
(a) For a reaction, A + B > Product, the rate law is given by,
Rate = k [A]1 [B]2 . What is the order of reaction?
(b) Write the unit of rate constat ‘k’ for the first order reaction.
Answer :
(a)Third oeder (b)s-1

 

Question 18:
Write two differences between ‘order of reaction’ and ‘molecularity of reaction’.
Answer :
Refer Ans.to Q.1.

 

Question 19:
For a chemical reaction R —> P, the variation in the concentration (R) vs.

(i) Predict the order of the reaction.
(ii) What is the slope of the curve?
Answer :
(a)Zero order reaction
(b) slope = -k

 

Question 20:
Define half-life of a reaction. Write the expression of half-life for
(i) zero order reaction and (ii) first order reaction

Answer :

 

Question 21:
Define the following terms:
(i) Pseudo first order reaction
(ii) Half life period of reaction (t1/2)
Answer :
(i) Pseudo first order reaction: The reaction which is bimolecular but order is one is called pseudo first order reaction.
(ii) Half life period of reaction (t1/2): It is time in which 50% of the reaction is completed, i.e. 50% of reactants changes into products.

Question 22:
Explain the following terms:
(i) Rate constant (k)
(it) Half life period of a reaction (t1/2)
Answer :
(i) Refer Ans. to Q.8 (a) (ii)
(ii) Refer Ans. to Q.21 (ii)

 

Short Answer Type Questions [II] [3 Marks]

Question 23:
The following data were obtained during the first order thermal decomposition of S02Cl2 at a constant volume:
Calculate the rate constant.
(Given: log 4 = 0.6021, log 2 = 0.3010)
Answer :

Short Answer Type Questions [I] [2 Marks]

Question 24:
(а) For a reaction A + B —> P, the rate law is given by
r = k [A1/2 ]  [A]2 
What is the order of this reaction?
(b) A first order reaction is found to have a rate constant k = 5.5 X 10-14 s-1. Find the half life of the reaction.
Answer :

Question 25:
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 1/10 th of its initial value?
Answer :

 

Question 26:
(i) If half life period of first order reaction is x and 3/4 th life period of same reaction is ’y’, how are V and ’y’ related to each other?
(ii) In some cases it is found that a large number of colliding molecules have energy more than threshold energy but yet the reaction is slow. Why?
Answer :

 

Question 27:
With the help of diagram explain the role of activated complex in a reaction.
Answer :

Activated complex is intermediate compound between reactants and products as shown in the diagram. It is highly unstable as it has highest energy. It readily changes into product. Those molecules which can form activated complex, can lead to formation of products, e.g.

Short Answer Type Questions [III] [2 Marks]

Question 28:

The rate of a reaction becomes four times when the temperature changes from 293 K to 313 K. Calculate the energy of activation (Ea) of the reaction assuming that it does not change with temperature.
[R = 8.314 J K-1 mol-1, log 4 = 0.6021]

Answer :

Question 29:

For a chemical reaction R -» P, the variation in the concentration ln[R] vs. time t plot is given as For this reaction
(i)What is the order of the reaction?
(ii) What is the slope of the line?
(iii) What is the unit of rate constant ‘k’?

Answer :

Question 30:

(a) A reaction is second order in A and first order in B.
(i) Write the differential rate equation.
(ii) How is the rate affected on increasing the concentration of A three times?
(iii) How is the rate affected when the concentrations of both A and B are doubled?
(b) A first order reaction takes 40 minutes for 30% decomposition. Calculate t1/2 for this reaction.

(Given: log 1.428 = 0.1548)

Answer :

Question 31:

Answer :

2012

Short Answer Type Questions [I]  [2 Marks]

Question 32:

Answer :

Question 33:

A reaction is of second order with respect to a reactant. How is its rate affected if the concentration of the reactant is (i) doubled (ii) reduced to
Answer :

Question 34:

Define the following terms:
(i) Order of a reaction.
(ii) Activation energy of a reaction.
Answer :
(i) The order of reaction is defined as the sum of powers to which concentration terms are raised in rate law or rate equation.
(ii) Refer Ans. to Q.13 (a) (i)

Question 35:

Answer :

Question 36:

Answer :

 

Question 37:

Answer :

 

2011
Very Short answer Type Questions [I][2 Marks]

Question 38:
Define order of a reaction.
Answer :
Refer Ans. to Q.34 (i)

 

Question 39:
Define ‘activation energy’ of a reaction.
Answer :
Refer Ans. to Q.13 (a) (i)

 

Short answer Type Questions [I][2 Marks]

Question 40:

Answer :
Ref Ans.to.Q.32.

 

Question 41:
Distinguish between ‘rate expression’ and rate constant’ of a reaction.
Answer :

 

Question 42:
Rate constant is equal to the rate of reaction when molar cone, of reactants is unity. Its unit depends upon order of reaction.
A reactant has a half-life of 10 minutes.
(i) Calculate the rate constant for the first order reaction.
(ii) What fraction of the reactant will be left after an hour of the reaction has occurred?
Answer :

 

Question 43:
A reaction is of second order with respect to a reactant. How is the rate of reaction affected if the concentration of the reactant is reduced to half? What is the unit of rate constant for such a reaction?
Answer :

Question 44:

Answer :

 

Question 45:

Answer :

Question 46:

Answer :

 

Question 47:
Explain the terms:
(i) Rate determining step of a reaction
(ii) Molecularity of a reaction
Answer :

 

Question 48:
What are pseudo first order reactions? Give one example of such reactions.
Answer :

 

Question 49:

Answer :

 

Short Answer Type Questions [II] [3 Marks]

Question 50:

Answer :

 

Question 51:

Answer :


Question 52:

Answer :

2010
Very Short Answer Type Questions[1 Mark]

Question 53:

Answer :
It is a second oreder reaction
Question 54:
Why does the rate of a reaction not remain constant throughout the reaction
process?
Answer :
It is because the concentration of reactants goes on decreasing with time,therefore, rate of reaction decreases with time and not remain constant.

Question 55:

Answer :
Ref Ans to Q.32
Question 56:
A reaction is of first order in reactant A and of second order in reactant B. How is the rate of this reaction affected when
(i) the concentration of B alone is increased to three times,
(ii) the concentrations of A as well as B are doubled?
Answer :

 

Question 57:

Answer :

Question 58:
Define:
(i)Elementary reaction in a process (ii) Rate of a reaction.
Answer :
(i)Elementary reaction in a process: Some reactions occur by a series of elementary steps and such simple steps are called elementary reactions in a process.
(ii)Rate of a reaction: It is defined as the change in concentration of reactant (or product) per unit time.

Question 59:
Show that for a first order reaction, the time required for half the change (half-life period) is independent of initial concentration.
Answer :

Question 60:

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Question 61:

Answer :

Question 62:

Answer :

Question 63:

Answer :

2009
Very Short Answer Type Question [1 Mark]

Question 64:
Why does the rate of a reaction not remain constant throughout the reaction
process?
Answer :
Refer Ans. to Q.54.
Question 65:
A reaction is of second order with respect to a reactant. How is its rate affected if the concentration of the reactant is (i) doubled (ii) reduced to half?
Answer :
Refer Ans. to Q.33.
Question 66:
Define the following terms:
(i) Order of a reaction.
(ii) Activation energy of a reaction.
Answer :
Refer Ans. to Q.34.
Question 67:
Define:
(i) Elementary reaction in a process
(ii) Rate of a reaction
Answer :
Refer Ans. to Q.58.
Question 68:

Answer :

Question 69:
A first order reaction has a rate constant of 0.0051 min-1. If we begin with 0.10 M concentration of the reaction, what concentration of reactant will remain in solution after 3 hours?
Answer :
Refer Ans. to Q.52 (b)

 

Question 70:


Answer :
Ref Ans Q.60(b)
Question 71 :

Answer :