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NEET-XII-Chemistry

04: Chemical Kinetics

 page 3
 
  • Qstn #9-ii
    How is the rate affected on increasing the concentration of B three times?
    Ans : If the concentration of B is increased three times, then



    Therefore, the rate of reaction will increase 9 times.
  • Qstn #9-iii
    How is the rate affected when the concentrations of both A and B are doubled?
    Ans : When the concentrations of both A and B are doubled,



    Therefore, the rate of reaction will increase 8 times.
  • Qstn #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-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?
    Ans : Let the order of the reaction with respect to A be x and with respect to B be y.

    Therefore,



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



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



    = 1.496

    = 1.5 (approximately)

    Hence, the order of the reaction with respect to A is 1.5 and with respect to B is zero.
  • Qstn #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.
    Ans : Let the order of the reaction with respect to A be x and with respect to B be y.

    Therefore, rate of the reaction is given by,



    According to the question,



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



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



    Therefore, the rate law is

    Rate = k [A] [B]2



    From experiment I, we obtain



    = 6.0 L2 mol-2 min-1

    From experiment II, we obtain



    = 6.0 L2 mol-2 min-1

    From experiment III, we obtain



    = 6.0 L2 mol-2 min-1

    From experiment IV, we obtain



    = 6.0 L2 mol-2 min-1

    Therefore, rate constant, k = 6.0 L2 mol-2 min-1
  • Qstn #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


    Ans : The given reaction is of the first order with respect to A and of zero order with respect to B.

    Therefore, the rate of the reaction is given by,

    Rate = k [A]1 [B]0

    ⇒ Rate = k [A]

    From experiment I, we obtain

    2.0 × 10-2 mol L-1 min-1 = k (0.1 mol L-1)

    ⇒ k = 0.2 min-1

    From experiment II, we obtain

    4.0 × 10-2 mol L-1 min-1 = 0.2 min-1 [A]

    ⇒ [A] = 0.2 mol L-1

    From experiment III, we obtain

    Rate = 0.2 min-1 × 0.4 mol L-1

    = 0.08 mol L-1 min-1

    From experiment IV, we obtain

    2.0 × 10-2 mol L-1 min-1 = 0.2 min-1 [A]

    ⇒ [A] = 0.1 mol L-1
  • Qstn #13
    Calculate the half-life of a first order reaction from their rate constants given below:
  • Ans : Half life,



    = 3.47

    ×10 -3 s (approximately)
  • Ans : Half life,



    = 0.35 min (approximately)
  • Qstn #13-iii
    4 years-1
    Ans : Half life,



    = 0.173 years (approximately)
  • Qstn #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.
    Ans : Here,





    It is known that,



    = 1845 years (approximately)

    Hence, the age of the sample is 1845 years.
  • Qstn #15
    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


  • Qstn #15-ii
    Find the half-life period for the reaction.
    Ans : Time corresponding to the concentration, is the half life. From the graph, the half life is obtained as 1450 s.
  • Qstn #15-iii
    Draw a graph between log [N2O5] and t.
    Ans :



    t(s)

    0
    		 1.63
    		 - 1.79
    400
    		 1.36
    		 - 1.87
    800
    		 1.14
    		 - 1.94
    1200
    		 0.93
    		 - 2.03
    1600
    		 0.78
    		 - 2.11
    2000
    		 0.64
    		 - 2.19
    2400
    		 0.53
    		 - 2.28
    2800
    		 0.43
    		 - 2.37
    3200
    		 0.35
    		 - 2.46




  • Qstn #15-iv
    What is the rate law?
    Ans : The given reaction is of the first order as the plot, v/s t, is a straight line. Therefore, the rate law of the reaction is


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