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Evaluation Test (Class 12) - Chemical Kinetics

Question 1 to 5 carries 1 mark each, from 6 to 9 carries 2 marks each, from 10 to 13 carries 3 marks each and question 14 carries 5 marks
Total Marks: 30                                                                                   Duration: 1 hour
  1. The reaction \ceA+B>C has zero order. What is the rate of equation? ...
    Solution
    Rate[A][B]

  2. For a reaction \ceA+H2O>B , Rate[A]. What is its (i) molecularity (ii) order of reaction? ...
    Solution
    Order= 1, Molecularity= 2


  3. When is the rate of reaction equal to specific reaction rate? ...
    Solution
    When concentration of each of the reactant is unity, the rate of reaction becomes specific rate.


  4. Why instantaneous rate is preferred over average rate? ...
    Solution
    The rate of reaction at any time depends upon one of the reactants at that time which is not constant but goes on decreasing with time continuously, so that instantaneously rate gives more correct information than average rate.


  5. For the reaction \ce2X>X2, the rate of reaction becomes three times when the concentration of X is increased 27 times. What is the order of reaction? ...
    Solution
    3


  6. (a) The rate of reaction in the remaining mixture is not affected when a portion is removed for analysis of products, why? (b) Even an exothermic reaction has an activation energy, why? ...
    Solution
    (a) Because the rate of reaction depends upon concentration which remains constant even if a portion is removed for analysis.
    (b) Every reaction needs activation energy and it is required so that reactant can move together, can overcome forces of repultion and start breaking bond.


  7. (a) Give one example of a reaction having identical values of molecularity and order of reaction. (b) What factors determine whether a collision between two molecules will lead to a chemical reaction? ...
    Solution
    (a) Decomposition of hydrogen peroxide
    \ceH2O2>H2O+12O2.
    for this reaction order and molecularity is identical which is one.
    (b) Temperature and shape of molecule is responsible for effective collision..


  8. Two reactions have identical values of Ea. Dos this ensure that they will have same rate constant if run at the same temperature? Explain. ...
    Solution
    No,according to Arhenius equation, k=AeEaRT rate constant will depend on A,EaandT as well.


  9. "The increase in reaction rate with rise in temperature is due to increase in the number of collisions." Is this statement correct? If not, correct then comment. ...
    Solution
    Yes, this is correct according to collision theory, because with increase in temperature number of collision of molecules also get increase. Further, increase in concentration of reactant increase frequency of collision and so that increase reaction rate.


  10. For a chemical reaction \ceA>B, the change in concentration of A in 40s is -0.004 mol/litre. What is the rate of chemical reaction? ...
    Solution
    Change in conc= 0.004molL1
    Change in time = 40 sec
    Rateofchemicalreaction=ChangeinconcChangeintime
    =0.004molL140sec=1×104molL1s1


  11. For a reaction, the energy of activation is zero. What is the value of rate constant at 370 K, if k=1.5×104s1at370K?R=8.31JK1mol1 ...
    Solution
    According to Arhenius equation
    logk2k1=Ea2.303RT2T1T1T2
    As Ea=0andk2k1=1
    =>k1=k2=1.5×104s1


  12. Rate constant for first order reaction is 5.70×105. What percentage of initial reaction will react in 5 hours? ...
    Solution
    k=5.70×105,t=5h
    Let initial conc = 100 , Change in conc = x
    k=2.303tlogaax
    5.70×105=2.3035×3600×log100100x
    on solving , we get x=64.66
    so percentage of initial concentration of amount reacted is 64.66%


  13. The pressure of a gas decomposing at the surface of a solid catalyst has been measured at different times and results are given below:
  14. Per second0100200300
    Pressure (Pascals)4.0×1033.5×1031.0×1032.5×103
    Determine the order of reaction. ...
    Solution
    This reaction is zero order
    As it is zero order, then k=1t×([A]0[A])
    in this case [A]0=4.0×103

    Per secondPressure (Pascals)k=1t×([A]0[A])
    1003.5×1035PaSec1
    2001.0×1035PaSec1
    3002.5×1035PaSec1

    As the value of k is constant, therefore the reaction is of zero order and its rate constant is k=5PaSec1

  15. (a) What is the difference between a unimolecular and a bimolecular elementry step? (b) If 1 % of the reactant decomposes in first minute in a first order reaction, how much of it will remain undecomposed after one hour?
    ...
    Solution
    (a) Unimolecular elementary steps involves only one reactant and bimolecular elementary step involves two reactants.
    (b) for the first order reaction
    t=2.303klogaax
    initially, t=1min,k=?,a=100,ax=99
    then value of k is
    k=2.303tlogaax
    k=2.3031log10099
    k=0.010min1
    now to calculate the amount of reactant that would remain undecomposed
    60min=2.3030.010log100ax
    log100ax=0.26
    ax=55.5
    So, 55.5% of reactant is remains undecomposed after 1 hour.

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