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NEET-XII-Chemistry

04: Chemical Kinetics

 
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    4 - Chemical Kinetics
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    Section : I
    SECTION I Page No 98:
  • Qstn #1
    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.
    Ans : Average rate of reaction



    = 6.67 × 10-6 M s-1
  • Qstn #2
    In a reaction, 2A → Products, the concentration of A decreases from 0.5 mol L-1 to 0.4 mol L-1 in 10 minutes. Calculate the rate during this interval?
    Ans : Average rate



    = 0.005 mol L-1 min-1

    = 5 × 10-3 M min-1

    SECTION I SECTION I Page No 103:
  • Qstn #3
    For a reaction, A + B → Product; the rate law is given by,. What is the order of the reaction?
    Ans : The order of the reaction



    = 2.5
  • Qstn #4
    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?
    Ans : The reaction X → Y follows second order kinetics.

    Therefore, the rate equation for this reaction will be:

    Rate = k[X]2 (1)

    Let [X] = a mol L-1, then equation (1) can be written as:

    Rate1 = k .(a)2

    = ka2

    If the concentration of X is increased to three times, then [X] = 3a mol L-1

    Now, the rate equation will be:

    Rate = k (3a)2

    = 9(ka2)

    Hence, the rate of formation will increase by 9 times.

    SECTION I SECTION I Page No 111:
  • Qstn #5
    A first order reaction has a rate constant 1.15 10-3 s-1. How long will 5 g of this reactant take to reduce to 3 g?
    Ans : From the question, we can write down the following information:

    Initial amount = 5 g

    Final concentration = 3 g

    Rate constant = 1.15 10-3 s-1

    We know that for a 1st order reaction,



    = 444.38 s

    = 444 s (approx)
  • Qstn #6
    Time required to decompose SO2Cl2 to half of its initial amount is 60 minutes. If the decomposition is a first order reaction, calculate the rate constant of the reaction.
    Ans : We know that for a 1st order reaction,



    It is given that t1/2 = 60 min



    SECTION I SECTION I Page No 116:
  • Qstn #7
    What will be the effect of temperature on rate constant?
    Ans : The rate constant of a reaction is nearly doubled with a 10° rise in temperature. However, the exact dependence of the rate of a chemical reaction on temperature is given by Arrhenius equation,



    Where,

    A is the Arrhenius factor or the frequency factor

    T is the temperature

    R is the gas constant

    Ea is the activation energy
  • Qstn #8
    The rate of the chemical reaction doubles for an increase of 10 K in absolute temperature from 298 K. Calculate Ea.
    Ans : It is given that T1 = 298 K

    ∴T2 = (298 + 10) K

    = 308 K

    We also know that the rate of the reaction doubles when temperature is increased by 10°.

    Therefore, let us take the value of k1 = k and that of k2 = 2k

    Also, R = 8.314 J K-1 mol-1

    Now, substituting these values in the equation:



    We get:





    = 52897.78 J mol-1

    = 52.9 kJ mol-1

    Note: There is a slight variation in this answer and the one given in the NCERT textbook.
  • Qstn #9
    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?
    Ans : In the given case:

    Ea = 209.5 kJ mol-1 = 209500 J mol-1

    T = 581 K

    R = 8.314 JK-1 mol-1

    Now, the fraction of molecules of reactants having energy equal to or greater than activation energy is given as:

    x=e-Ea/RT⇒Inx=-EaRT⇒logx=-Ea2.303RT⇒logx=-209500Jmol-12.303×8.314JK-1mol-1×581=-18.8323Now,x=Antilog-18.8323=1.471×10-19
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    Section : II
    SECTION I Page No 117:
  • Qstn #1
    From the rate expression for the following reactions, determine their order of reaction and the dimensions of the rate constants.
  • Qstn #1-i
    3 NO(g) → N2O (g) Rate = k[NO]2
    Ans : Given rate = k [NO]2

    Therefore, order of the reaction = 2

    Dimension of


  • Qstn #1-ii
    H2O2 (aq) + 3 I- (aq) + 2 H+ → 2 H2O (l) + Rate = k[H2O2][I-]
    Ans : Given rate = k [H2O2] [I-]

    Therefore, order of the reaction = 2

    Dimension of


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