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Chemical Kinetics Test - 20

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Chemical Kinetics Test - 20
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  • Question 1
    1 / -0

    The rate of a reaction is expressed in different ways as follows :

     = 

    The reaction is :

    Solution

    B + 3D = 2C + 4A

  • Question 2
    1 / -0

    The rate constant for the forward reaction A(g)  2B(g) is 1.5 × 10-3 s-1 at 100 K. If 10-5 moles of A and 100 moles of B are present in a 10 litre vessel at equilibrium then rate constant for the backward reaction at this temperature is

    Solution


    Kf = = 1.5 × 10–3 s–1
    At eq.m, Rf = Rb
    Kf  [A] = Kb[B]2
    On solving : Kb = 1.5 × 10-11

  • Question 3
    1 / -0

    Reaction A + B → C + D follow's following rate law : rate = k[A]+1/2[B]1/2. Starting with initial conc. of 1 M of A and B each, what is the time taken for concentration of A of become 0.25 M.

    Given : k = 2.303 × 10-3 sec-1.

    Solution

    Rate= K[A]1/2[B]1/2 

    Overall order of reaction is = 1/2+1/2=1

    ∴ It is a 1st order reaction.

     

    For concentration of A to become 0.5M:-t1/2​=0.693​/k

     

    It will require 2 half times for the concentration of A to become .0.25M.

    ∴ Time taken= 2×t1/2 sec

    ​=2×300

    =600sec

  • Question 4
    1 / -0

    Consider the following first order competing reactions :

    X  A+ B      and y  C+D

    if 50% of the reaction of X was completed when 96% of the reaction of Y was completed, the ratio of their rate constants   is

    Solution


  • Question 5
    1 / -0

    Units of rate constant for first and zero order reactions in terms of molarity (M) are respectively.

    Solution

    sec–1, Msec–1

  • Question 6
    1 / -0

    The rate of a chemical reaction doubles for every 10°C rise of temperature. If the temperature is raised by 50°C, the rate of the reaction increases by about

    Solution

    For every 10°C rise of temperature, the rate is doubled. Thus, the temperature coefficient of the reaction = 2
    When temperature is increased by 50°, rate becomes
    =2(50/10) = 25 times = 32 times

  • Question 7
    1 / -0

    For a reaction pA + qB → products, the rate law expression is r = k[A]l[B]m, then :

    Solution

    pA + qB  → Products r = k[A]l[B]m
    (p + q) may or may not be equal to (l + m) (order may or may not be equal to stoichiometric coefficient for a complex reaction.

  • Question 8
    1 / -0

    In the reaction : A + 2B → 3C + D, which of the following expression does not describe changes in the conc. of various species as a function of time :

    Solution

  • Question 9
    1 / -0

     A first order reaction is 87.5% complete in an hour. The rate constant of the reaction is

    Solution

    The correct answer is Option A.
         KT  =   ln (a0 / at)
    ⇒ K*60 = ln (a0 / at) 
    87.5 % completed in 1 hour.
    Therefore, at = a0 - 0.875 a0
                        at = 0.125 a0
         K * 60 = ln (a0 / 0.125 a0)
    ⇒ K * 60 = ln 8
    ⇒ K = 2.079 / 60
    ⇒ K = 0.0346 min-1

  • Question 10
    1 / -0

    Half-life period of a second order reaction is

    Solution

    The correct answer is Option B.
     
    Relation between half-life period of a reaction and initial concentration is as follows: 
    For first order reaction         (Half life α a0 )
    For second order reaction    (Half life ∝ 
    1/a)
    For third order reaction         (Half life ∝ 1a2)

  • Question 11
    1 / -0

    A first order reaction is 50% completed in 20 minutes at 27°C and in 5 min at 47°C. The energy of activation of the reaction is

    Solution

    t1/2 = 20 min at 300 K
    t’1/2 = 5 min at 320 K



    On solving, Ea = 55303.12 J
    = 55.3 KJ

  • Question 12
    1 / -0

    For the first order reaction A → B + C, carried out at 27°C if 3.8 × 10-16% of the reactant molecules exists in the activated state, the Ea (activation energy) of the reaction is

    Solution

    A → B + C

    Ea = 100 KJ/mol

  • Question 13
    1 / -0

    The rate constant, the activation energy and the Arrhenius parameter (A) of a chemical reaction at 25°C are 3.0 × 10-4 s-1, 104.4 kJ mol-1 and 6.0 × 10-4s-1respectively. The value of the rate constant at T → ∞ is

    Solution

    At temperature = ∞
    Rate constant = Arrhenious constant.

  • Question 14
    1 / -0

    The following mechanism has been proposed for the exothermic catalyzed complex reaction.

    If k1 is much smaller than k2. The most suitable qualitative plot of potential energy (P.E.) versus  reaction coordinate for the above reaction.

    Solution


    Since
    K1 <<< K2 = most Imp. peack will be higher

  • Question 15
    1 / -0

    The activation energy of a reaction at a given temperature is found to be 2.303 RT J mol–1. The ratio of rate constant to the Arrhenius factor is

    Solution

    The correct answer is option A
    Arrhenius equation is,
    rate constant, k=Ae−Ea​/RT
    k=Ae−2.303RT/RT ⇒ k/A = e-2.303
    On solving, we get
    k/A = 10-1 or 0.1

  • Question 16
    1 / -0

    In a reaction, the thershold energy is equal to :

    Solution

    In a reaction, the threshold energy is equal to : Activation energy + Normal energy of reactants.

  • Question 17
    1 / -0

    Consider  + heat, If activation energy for forward reaction is 100 kJ/mole then activation energy for backward reaction and heat of reaction is :

    Solution


    Eaf = 100 KJ/mol
    Eab = ? heat of reaction  = ?

    Only one option because
    Eb = Ea + ΔH.O. reaction

  • Question 18
    1 / -0

    The first order rate constant k is related to temperature as log k = 15.0 - (106/T). Which of the following pair of value is correct ?

    Solution


    log A = 15
    ⇒ A = 1015

    Ea = 1.9 × 104  KJ

  • Question 19
    1 / -0

    When a graph between log K and 1/T is drawn a straight line is obtained. The temperature at which line cuts y-axis and x-axis.

    Solution


    When line cuts y axis

    When it cut  X -axis
    log K = 0

  • Question 20
    1 / -0

    The rate constant, the activation energy and the frequency factor of a chemical reaction at 25°C are 3.0 × 10-2 s-1, 104.4 KJ mol-1 and 6.0 × 1014 s-1 respectively. The value of the rate constant as T → ∞ is :

    Solution

    K = 3 × 10–2 s–1
    Ea = 104.4 KJ/mol
    A = 6 × 1014
    Value of rate constant at T = ∞ will be equal to frequency factor  i.e. A = 6 × 1014 s-1

  • Question 21
    1 / -0

    The rate data for the net reaction at 25°C for the reaction X + 2Y → 3Z are given below :

    [X0]   [Y0]   Time required for [Z] to increase by 0.005 mol per litre.

    0.01   0.01   72 sec

    0.02   0.005  36 sec

    0.02   0.01    18 sec

    The intial rate (as given by Z) is :

    Solution


    Since,
    Rate becomes four times by doubling the concentration of A
    ⇒ Order w.r.t  A is 2
    Also,
    by doubling the concentration of B, the rate becomes double
    ⇒ Order w.r.t. B is 1.

  • Question 22
    1 / -0

    The rate of production of NH3 in N2 + 3H2 → 2NH3 is 3.4 kg min-1. The rate of consumption of H2 is :

    Solution


    = 5.1 Kgmin-1

  • Question 23
    1 / -0

    For a given reaction of first order it takes 20 min. for the conc. to drop from 1.0 M to 0.60 M. The time required for the conc. to drop from 0.60 M to 0.36 M will be :

    Solution


    Also,

    Since, Rate constant will be equal

    On solving, t = 20 min

  • Question 24
    1 / -0

    For a first order reaction, the concentration of reactant :

    Solution

    For a first order reaction, the concentration of the reactant varies exponentially with time (A = A0e–kt)

  • Question 25
    1 / -0

    Radioactivity of a sample (z = 22) decreases 90% after 10 years. What will be the half-life of the sample?

    Solution

    Reaction is or zero order hence, Option C will be correct.

  • Question 26
    1 / -0

    Mathematical representation for t1/4 life for first order reaction is over is given by :

    Solution

    t1/4 = time taken for   decomposition.

  • Question 27
    1 / -0

    For a reaction A → Products, the conc. of reactant C0, aC0, a2C0, a3C0............ after time interval 0, t, 2t ............ where 'a' is constant. Then :

    Solution

    If we calculate K, it comes constant every time i.e. reaction is of first order.

  • Question 28
    1 / -0

     Half-life is independent of conc. of A. After 10 minutes volume N2 gas is 10 L and after complete reaction 50 L. Hence rate constant in min-1 :

    Solution


    Half life is independent of concentration
    ⇒ reaction is of first order


  • Question 29
    1 / -0

    In a zero-order reaction for every 10° rise of temperature, the rate is doubled. If the temperature is increased from 10°C to 100°C, the rate of the reaction will become 

    Solution

    For rise in temperature, n=1
    Therefore rate = 2n
    = 21
    = 2
    When temperature is increased from 10o C to 100oC    
    change in temperature = 100 − 10 = 90oC 
    therefore n=9 therefore, rate = 29
    = 512 times.

  • Question 30
    1 / -0

    In acidic medium the rate of reaction between (BrO3)- & Br- ions is given by the expression, –[d(BrO3-) /dt] = K[BrO3-][Br-][H +]2 It means :

    Solution


    D option is correct.

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