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In some cases, it is found that a large ...

In some cases, it is found that a large number of colliding molecules have energy more than thereshold value, yet the reaction is slow. Why?

Text Solution

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This is because of improper orientation of the colliding molecules at the time of collision.
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(a) The decomposition of A into products has a value of K as 4.5 xx 10^(3)s^(-1) " at "10^(@)C and energy of activation 60kJ "mol"^(-1) . At what temperature would K be 1.5 xx 10^(4) s^(-1) ? (b) (i) If half life period of a first order reaction is x and 3//4^(th) life period of the same reaction is y, how are x 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, yet the reaction is slow. Why?

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Knowledge Check

  • For a certain reaction a large fraction of molecules has energy more than the threshold energy, still the rate of reaction is very slow. The possible reason for this could be that

    A
    the colliding molecules could be large in size
    B
    the colliding molecules must not be properly oriented for effective collisions
    C
    the rate of reaction could be independent of the energy
    D
    one of the reactant cound be in excess.

  • Arrhenius studied the effect of temperature on the rate of a reaction and postulated that rate constant varies with temperature exponentially as k=Ae^(-E_(a)//RT) . For most of the reactions, it was found that the temperature coefficient of the reaction lies between 2 to 3. The method is generally used for finding the activation energy of a reaction. Keeping temperature constant, the effect of catalyst on the activation energy has also been studied by studying how much the rate of reaction changes in the presence of catalyst. In most of the cases, it is observed that catalyst lowers the activation energy barrier and increases the rate of reaction. If x is the fraction of molecules having energy greater than E_(a) , it will be given by

    A
    `x=-(E_(a))/(RT)`
    B
    ln `x=-(E_(a))/(RT)`
    C
    `x=e^(E_(a)//RT)`
    D
    Any of these.

  • Assertion (A) : Catalyst alters the individual energies of the reactants and products Reason (R) : As such more number of molecules are able to cross the barrier per unit time. Delta H or Delta U of the reaction remains unaltered.

    A
    Both A and R are true and R is the correct explanation of A
    B
    Both A and R are true but R is not a correct explanation of A
    C
    A is true but R is false
    D
    A is false but R is true

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