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1 July, 12:01

If the kpseudo of the reaction of crystal violet with a substantially larger amount of naoh (0.1 m) is 0.5 s-1 what is k for the overall reaction? (show inverse units as unit^-1)

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  1. 1 July, 13:20
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    5 s-1

    Explanation:

    In order to explain the meaning of pseudo rate constant, consider the following;

    Let's say we have a rate law, Rate = k[A][B]. k is a second order rate constant. If B is in a large excess over A, then it logically follows that [B] will not change significantly during the reaction compared to A so it can be approximated as constant. Then, [B] can be absorbed into the rate constant, so we write Rate = k'[A] where k'=k[B] and k' is now a pseudo-first-order rate constant.

    The rate law is now first-order since it looks like only one reactant [A], but pseudo - since we had to make an approximation and rely on having excess B.

    Let us apply what we just learnt to the question at hand. We can write

    k' = k[OH^-]

    From the question

    k' = 0.5 s-1

    [OH^-] = 0.1 M

    Where;

    k' = pseudo rate constant

    k = overall rate constant

    [OH^-] = hydroxide ion concentration

    Then;

    0.5 = k (0.1)

    k = 0.5/0.1

    k = 5 s-1
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