NEET Chemistry NEET PYQ-Chemical Kinetics

  • question_answer
    When initial concentration of a reactant is doubled in a reaction, its half-life period is not affected. The order of the reaction is [NEET 2015 ]

    A) zero

    B) first

    C) second

    D) more than zero but less than first

    Correct Answer: A

    Solution :

    [a] For a zero order reaction \[{{t}_{1/2}}\] is directly proportional to the initial concentration of the reactant \[{{[R]}_{0}}\]
    \[{{t}_{1/2}}\propto {{[R]}_{0}}\]
    For a first order reaction
    \[k=\frac{2.303}{t}\log \frac{{{[R]}_{0}}}{[R]}\,\,at\,\,\,{{t}_{1/2}},[R]=\frac{{{[R]}_{0}}}{2}\]
    So, the above equation becomes
    \[K=\frac{2.303}{{{t}_{1/2}}}\log \frac{{{[R]}_{0}}}{{{[R]}_{0}}/2}\]
    \[{{t}_{1/2}}=\frac{2.303}{K}=\log 2=\frac{2.303}{K}\times .3010\]
    \[{{t}_{1/2}}=\frac{.693}{k}\]
    i.e., half-life period is independent of initial concentration of a reactant.


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