VIT Engineering VIT Engineering Solved Paper-2010

  • question_answer
    For a reaction of type \[A+B\xrightarrow{{}}\] products, it is observed that doubling concentration of A causes the reaction rate to be four times as great, but doubling amount of B does not affect the rate. The unit of rate constant is

    A)  \[{{s}^{-1}}\]

    B)  \[{{s}^{-1}}\,mol\,\,{{L}^{-1}}\]

    C)  \[{{s}^{-1}}\,mo{{l}^{-1}}\,\,L\]

    D)  \[s\,\,{{s}^{-1}}\,mo{{l}^{-2}}\,\,{{L}^{2}}\]

    Correct Answer: C

    Solution :

     Let the initial rate is R and order with respect to A is x and B is y. Thus, rate law is, rate,.   \[R={{[A]}^{x}}{{[B]}^{y}}\]                  ?...(i) On doubling the concentration of A, rate becomes 4R, \[4R={{[2A]}^{x}}{{[B]}^{y}}\]         ?...(ii) On doubling concentration of B, rate remains R, \[R={{[A]}^{x}}{{[2B]}^{y}}\]             ??.(iii) From Eq. (i) and (ii), we get \[\frac{R}{4R}={{\left( \frac{1}{2} \right)}^{x}}\] \[{{\left( \frac{1}{2} \right)}^{2}}={{\left( \frac{1}{2} \right)}^{x}}\] \[\therefore \] \[x=2\] From Eq. (i) and (iii), we get \[\frac{R}{R}={{\left[ \frac{1}{2} \right]}^{y}}\] \[1={{\left( \frac{1}{2} \right)}^{y}}\] \[\therefore \] \[y=0\] Thus, actual rate law is, rate, \[R={{[A]}^{2}}{{[B]}^{0}}\] The order of this reaction is 2 and for second order reaction units of rate constant are \[\mathbf{mo}{{\mathbf{l}}^{\mathbf{-1}}}\,\,\mathbf{L}\,\,{{\mathbf{s}}^{\mathbf{-1}}}\].


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