Q72 / 180 · AIIMS 2008 · Chemistry

For the reaction, \(2 \mathrm{NO}_2+\mathrm{F}_2 \rightarrow 2 \mathrm{NO}_2 \mathrm{~F}\), following mechanism has been provided. \[ \begin{aligned} & \mathrm{NO}_2+\mathrm{F}_2 \stackrel{\text { Slow }}{\longrightarrow} \mathrm{NO}_2 \mathrm{~F}+\mathrm{F} \\ & \mathrm{NO}_2+\mathrm{F} \stackrel{\text { Fast }}{\longrightarrow} \mathrm{NO}_2 \mathrm{~F} \end{aligned} \] Thus, rate expression of the above reaction can be written as

  • A
    \(r=k\left[\mathrm{NO}_2\right]_2\left[\mathrm{~F}_2\right]\)
  • Correct answer
    B
    \(r=k\left[\mathrm{NO}_2\right]\left[\mathrm{F}_2\right]\)
  • C
    \(r=k\left[\mathrm{NO}_2\right]\)
  • D
    \(r=k\left[\mathrm{~F}_2\right]\)

Solution

Slowest step of mechanism decides the rate expression. Thus rate $=k\left[\mathrm{NO}_2\right]\left[\mathrm{F}_2\right]$.

Hence, the correct answer is option: 'B'.

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