Q82 / 200 · NEET 2007 · Chemistry
The reaction of hydrogen and iodine monochloride is given as:
$\mathrm{H}_{2(g)}+2 \mathrm{ICl}_{(g)} \rightarrow 2 \mathrm{HCl}_{(g)}+\mathrm{I}_{2(g)}$
This reaction is of first order with respect to $\mathrm{H}_{2(g)}$ and $\mathrm{lCl}_{(g)+}$ following mechanisms were proposed
Mechanism A:
$\mathrm{H}_{2(g)}+2 \mathrm{ICl}_{(g)} \rightarrow 2 \mathrm{HCl}_{(g)}+\mathrm{H}_{2(g)}$
Mechanism B:
$\begin{aligned}
& \mathrm{H}_{2(g)}+\mathrm{ICl}_{(g)} \rightarrow \mathrm{HCl}_{(g)}+\mathrm{HI}_{(g)} \text { :slow } \\
& \mathrm{Hl}_{(g)}+\mathrm{lCl}_{(g)} \rightarrow \mathrm{HCl}_{(g)}+\mathrm{l}_{2(g)} \text {; fast }
\end{aligned}$
Which of the above mechanism(s) can be consistent with the given information about the reaction?
- Correct answerAA and B both
- Bneither A nor B
- CA only
- DB only
Solution
The slowest step is the rate determining step thus the mechanism $\mathrm{B}$ will be more consistent because it involve one molecule of $\mathrm{H}_2$ and one molecule of ICl. It can be expressed as
$r=k\left[\mathrm{H}_2\right][\mathrm{ICl}]$
Related Theory
The rate determining step in a reaction mechanism is characterised by high activation energy. Usually, the first step is the slow step since it has the highest activation energy.
Hence, the correct answer is option: 'A'.