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Reaction Mechanism and Rate Law
Concepts tested here
- Rate determining step
All Questions
2007 AIPMT-PRE 1 question
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The reaction of hydrogen and iodine monochloride is given as:
$H_2(g) + 2ICl(g) \rightarrow 2HCl(g) + I_2(g)$
The reaction is of first order with respect to $H_2(g)$ and $ICl(g)$; following mechanisms were proposed.
Mechanism A:
$H_2(g) + 2ICl(g) \rightarrow 2HCl(g) + I_2(g)$
Mechanism B:
$H_2(g) + ICl(g) \rightarrow HCl(g) + HI(g)$; slow
$HI(g) + ICl(g) \rightarrow HCl(g) + I_2(g)$; fast
Which of the above mechanism(s) can be consistent with the given information about the reaction?The reaction is first order with respect to both $H_2$ and $ICl$, so the experimental rate law is $r = k[H_2][ICl]$.
Mechanism A is a single step involving one molecule of $H_2$ and two molecules of $ICl$. It would give $r = k[H_2][ICl]^2$, i.e. second order in $ICl$, which does not agree with the given information.
In mechanism B, the slowest step is the rate determining step.
The slow step involves one molecule of $H_2$ and one molecule of $ICl$, so the rate can be expressed as $r = k[H_2][ICl]$.
This shows that the reaction is first order with respect to both $H_2$ and $ICl$, which agrees with the given information.
So only mechanism B is consistent.