Prediction of reaction mechanism: The following reaction has a second - order rate law : `H_(2)(g)+2ICI(g)rarrI_(2)(g)+2HCI(g)`
`Rate = k[H_(2)][ICI]`
Devise a possible reaction mechanism
Strategy: The reaction doesn't occur in a single elementary step because, if did, the rate law would be third order: Rate `= k[H_(2)][ICI]^(2)`. The observed rate law would be obtained if the rate determining step involves the bimolecular reaction of `H_(2)` and ICI.

Explanation

A plausible sequence of elementary steps is
`H_(2)(g)+ICI(g)overset(k_(1))rarrHI(g)+HCI(g)`, Slower (rate-determining)
`HI(g)+ICI(g)overset(k_(2)rarrI_(2)(g)+HCI(g)`, Faster
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`H_(2)(g)+2ICI(g)rarrI_(2)(g)+2HCI(g)`, Overall reation
The rate law predicted by this mechanism, Rate `= k_(1)[H_(2)][ICI]`, agrees with the observed rate law.