Question
What is reaction intermediate? Explain with an example.
✓
Answer
Reaction intermediate : The additional species other than the reactants or products formed in the mechanism during progress of the reaction is called reaction intermediate.
Features of reaction intermediate :
Step I : $NO _2 Cl _{( g )} \xrightarrow{k_1} NO _{2( g )}+ Cl _{( g )}$ (slow, unimolecular)
Step II : $NO _2 Cl _{( g )}+ Cl _{( g )} \xrightarrow{k_2} NO _{2( g )}+ Cl _{2( g )}$ (fast, bimolecular) $2 NO _2 Cl _{( g )} \rightarrow 2 NO _{2( g )}+ Cl _{2( g )}$ (overall reaction)
$Cl$ formed in Step I is removed in Step II, Hence $Cl$ is a reaction intermediate.
- The reaction intermediate appears in the reaction mechanism but does not appear in the overall reaction or in the products.
- It is always formed in one step and consumed in the subsequent step in the mechanism.
- Its concentration is very small and cannot be determined easily.
- Rate of the reaction is independent of concentration of this intermediate.
- The life period of the reaction intermediate is extremely small, hence cannot be isolated.
- The composition of the reaction intermediate, decides the mechanism of the reaction.
- Consider decomposition of gaseous $NO _2 Cl .2 NO _2 Cl _{( g )} \rightarrow 2 NO _{2( g )}+ Cl _{2( g )}$
Step I : $NO _2 Cl _{( g )} \xrightarrow{k_1} NO _{2( g )}+ Cl _{( g )}$ (slow, unimolecular)
Step II : $NO _2 Cl _{( g )}+ Cl _{( g )} \xrightarrow{k_2} NO _{2( g )}+ Cl _{2( g )}$ (fast, bimolecular) $2 NO _2 Cl _{( g )} \rightarrow 2 NO _{2( g )}+ Cl _{2( g )}$ (overall reaction)
$Cl$ formed in Step I is removed in Step II, Hence $Cl$ is a reaction intermediate.
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