Question
Explain how the molecularity of a complex reaction determined? And give the points showing difference between molecularity of a reaction and order of a reaction.
✓
Answer
→ Term molecularity is only applicable to elementary reactions, even though to determine molecularity of a complex reaction it is necessary to know the mechanism of that reaction.
→ Through mechanism of a reaction we can have information about slowest step of reaction.
→ The overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step.
→ Consider the decomposition of hydrogen peroxide which is catalysed by iodide ion in an alkaline medium $2 H _2 O _2 \xrightarrow[\text { Alkaline medium }]{ I ^{-}} 2 H _2 O + O _2$
→ The rate equation for this reaction is found to be Rate $=\frac{- d \left[ H _2 O _2\right]}{ dt }= k \left[ H _2 O _2\right]\left[ r ^{-}\right]$
→ This reaction is first order with respect to both $H _2 O _2$ and $I ^{-}$.
→ Mechanism involve two steps.
(1) $H _2 O _2+ I ^{-} \xrightarrow{\text { Slow Step }} H _2 O + IO ^{-}$
(2) $H _2 O _2+ IO ^{-} \longrightarrow H _2 O + I ^{-}+ O _2$
→ The first step, being slow, is the rate determining step.
→ Thus, the rate of formation of intermediate will determine the rate of this reaction.
→ Thus, it can be said that, for complex reaction, order is given by the slowest step and molecularity of the slowest step is same as the order of the overall reaction.
Difference between molecularity and order of a reaction:
(i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non integer.
(ii) Order is applicable to elementary as well as complex reactions whereas molecularity is applicable only for elementary reactions. For complex reaction molecularity has no meaning.
(iii) For complex reaction, order is given by the slowest step and generally molecularity of the slowest step is same as the order of the overall reaction.
→ Through mechanism of a reaction we can have information about slowest step of reaction.
→ The overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step.
→ Consider the decomposition of hydrogen peroxide which is catalysed by iodide ion in an alkaline medium $2 H _2 O _2 \xrightarrow[\text { Alkaline medium }]{ I ^{-}} 2 H _2 O + O _2$
→ The rate equation for this reaction is found to be Rate $=\frac{- d \left[ H _2 O _2\right]}{ dt }= k \left[ H _2 O _2\right]\left[ r ^{-}\right]$
→ This reaction is first order with respect to both $H _2 O _2$ and $I ^{-}$.
→ Mechanism involve two steps.
(1) $H _2 O _2+ I ^{-} \xrightarrow{\text { Slow Step }} H _2 O + IO ^{-}$
(2) $H _2 O _2+ IO ^{-} \longrightarrow H _2 O + I ^{-}+ O _2$
→ The first step, being slow, is the rate determining step.
→ Thus, the rate of formation of intermediate will determine the rate of this reaction.
→ Thus, it can be said that, for complex reaction, order is given by the slowest step and molecularity of the slowest step is same as the order of the overall reaction.
Difference between molecularity and order of a reaction:
(i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a non integer.
(ii) Order is applicable to elementary as well as complex reactions whereas molecularity is applicable only for elementary reactions. For complex reaction molecularity has no meaning.
(iii) For complex reaction, order is given by the slowest step and generally molecularity of the slowest step is same as the order of the overall reaction.
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