Molecularity of the Reaction





The number of molecules taking part in the reaction as represented by the simple chemical equation is called molecularity of the reaction or It is number of species which undergo collision to result into a chemical reaction. I.e. to convert into products.

E.g.

CH_3COOC_2H_5 + \underset{ ( \text{large excess} ) }{H_2O} \overset{H^+}{\rightarrow} CH_3COOH + C_2H_5OH

 

\underset{\text{Sucrose}}{ C_{12} H_{22} O_{11}} + \underset{ ( \text{large excess} ) }{H_2O} \overset{H^+}{\rightarrow} \underset{\text{Glucose}}{C_6H_{12}O_6} + \underset{\text{Fructose}}{C_6H_{12}O_6}

 

In both these reactions molecularity is 2 while the order of the reaction is 1 as the concentration of only CH_3COOC_2H_5 in the first case and C_{12}H_{22}O_{11} in the second case is effected.

Some more examples of different molecularity of the reactions are given below:

N_2O_5 \to N_2O_4 + \dfrac{1}{2} O_2 (unimolecular)

PCl_5 \to PCl_3 + Cl_2 (unimolecular)

2HI \to H_2 + I_2 (bimolecular)

2NO + O_2 \to 2NO_2 (Termolecular)

In the case of complex reactions (or the reactions which take place in two or more steps, the molecularity of the reaction is determined by number of the molecules taking part in the reaction; while order of the reaction is determined by rate of the slowest reaction.

It is important to note that order of reaction and molecularity of reaction are rarely equal to or greater than three.

Now we consider some examples:

(i)                  2FeCl_3 + 6KI \to 2FeCl_2 + 6KCI + I_2

The molecularity of this reaction has no significance. The overall kinetics of the reaction depends upon rate determining step. We know that the order of reaction is determined by slowest step and generally called as rate determining step.

The above reaction may be represented as:

(a)    FeCl_3 + KCl \to FeI_2 + 2KCl + Cl (slow)

(b)   2KI + 2Cl \to 2KCl + I_2 (fast)

This reaction is of third order.

 

(ii)                2NO +2H_2 \to N_2 + 2H_2O.

This reaction takes place in following two steps:

(a)    2NO + H_2 \to N_2 + H_2O_2 (slow)

(b)   H_2O_2 + H_2O \to 2H_2O (fast)

\therefore \text{Rate} = k[NO]^2[H_2]

 

This reaction is of third order.

 

(iii)               2NO_2 + F_2 \to 2NO_2F

This reaction takes place in following two steps:

(a)    NO_2 + F_2 \to NO_2F + F (Slow)

(b)   NO_2 + F\to NO_2F (fast)

\therefore \text{Rate} = k[NO_2][F_2]

 

This reaction of the second order.



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