# 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.

Related posts:

1. Third Order Reaction A reaction is said to be of third order if...
2. Second order Reaction A reaction is said to be the second order if...
3. Determination of order of Reaction There are at least four different methods to ascertain the...
4. Order of Reaction The order of reaction may be defined as the sum...
5. Zero Order Reaction Zero Order Reaction If the rate of reaction is independent...