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pls explain the difference between molecularity nd order of reaction...more clearly..im not getting ..pls help.!!

1) The order of reaction with respect to a reactant is defined as the power (exponent) of the concentration of the reactant to which the rate of reaction is directly proportional. The overall order of reaction is the sum of all the exponents of all the reactants. Molecularity on the other hand is defined as the number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction.

2) The order of reaction is applicable to both complex reactions as well as elementary reactions while molecularity is defined only for elementary reactions. Molecularity has no meaning for complex reactions.

3) Order of a reaction is always determined experimentally and can any value - zero, integral values, fractions (and even negative values in certain cases). Molecularity always has simple integral values. Thus molecularity is 1 for unimolecular, 2 for bimolecular, three for termolecular elementary reactions.

4) The order of a reaction is the same as its molecularity and order with respect to each reactant is equal to its stoichiometric coefficient in the chemical equation representing the elementary reaction. Thus we can say that the order of a reaction is the same as its molecularity but the converse is not true.

For example, for the reaction

molecularity is 1. This is an elementary reaction.

Where as the reaction 

 

The given decomposition reaction of H2O2 takes place in two steps:

 

 

The order of reaction would be determined experimentally and is given by the slowest step.

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Molecularity in chemistry is the number of colliding molecular entities that are involved in a single reaction step (Elementary reaction). While the order of a reaction is derived experimentally. For most elementary reactions the order and molecularity are the same which is equal to the stioichiometric coefficients. But they may not be same for multi-step reactions.

Below are examples of some elementary reactions whose molecularity and order are same:

The molecularity of a reaction is the number of reactant molecules taking part in a single step of the reaction.

e.g., PCl5 → PCl3 + Cl2 (Unimolecular) ie, first order reaction

2HI → H2 + I2 (Bimolecular), second order reaction

2SO2 + O2 → 2SO3 (Trimolecular)

NO + O3 → NO2 + O2 (Bimolecular)

2CO + O2 → 2CO2 (Trimolecular)

2FeCl3 + SnCl2 → SnCl2 + 2FeCl2 (Trimolecular)

 

Below are examples where molecularity and order are different.

Hydrolysis of ethyl acetate in acetic medium

CH3 COOC2H5   + H2O  -------> Acetic acid + Ethanol

It is a bimolecular reaction(Molecularity = 2) as two molecules are taking part in this reaction. But here the water is in excess and there is no measurable change in its concentration, only the concentration of ester changes. Therefore the rate of the reraction depends only on the conc of ester and hence it is a first order reaction.


Consider the decomposition of Nitrogen Pentoxide.

2N2O5 ----> 4NO2 + O2

(Note that Molecularity exists only for a single step (Elementary reaction). The above reaction is not elementary as it is actually taking place in three different steps. Therefore the molecularity of above reaction is not 2 but is observed for the following elementary steps.

1. N2O5 ---> N2O3 (slow step)  <----- (Molecularity = 1)

2. N2O3 -----> NO + NO2 (fast)  <------ (Molecularity = 1)

3. NO + N2O5 -----> 3NO2 (fast)  <------(Molecularity = 2)

Here, The order of the reaction is determined by the molecularity of the slowest step (step1) which is called the rate determining step.

Here the molecularity of step 1 is 1. Hence the order of the overall reaction is 1

And Rate = k[N2O5]1    

 

 

Also note that order may also be zero or fractional number, but molecularity will always be a whole positive number.

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