What are the difference between Order and Molecularity?

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 H_{2}O_{2}_{ } takes place in two steps:

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

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