Write the difference between Order and Molecularity of a reaction.

Explanation

The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction which must collide.simultaneoiisly in order to bring "about alchemical reaction is called molecularity of a reaction.
(i) Order of a reaction is aruexperimental quality. It can be zero and even a fraction but molecularity cannot be zero or 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 motecularity of the slowest step is taken as the order of the overall reaction.
Dissociation of hydrogen iodide into `H_(2)` and `l_(2)` is a bimoleciilar .reaction.
`2HI rarr H_(2)+I_(2)`
Formation of `NO_(2)` From NO and `O_(2)` is a trimolecular reaction
`2NO+O_(2) rarr 2NO_(2)`