What is "molecularity" of a reaction ? How is it different from the 'order' of a reaction? Name one bimolecular and one trimolecular gaseous reactions.

Explanation

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 is called molecularity of a reaction.
i) Order of a reaction is an experimental 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 molecularity of the slowest step is taken as the order of the overall reaction.
Dissociation of hydrogen-iodide into`H_(2) and I_(2)` is a bimolecular reaction.
`2HI+H_(2)+I_(2)`
Formation of `NO_(2)` from NO and `O_(2)` is a trimolecular reaction
`2NO+O_(2) to 2NO_(2)`