Order and molecularity of a reaction
Molecularity in chemistry is the number of molecules that come together to react in an elementary reaction and is equal to the sum of stoicheometric coefficients of reactants in this elementary reaction. A chemical reaction that takes place in one and only one step i.e., all that occurs in a single step is called elementary reaction while a chemical reaction occurring in the sequence of two or more steps is called complicated reaction. The sequence of steps through which a complicated reaction takes place is called reaction – mechanism. Each step in a mechanism is an elementary step reaction.
PCl5 → PCl3 + Cl2 Unimolecular
2HI → H2 + I2 Bimolecular
2SO2 + O2 → 2SO2 Trimolecular
NO + O3 → NO3 + O2 Bimolecular
2CO + O2 → 2CO2 Trimolecular
2FeCl3 + SnCl2 → SnCl2 + 2FeCl2 Trimolecular