Difference between Order and Molecularity

(i) Order is an experimental property while molecularity is the theoretical property.

(ii) Order concerns with kinetics (rate-law) while molecularity concerns with mechanism.

(iii) Order may be any number, fractional, integral or even zero whereas molecularity is always an integer excepting zero.

Note that a zero order reaction is one in which concentration of reactant remains throughout constant and as such rate of reaction remains throughout constant equal to the rate constant.

(iv) Order may change with change in experimental condition while molecularity can’t.

Example

This reaction follows 1st order kinetics at high gas pressure and 2nd order kinetics at low gas pressure of cyclopropane.