`=>` Molecularity of a reaction helps in understanding its mechanism.
`text(Definition) :` 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.
`=>` The reaction can be unimolecular when one reacting species is involved, for example, decomposition of ammonium nitrite.
`NH_4NO_2 → N_2+2H_2O`
`=>` Bimolecular reactions involve simultaneous collision between two species, for example, dissociation of hydrogen iodide.
`2HI → H_2 +I_2`
`=>` Trimolecular or termolecular reactions involve simultaneous collision between three reacting species, for example,
`2NO +O_2 → 2NO_2`
`=>` The probability that more than three molecules can collide and react simultaneously is very small. Hence, the molecularity greater than three is not observed.
`=>` It is, therefore, evident that complex reactions involving more than three molecules in the stoichiometric equation must take place in more than one step.
`KClO_3 + 6FeSO_4 + 3H_2SO_4 → KCl + 3Fe_2 (SO_4)_3 + 3H_2O`
● This reaction which apparently seems to be of tenth order is actually a second order reaction. This shows that this reaction takes place in several steps.
● Which step controls the rate of the overall reaction can be answered if we go through the mechanism of reaction, for example, chances to win the relay race competition by a team depend upon the slowest person in the team.
● Similarly, the overall rate of the reaction is controlled by the slowest step in a reaction called the rate determining step.
`=>` Consider the decomposition of hydrogen peroxide which is catalysed by iodide ion in an alkaline medium.
`2H_2O_2 undersettext(Alkaline medium) overset(-I) → 2H_2O +O_2`
● The rate equation for this reaction is found to be
`text(Rate) = (- d [H_2O])/(dt) = kappa [H_2O_2] [I^(-)]`
● This reaction is first order with respect to both `H_2O_2` and `I^(-)`.
● Evidences suggest that this reaction takes place in two steps
(a) `H_2O + I^(-) → H_2O +I O^-`
(b) `H_2O_2+I O^(-) → H_2O + I^- + O_2`
● Both the steps are bimolecular elementary reactions. Species `IO^-` is called as an intermediate since it is formed during the course of the reaction but not in the overall balanced equation.
● The first step, being slow, is the rate determining step. Thus, the rate of formation of intermediate will determine the rate of this reaction.
`=>` From the above discussion, we conclude the following :
(i) Order of a reaction is an experimental quantity. It can be zero and even a fraction but molecularity cannot be zero or a 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 generally, molecularity of the slowest step is same as the order of the overall reaction.