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MOLECULARITY OF REACTION
In general, molecularity of simple reactions is equal to the sum of the number of molecules of reactants involved in the balanced stoichiometric equation.
The molecularity of a reaction is the number of reactant molecules taking part in a single step of the reaction.
e.g., PCl5 → PCl3 + Cl2 (Unimolecular)
2HI → H2 + I2 (Bimolecular)
2SO2 + O2 → 2SO3 (Trimolecular)
NO + O3 → NO2 + O2 (Bimolecular)
2CO + O2 → 2CO2 (Trimolecular)
2FeCl3 + SnCl2 → SnCl2 + 2FeCl2 (Trimolecular)
The minimum number of reacting particles (molecules, atoms or ions) that come together or collide in a rate determining step to form product or products is called the molecularity of a reaction.
For example, decomposition of H2O2 takes place in the following two steps:
H2O2 → H2O + 1/2O2 (overall reaction)
Step 1: H2O2 → H2O + [O] (Slow)
Step 2: [O] + [O] → O2 (fast)
The slowest step is rate-determining. Thus from step 1, reaction appears to be unimolecular.
(i) Molecularity is a theoretical concept.
(ii) Molecularity cannot be zero, -ve, fractional, infinite and imaginary.
(iii) Molecularity cannot be greater than three because more than three molecules may not mutually collide with each other.
There are some chemical reactions whose molecularity appears to be more than three from stoichiometric equations, e.g. in
4HBr + O2 → 2H2O + 2Br2
2MNI4- + 16H+ + 5C2 O42- → 2Mn2+ + 10CO2 + 8H2O
In the first reaction molecularity seems to be '5' and in the second reaction molecularity seems to be '23'. Such reactions involve two or more steps; each step has its own molecularity not greater than three, e.g., in first reaction.
HBr + O2 → HOOBr
HOOBr + HBr → 2HOBr
[HOBr + HBr → H2O + Br2] × 2
4HBr + O2 → 2H2O + Br2
Molecularity of each of the above steps is 2.