Question

Which statement about molecularity of reaction is not correct?
A. It can be obtained from a reaction mechanism.
B. It may be either whole number or fractional.
C. It depends on the elementary step of reaction.
D. It is the number of molecules of reactant taking part in a single step reaction.

Answer 1

As stated in collision theory, the number of molecules that are reactant molecules collide in order form a product. The number molecules may sometimes be one, sometimes be two etc.

Complete step by step answer:
Molecularity is the number of molecules of reactants that collide simultaneously in order to give product.
Option A: It states that molecularity can be obtained from a reaction mechanism, if we blindly look into this option it may seem to be wrong but it is not. You might get a clarity if I put the definition in other words molecularity can also defined as A mechanism in which reactant species combine to form a product, in case of complex mechanism the molecularity of the rate determining step is the overall molecularity. Therefore statement in option A is correct.

Option B: Basically it depends on elementary steps of reaction and reaction mechanism. If only 1 molecule is involved in the reaction, then the reaction is said to be unimolecular. If two molecules are involved the reaction is said to be bimolecular, if 3 molecules are involved the reaction is said to be trimolecular. From this we can come to a conclusion the molecularity never be represented in numbers at first place and it cannot be fractional. It is represented as unimolecular, bimolecular, trimolecular etc. Therefore statement B does not meet the correct representation of molecularity.

Option C: The statement is correct as molecularity depends on the elementary reaction as mentioned in explanation of option A and it also gives clarity of how many molecules are participating in a chemical reaction.

Option D: It states that it is the number of molecules of reactant taking part in a single step .It exactly satisfies and resembles the definition of molecularity. Therefore option D is also a correct statement.
Let A and B be the reacting species which collide simultaneously to give product C and D.
$A + B \to C + D$
So, the correct answer is “Option B”.

Note: Molecularity of a reaction never can be fractional. According to collision theory the number of molecules colliding cannot be in fractional, it can only be a whole number.