Question

The reactions involving two different reactants:
(A) can never be a second order reaction
(B) can never be a unimolecular reaction
(C) can never be a bimolecular reaction
(D) can never be a first order reaction

Answer 1

Hint: A reaction involving some number of reactants can be of any order as it depends upon the experimental results; whereas, the molecularity of the same will depend upon the practical values.

Complete Solution :
Let us study the concept of rate of reaction, its order and molecularity in detail;
- Rate of reaction-
The speed at which the reaction takes place is known as the rate of reaction.
- Order of reaction-
It is the basic relationship between the rate of the reaction and the concentration of reactants taking part in it. Order is the sum of powers raised to the concentration of reactants in the rate law.
It can be zero or an integer.
- Molecularity of reaction-
It is the number of reactants taking part into the elementary reaction. It is the number of actually colliding numbers of molecules to form the required product. It is independent of the rate of reaction.
It can never be zero or an integer.

Illustration-
We have given that the two reactants are reacting together;
For example,
Let us say we have a reaction between A and B as;
$A+B\to products$ ; it is a bimolecular reaction.
It can be second order if,
$\frac{dx}{dt}=k\left[ A \right]\left[ B \right]$
It can be first order if (considering B in excess),
$\dfrac{dx}{dt}=k\left[ A \right]$
Thus, we can say that the reaction having two reactants can be bimolecular, can have first order rate equation, can have second order rate equation but cannot be unimolecular.
So, the correct answer is “Option B”.

Note: In a unimolecular reaction, only one molecule is involved. Molecularity can never be one if two reactants are involved as it is a theoretical concept.