Question

A reaction 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

A reaction involving two different reactants could be well understood by an example. In unimolecular reaction only one type of molecule reacts, in bimolecular reaction only two types of molecules react with each other. First order and second order reactions depend on the rate of the reaction and not on the molecularity of the reaction.

Complete answer:
The correct answer to this question is option B, a reaction involving two different reactants can never be a unimolecular reaction. Unimolecular reaction is a reaction in which only one type of molecule can react. But, in a reaction in which two different reactants are reacting, it cannot be a unimolecular reaction.
Let us understand this better with an example. Suppose a reaction is occurring and the two different reactants are A and B, the product obtained is C.
$A+B\to C$
Now, according to the definition of unimolecular reaction, only one type of molecule is allowed to react, but here two different types of molecules are reacting with each other.
According to the definition of a bimolecular reaction, only two different types of molecules are allowed to react, here in this reaction A and B two different types of reactants react with each other. Therefore, the reaction given as an example is a bimolecular reaction.
Now, according to the definition of first and second order reaction, it depends on the rate of the reaction. The rate of the reaction given above is given by,
$r=K{{[A]}^{n}}{{[B]}^{m}}$
The order of the reaction can be given by n plus m. Thus, the order of the reaction is given by addition of n and m, and it does not depend on the types of molecules that react. The order of the reaction can be found out by calculations, but it is clear that the reaction can be of first order or second order.
Therefore, a reaction involving two different reactants can never be a unimolecular reaction.

The Correct answer is Option C

Note:
The understanding of unimolecular and bimolecular reactions is necessary here to get the correct answer, without it we cannot find the correct answer. Also, if the value of n or m in the reaction is zero then the reaction is of first order and if the value of n and m is one, then the reaction can be of second order. Thus, the order of any reaction depends on the rate of the reaction.