Consider the reaction below:
2 a → 2 b + c
if the rate of reaction is directly proportional to the concentration of a, this reaction is
a) a first-order reaction
b) a zero-order reaction
c) a third-order reaction
d) a second-order reaction

(A) first order reaction

Explanation:

A first order reaction is a type of reaction in which the rate of the reaction depends only on the concentration on one of the reactants. Since A is the only reactant we have, it is right to deduce that this reaction is a first order reaction.

Note: while the order of this reaction is 1, its molecularity is 2. The molecularity of a reaction is the number of moles of reactants that is actually reacting.

(B) is wrong

This is because a zero order reaction is one in which the rate of reaction is not influenced by the concentrations of the reactants and hence remains constant irrespective. Since we were not furnished with this idea in the question, it is only right that we reject this answer.

(C) is wrong.

C is outrightly wrong as we have only one reactant.

(D) is wrong

We have only one reactant.