In the rate equation

Rate = k [A]^x [B]^y

x and y indicate how sensitive the rate is to the change in concentration of A and B. Sum of these exponents, i.e., x + y gives the overall order of a reaction whereas x and y represent the order with respect to the reactants A and B respectively.

Hence, the sum of powers of the concentration of the reactants in the rate law expression is called the order of that chemical reaction. Order of a reaction can be 0, 1, 2, 3 and even a fraction. A zero-order reaction means that the rate of reaction is independent of the concentration of reactants.

Another property of a reaction called molecularity helps in understanding its mechanism. The number of reacting species (atoms, ions or molecules) taking part in an elementary reaction, which must collide simultaneously in order to bring about a chemical reaction is called molecularity of a reaction.

What is the molecularity of the following reaction?

2FeCl₃ + SnCl₂ → SnCl₄ + 2FeCl₂

3

For the elementary reaction; 2FeCl₃ + SnCl₂ → SnCl₄ + 2FeCl₂ 

Molecularity is 3 as total three reactant molecules are simultaneously colliding with each other to form products.