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.

Identify the reaction order from the rate constant, k = 5.5 x 10^-7 litre mol^-1 s^-1.

2

Unit of rate constant = (mole)^1-n(litre)^n-1time^-1, n = order of reaction

rate constant, k = 5.5 x 10^-7 litre mol^-1 s^-1 

comparing the powers of litre 

n - 1 = 1

n = 1 + 1 

n = 2