Question

A reaction proceeds in three stages. The first stage is slow and involves two molecules of reactants. The second and third stage is fast. The overall order of the reaction is:
A.First order
B.Second order
C.Third order
D.Zero order

Answer 1

The rate of reaction depends upon the slowest step among the various steps.
The above is the complex type of reaction, in which various steps are involved in the formation of products.

Complete step by step answer:
Each step in a complex reaction is elementary, that is the order is the same as the molecularity of the reactants. Molecularity refers to the number of moles or molecules of reactants reacting.
The rate of reaction always depends upon the slowest step. Let us understand this statement with the help of an example. Suppose there is a factory that makes pens. The three components of a pen, the cap, refill and cover are made separately. The cap takes 5 minutes to prepare whereas refill takes 2 minutes but cover takes 10 minutes. The whole pen will only be ready when all the three components are prepared, which will be after 10 mins. It does not matter when other components are prepared. Hence rate depends on the slowest step.
It is given to us that the first step is the slowest step. So the rate will depend upon the first step and it has two molecules so the order will be 2. Hence this is a second order reaction.

Hence Option B is correct.
Note:
The first order reactions are those reactions in which the rate of reaction is directly proportional to the concentration of reactant. The zero order reactions are those reactions in which the rate of the reaction does not depend on the rate of reaction. For third order reactions, the rate depends on the second power of the reactant molecule.