What are consecutive reactions? Give an example.

Hint: The consecutive reactions can be elaborated as the reactions which occur in several steps and involve multiple intermediates during a reaction. One can elaborate on the kinetics of these reactions with examples.

Complete step by step answer:

1) First of all let's learn about the consecutive reactions which contain several steps of the reaction during which the product formed of the primary step is going to be the reactant of the subsequent step then on.
2) The consecutive reactions also can be mentioned as complex reactions. Usually, the product formed of the primary reaction is named the intermediate during a three-step reaction, the product formed of first and second reaction is that of the intermediates, and therefore the final step we obtain the specified product. Another way to define consecutive reaction is that the reaction during which the reactants form intermediates then these intermediates react to make the ultimate products.
3) In order to elaborate the consecutive reactions let us consider a consecutive occurrence of two first-order reversible reactions $X \to Y \to Z$. Here X, Y, and Z are the chemical species in that particular step. The change in concentration of those substances is often calculated by integrating the system of the two kinetic equations. Such calculations show how the concentration of Y first increases within the first reaction then the concentration decreases because it's consumed within the second reaction.
4) Now let's see examples of consecutive reactions: the thermal cracking and chlorination of hydrocarbons.
$C{H_4} + C{l_2}\xrightarrow{\Delta }C{H_3}Cl$
$C{H_3}Cl + C{l_2}\xrightarrow{\Delta }C{H_2}C{l_2}$
$C{H_2}C{l_2} + C{l_2}\xrightarrow{\Delta }CHC{l_3}$
$CHC{l_3} + C{l_2}\xrightarrow{\Delta }CC{l_4}$
The chlorination of methane occurs in four steps which is the consecutive reaction. Methane when reacted with chlorine within the presence of heat we get chloromethane. The chloromethane when reacted with chlorine within the presence of heat we get dichloromethane. In the next step, we get chloroform. The last step will form carbon tetrachloride. The consecutive reaction stops when all the hydrogen atoms get replaced by the chlorine atom.

Note:
If the reactions are reversible then more complex consecutive reactions are obtained and to calculate the concentration of the intermediate becomes more complex. The consecutive reactions concept must not be confused with parallel reactions. A parallel reaction is where two reactions occur simultaneously and yield two or more different products.