Question

Which is an example of autocatalysis?
A.Hydrolysis of methyl acetate
B.Decomposition of TNG
C.Oxidation of oxalic acid by ${\text{KMn}}{{\text{O}}_{\text{4}}}$
D.All of the above

Answer 1

A reaction is said to be undergoing autocatalysis if one of the products of the reaction produced during the reaction acts as a catalyst and catalyses the reaction. They do not require the addition of an external catalyst. The rate of such reactions is slow in the beginning but as the reaction proceeds a product is formed which in turn acts as a catalyst and the rate of reaction is increased.

Complete step by step answer:
Let us now look at each of the given chemical reactions and their catalytic mechanics:
Hydrolysis of methyl acetate:
${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOC}}{{\text{H}}_{\text{3}}}{\text{ + }}{{\text{H}}_{\text{2}}}{\text{O}}\xrightarrow{{{\text{HCl}}}}{\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH + C}}{{\text{H}}_{\text{3}}}{\text{OH}}$
The hydrolysis of methyl acetate occurs in the presence of dilute HCl as a catalyst. But is added only in a small amount at the beginning of the reaction to initiate the reaction, after that the acid produced (acetic acid) acts as a catalyst this is an example of autocatalysis
Since the reaction product acts
Decomposition of TNG:
TNG stands for trinitroglycerin.
${\text{4}}{{\text{C}}_{\text{3}}}{{\text{H}}_{\text{5}}}{{\text{N}}_{\text{3}}}{{\text{O}}_{\text{9}}}{\text{(s)}} \to {\text{6}}{{\text{N}}_{\text{2}}}{\text{(g) + 12CO(g) + 10}}{{\text{H}}_{\text{2}}}{\text{O(g) + 7}}{{\text{O}}_{\text{2}}}{\text{(g)}}$
In this reaction, the gases produced during the reaction acts as a catalyst. And thus it is also an example of autocatalysis
Oxidation of oxalic acid by ${\text{KMn}}{{\text{O}}_{\text{4}}}$:
${\text{2KMn}}{{\text{O}}_{\text{4}}}{\text{ + 2}}{{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{ + 5}}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{4}}}{\text{.2}}{{\text{H}}_{\text{2}}}{\text{O}} \to {{\text{K}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}{\text{ + 2MnS}}{{\text{O}}_{\text{4}}}{\text{ + 18}}{{\text{H}}_{{\text{2O}}}}{\text{ + 10C}}{{\text{O}}_{\text{2}}}$
The produced during the reaction acts as a catalyst for the reaction and thus it is also an example of autocatalysis.

Since all the given reactions are examples of autocatalysis, the correct answer is D.

Note: Sometimes there is a myth among students that a catalyst always increases the rate of a chemical reaction. This is incorrect. A catalyst can also decrease the rate of the chemical reaction and such substances are known as inhibitor or poison. They decrease the rate of the chemical reaction. E.g., during the production of sodium sulphate by oxidising sodium sulphite, ethanol acts as an inhibitor.