Question

Which is an example of auto catalyst?
A. Hydrolysis of ethyl acetate
B. Decomposition of TNG
C. Oxidation of oxalic acid
D. All of the above

Answer 1

Hint: In some reactions, one of the products acts as a catalyst; as initially the reaction is slow but as the reaction proceeds and the products are formed, the reaction rate increases. This phenomenon is known as auto-catalysis.

Complete answer:
Before discussing the options of auto catalyst, let us see what is auto catalyst. Autocatalysis is the reaction which is catalyzed by one of its products or catalysts are generally foreign agents but sometimes reaction’s products formed may act as a catalyst and the catalyst is called auto catalyst. Let us get some briefing of the examples now:

A. Hydrolysis of ethyl acetate: Hydrolysis is a chemical decomposition which involves breaking of a bond and the addition of elements like water. In hydrolysis of ethyl acetate, an alkali which is sodium hydroxide. $\text{HCl}$is used as a catalyst here which initiates the reaction.
$\text{C}{{\text{H}}_{3}}\text{COO}{{\text{C}}_{2}}{{\text{H}}_{5}}+\text{NaOH}\left( \text{aq} \right)\xrightarrow{\text{HCl/}{{\text{H}}^{+}}}\text{C}{{\text{H}}_{3}}\text{COONa}\left( \text{aq} \right)+\text{C}{{\text{H}}_{3}}\text{C}{{\text{H}}_{2}}\text{OH}$

B. Decomposition of TNG: TNG is abbreviated as trinitroglycerin wit molecular formula (${{\text{C}}_{3}}{{\text{H}}_{5}}{{\text{N}}_{3}}{{\text{O}}_{9}}$). In decomposition of trinitroglycerin reaction, gases formed act as catalysts for the reaction. $\text{4}{{\text{C}}_{3}}{{\text{H}}_{5}}{{\text{N}}_{3}}{{\text{O}}_{9}}\left( \text{s} \right)\to \text{6}{{\text{N}}_{2}}\left( \text{g} \right)+\text{12CO}\left( \text{g} \right)+\text{10}{{\text{H}}_{2}}\text{O}\left( \text{g} \right)+7{{\text{O}}_{2}}\left( \text{g} \right)$. This is an example of auto-catalysis.

C. Oxidation of oxalic acid$\text{2KMn}{{\text{O}}_{4}}+\text{2}{{\text{H}}_{2}}\text{S}{{\text{O}}_{4}}+\text{5}{{\text{H}}_{2}}{{\text{C}}_{2}}{{\text{O}}_{4}}\text{.2}{{\text{H}}_{2}}\text{O}\to {{\text{K}}_{2}}\text{S}{{\text{O}}_{4}}+2\text{MnS}{{\text{O}}_{4}}+18{{\text{H}}_{2}}\text{O}+\text{10C}{{\text{O}}_{2}}$ is a redox reaction. Oxalic acid is oxidised to carbon dioxide by $\text{KMn}{{\text{O}}_{4}}$which itself gets reduced to $\text{MnS}{{\text{O}}_{4}}$. The reaction produces $\text{M}{{\text{n}}^{+2}}$, which is the auto-catalyst of this reaction. When $\text{KMn}{{\text{O}}_{4}}$undergoes a reaction with an acidified oxalate solution, initially, the rate of the reaction is slow. There is an increment in rate of reaction due to the formation of $\text{M}{{\text{n}}^{+2}}$ ions.

The correct answer to this question is option ‘d’ (all of the above), which includes hydrolysis of ethyl acetate, decomposition of TNG and oxidation of oxalic acid are the examples of auto-catalysis.

Note: It is a myth that catalysts affect the reaction in a positive way only. There are some catalysts also which decrease the rate of reaction and such catalysts are called negative catalysts. On the other hand, the catalysts which accelerate the rate are positive catalysts.
For example: In production of sodium sulphate from sodium sulphite by oxidation but the presence of ethanol hampers the process, $\text{N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{3}}+{{\text{O}}_{2}}\xrightarrow{{{\text{C}}_{2}}{{\text{H}}_{5}}\text{OH}}\text{N}{{\text{a}}_{2}}\text{S}{{\text{O}}_{4}}$.