Question

Which one of the following reactions is an example of auto-catalysis?
(A) $2{\text{As}}{{\text{H}}_{\text{3}}}\left( {\text{s}} \right) \to 2{\text{As}}\left( {\text{s}} \right) + 3{{\text{H}}_{\text{2}}}\left( {\text{g}} \right)$
(B) ${{\text{N}}_{\text{2}}}\left( {\text{g}} \right) + 3{{\text{H}}_2}\left( {\text{g}} \right)\xrightarrow{{{\text{Fe}}\left( {\text{g}} \right)}}2{\text{N}}{{\text{H}}_{\text{3}}}\left( {\text{g}} \right)$
(C) ${\text{2S}}{{\text{O}}_{\text{2}}}\left( {\text{g}} \right) + {{\text{O}}_2}\left( {\text{g}} \right)\xrightarrow{{{\text{NO}}\left( {\text{g}} \right)}}2{\text{S}}{{\text{O}}_{\text{3}}}\left( {\text{g}} \right)$
(D) ${{\text{C}}_{{\text{12}}}}{{\text{H}}_{{\text{22}}}}{{\text{O}}_{{\text{11}}}}\left( {\text{l}} \right) + {{\text{H}}_{\text{2}}}{\text{O}}\left( {\text{l}} \right)\xrightarrow{{{{\text{H}}^ + }\left( {\text{l}} \right)}}{{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}{{\text{O}}_{\text{6}}}\left( {\text{l}} \right) + {{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}{{\text{O}}_{\text{6}}}\left( {\text{l}} \right)$

Answer 1

- In autocatalytic reactions, no external catalyst is required to speed up a chemical reaction.
- Atleast one of the products is a reactant in autocatalysis.

Complete step by step answer:
A chemical reaction is said to be undergoing autocatalysis if one of the products of the reaction acts as a catalyst for the same or for a coupled reaction.
Collectively autocatalytic reactions are a set of chemical reactions if a number of those reactions yield catalysts for other reactions that the entire set of chemical reactions is self sustaining given an input of energy.
Let us consider the first reaction $2{\text{As}}{{\text{H}}_{\text{3}}}\left( {\text{s}} \right) \to 2{\text{As}}\left( {\text{s}} \right) + 3{{\text{H}}_{\text{2}}}\left( {\text{g}} \right)$ .
It is the thermal decomposition reaction of arsine ${\text{As}}{{\text{H}}_{\text{3}}}$ to give free arsenic and hydrogen. Here, the free arsenic produced acts as a catalyst for the reaction and hence this reaction is an example of autocatalysis. So, option A is correct.
Now, let us check the next reaction ${{\text{N}}_{\text{2}}}\left( {\text{g}} \right) + 3{{\text{H}}_2}\left( {\text{g}} \right)\xrightarrow{{{\text{Fe}}\left( {\text{g}} \right)}}2{\text{N}}{{\text{H}}_{\text{3}}}\left( {\text{g}} \right)$ .
It is the Haber’s process for the manufacturing of ammonia on a commercial scale. It is an exothermic reaction and the favourable conditions for this reaction are low temperature, high pressure and presence of a catalyst. The catalyst used is iron oxide. This means this is not an autocatalytic reaction. So option B is wrong.
Now, let us check the next reaction ${\text{2S}}{{\text{O}}_{\text{2}}}\left( {\text{g}} \right) + {{\text{O}}_2}\left( {\text{g}} \right)\xrightarrow{{{\text{NO}}\left( {\text{g}} \right)}}2{\text{S}}{{\text{O}}_{\text{3}}}\left( {\text{g}} \right)$ .
Here, it can be seen that the catalyst used is nitric oxide which is not a product of the reaction and so option C is wrong.
Lastly, consider the reaction ${{\text{C}}_{{\text{12}}}}{{\text{H}}_{{\text{22}}}}{{\text{O}}_{{\text{11}}}}\left( {\text{l}} \right) + {{\text{H}}_{\text{2}}}{\text{O}}\left( {\text{l}} \right)\xrightarrow{{{{\text{H}}^ + }\left( {\text{l}} \right)}}{{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}{{\text{O}}_{\text{6}}}\left( {\text{l}} \right) + {{\text{C}}_{\text{6}}}{{\text{H}}_{{\text{12}}}}{{\text{O}}_{\text{6}}}\left( {\text{l}} \right)$ .
It is a hydrolysis reaction of sucrose to give glucose and fructose. The catalyst used is either an acid or an enzyme and so option D is also wrong.

Note:
A biological example of an autocatalytic reaction is glycolysis.
It is the degradation of one glucose molecule with the production of 2 molecules of ATP.
The reaction is:
${\text{Glucose + 2ADP + 2}}{{\text{P}}_{\text{i}}}{\text{ + 2NAD}} \to {\text{2}}\left( {{\text{pyruvate}}} \right){\text{ + 2ATP + 2NADH}}$
A portion of this process is auto-catalyzed by a kinase enzyme PFK.
The process of glycolysis is of great importance to the energy of living cells.