Question

A reaction in which one of the products of the reaction acts as a catalyst is called:

• A. a catalytic reaction
• B. a photochemical reaction
• C. an autocatalytic reaction
• D. a biochemical reaction

Hint:

In an autocatalytic reaction of the form \( A + P \rightarrow 2P \), the rate graph versus time typically forms a characteristic sigmoidal (S-shaped) curve, starting slowly, accelerating rapidly to a peak, and then slowing down as reactants are depleted.

Answer 1

The Correct Option is C

Concept: In chemical kinetics, catalysis involves increasing the rate of a chemical reaction by adding a substance known as a catalyst. Usually, catalysts are external agents added to the reaction mixture. However, in certain specific types of reactions, one of the chemical products formed during the course of the reaction possesses catalytic properties that accelerate the reaction further. This phenomenon is critical in understanding runaway reactions and metabolic pathways.

Step 1: Explaining Autocatalysis.
When a reaction produces a substance that acts as a catalyst for that same reaction, the process is explicitly known as autocatalysis. Initially, the reaction starts very slowly because the concentration of the catalyst (the product) is zero or negligible. As time passes and the product begins to accumulate, the reaction rate increases exponentially due to the self-catalyzing nature of the mixture. A typical example is the oxidation of oxalic acid by acidified potassium permanganate, where the generated \( \text{Mn}^{2+} \) ions act as an autocatalyst.

Step 2: Evaluating alternative options.

• Option (A): A general catalytic reaction involves an external substance explicitly added at the beginning to alter the reaction rate without being consumed.
• Option (B): A photochemical reaction is a chemical reaction initiated by the absorption of energy in the form of light (photons), not by its own products.
• Option (D): A biochemical reaction represents chemical processes occurring within living organisms, typically mediated by complex biological proteins called enzymes. Therefore, option (C) is the only precise scientific classification for this definition.