Autocatalysis

German chemist Wilhelm Ostwald introduced the concept of autocatalysis to characterise the rate of a reaction as a function of time. To understand autocatalysis, we must first understand catalysis. Catalysis is a chemical procedure in which the reaction rate is affected (usually an acceleration) by adding a substance known as a catalyst. 

According to the primary scientific definition, autocatalysis refers to catalysis by one or more products of a chemical reaction. The reaction involving autocatalysis results in a product that acts as a self-catalyst. Autocatalysis is also known as autoactivation, and in autocatalysis, one of the products performs as a catalyst.

Difference between Autocatalysis and Catalysis

Now, let us understand the difference between a catalyst and an autocatalyst. A catalyst is generally a foreign substance added in a reaction to alter or change the reaction rate. It is used to accelerate the reaction for fast completion, while in an autocatalytic reaction, the catalyst is a product of the chemical reaction and called an autocatalyst and is thus a self catalyst. This is why an autocatalytic reaction is considered a self catalysed reaction. Therefore, there is no need to add any foreign catalyst in an autocatalytic reaction. To further clarify the concept of a self-catalyst, let’s consider this example:

Consider the following chemical reaction:

A+B   →   C+D

 Here, we have reactants A and B that give rise to the products C and D.

Let us consider this reaction to be an autocatalytic reaction. According to the concept, in this chemical reaction, reactants A and B will form products in which one of them will act as a self-catalyst for the response, say C will act as a self-catalyst here, which will help increase the rate of the chemical reaction, and thus help the fast completion the chemical reaction. This whole phenomenon is known as autocatalysis.

Autocatalytic Reactions in Depth

The simplest autocatalytic reaction can be written as:

A + B ⇌ 2B

With the rate equations (for an elementary reaction)

d[A]/dt=-k+[A][B] + k-[B]2

d[B]/dt= +k+[A][B]-K-[B]2

In this reaction, the molecule of species A reacts with molecules of species B. After the reaction, we get

The graph given here is for these above equations; it is a Sigmoid Curve (precisely a logistic function). This shows that reactions proceed slowly at the start of the response. Still, with time due to the presence of a catalyst, the reaction rate increases progressively as the reaction proceeds, and then again, the concentration of reactants decreases. If any reaction follows the Sigmoid Curve, the reaction may be an autocatalytic reaction.

Examples of Autocatalysis 

The decomposition of arsine is catalysed by arsenic, which is produced in the reaction.

  2AsH3 → 2As + 3H2

The key feature of these chemical reactions is that they are nonlinear. The concentration-time curve graph of formation of autocatalyst has a Sigmoid form that results in the self-acceleration growth phase of the autocatalytic reaction.

Initially, the rate of reaction is very slow, but with time, the rate of reaction increases. Autocatalysis is one of the pathways for chiral symmetry and is also responsible for the formation of patterns in chemical reactions.

What is an Autocatalytic Reactor?

A reactor is an apparatus in which we can proceed with any chemical, biological, and physical reaction intentionally, purposefully, and in a well-controlled manner. Some examples of major autocatalytic reactors include CSTR, CSTR in series, plug flow reactors, and recycle reactors. Choosing an ideal reactor to proceed with the chemical reaction in an autocatalytic chemical reaction is crucial.

Conclusion 

This article discusses autocatalysis and its essential features. We can conclude that autocatalysis is a process in which one of the products of a reaction acts as a self catalyst that helps in acceleration and thus fast completion of the reaction. The reaction in which autocatalysis takes part is known as an autocatalytic reaction. The self catalyst in an autocatalytic reaction is named an Autocatalyst. The concentration-time graph of autocatalysis is generally in a Sigmoid form. An autocatalytic reaction usually proceeds slowly initially, but the reaction rate increases with time due to a small amount of catalyst.