Chain Reaction: Definition, Mechanism, Role

You can witness chain Reactions in many chemical reactions and different natural phenomena. For instance, nucleophilic substitution reactions in the field of organic chemistry and nuclear reactions in the field of nuclear physics are examples of chain reactions. On the other hand, various phenomena such as photosynthesis, where glucose is made from carbon dioxide and water, or ozone depletion in which the chlorofluorocarbons reduce ozone molecules into oxygen happen through a series of chemical reactions, that is chain reactions. Here, we will learn more about the mechanism associated with chain reactions and answer some important questions.

Key Takeaways- Chain reactions, chain carriers,free radical,reactive intermediate complex, branching chain reactions, elementary steps- chain initiation.

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What is a Chain Reaction?

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Chain reactions, as the name suggests is the series of reactions that occur one after the other. Sometimes, the growth of reactions occurs exponentially (as in case of nuclear fission). Chain reaction is a kind of reaction in which a set of reactions occurs in a consecutive manner until the reactants are totally exhausted. It consists of several elementary steps, with each step having a chain carrier. There is a negligible time gap between the occurrence of these elementary steps.

Max Bodenstein, a German chemist, first proposed the concept of chemical chain reactions in 1913. He stated that when two molecules react, they produce not only molecules of the final reaction products, but also certain unstable molecules that have a far higher chance of reacting with the parent molecules than the initial reactants.

Also read: Difference between mixture and solution

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Mechanism of Chain Reaction

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The elementary steps involved in a chain reaction are listed below:-

• initiation step
• chain propagation steps
• chain branching steps
• chain inhibition steps
• chain termination steps

In some chain reactions all the above elementary steps are involved while in others one or more steps are excluded.

1. Initiation step

The Initiation step involves the breaking up of the reactant(s) into a free Radical which is the unstable reaction intermediate.This free radical is highly reactive in nature. To understand this clearly, let’s take the example of Chlorination of ethane. It is a chain reaction in which the initiation occurs by the photo dissociation of chlorine molecules when exposed to light.

Cl₂ + hv→ Cl• + Cl•

2. Chain propagation step

In this elementary step, the reactive radical or particle (that was generated in the initiation step) reacts with one of the reactant molecules to form another reactive radical. In the Chlorination of ethane one radical of chlorine reacts with one molecule of ethane to give 1-chloroethane and a hydrogen radical. The second molecule of chlorine radical reacts with another molecule of ethane to give an ethyl radical.

Cl• + CH₃CH₃ → ClH₂CH₃ + H•

Cl• + CH₃CH₃ → CH₃CH₂• + HCl

In each chain propagation step when a radical is used up, it leads to the formation of a new radical, causing the continuation of the reactions. Once a radical is reacted, it gives heat and light which provides a suitable condition to promote the next reaction, which in turn would produce more heat and light. Hence appropriate conditions are established to continue the process.

3. Chain Inhibition Step

The elementary steps that do not lead to the formation of products are termed as Chain Inhibition Reactions or steps. The following reactions that occur during the Chlorination of ethane are chain inhibition reactions-

Cl• + ClCH₂CH₃ → CH₃CH₂• + Cl

Cl• + HCl → H• + Cl₂

H• + ClCH₂CH₃ → CH₃CH₃ + Cl•

These reactions are in a way that lead to the formation of the reactants, hence try to hinder the overall chain reaction.

4. Chain Termination Step

The Chain Termination Step comprises all the reactions that lead to the formation of the products and is the last step in a Chain reaction process. This process occurs when all the reactants have been used up and the remaining reactive radicals undergo changes to form neutral molecules.

For example:

Cl• + Cl• → Cl₂

H• + H• → H₂

H• + Cl• → HCl

CH₃CH₂• + •CH3₃CH₂ → CH₃CH₂-CH₂CH₃ (dimer)

The above products are obtained in small amounts in the Chain reaction (minor product). Always keep in mind that in a Chain reaction a number of products are obtained of which some are reactants. In other words, not all the reactants are consumed in a Chain reaction.

Also read: Collision Theory of Chemical Reaction

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Branching chain Reactions

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It is a special case under chain propagation reaction. In a Branching chain reaction, the number of radicals obtained as output is always more than the number of radicals consumed. These Branching chain reactions lead to a greater number of reactive components which increases the number of chain propagation reactions exponentially. This usually happens in case of explosions.

For example, when hydrogen and oxygen are mixed with each other, there happens a violent explosion. In it the following reactions may take place:-

H• + O₂ → HO• + •O•

Here one radical (H•) produces three radicals- an HO•, which is and •O•, which is a di-radical. Hence, in a Branching chain reaction the radicals obtained are always more than the radicals consumed.

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Role of Radical and Chain carriers in a Chain reaction

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Most chemical chain reactions have very reactive intermediates called free radicals. The intermediate product that maintains the chain reaction is called a chain carrier. The reactive chlorine radical is responsible for causing further reactions in the chlorination process. These atoms or fragments are usually derived from stable molecules due to photo- or heat-dissociation. The main function of these free radicals is to proceed the chain reaction in the forward direction hence, continuing the steps of chain reaction. 

In Polymerisation, explosion or spoilage, in all these phenomena, the chain carriers play a very important role in the completion of the chain reaction.

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Things to remember

• Most of the reactions in nature proceed not in one step, but take place through a series of steps/reactions. Those kinds of reactions are called Chain reactions.
• The chain reactions have certains steps that happen one by one. These steps are  chain initiation, chain propagation, chain branching, chain inhibition and chain termination.
• At each step of the chain reaction there are certain reactive intermediate complexes which are formed and these tend to start the next elementary step.
• The reactive intermediate complexes are mostly free radicals which are highly reactive in nature.
• The reactive free radicals that tend to continue the series of reactions are termed as Chain carriers.
• Polymerisation, explosion and spoilage of food are some of the different types of Chain reactions.
• Explosion of a bomb, catching of fire, reaction of ethane with chlorine and nuclear reactions are some examples of Chain reactions.

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