Chain Reactions Or Consecutive Reactions

Chain Reactions Or Consecutive Reactions

A series of successive elementary reaction. involving the generation and consumption of reactive intermediate species. Chain reactions are special type of consecutive runs that involves multiple steps.

Consecutive runs: The reactions in which intermediate species involved which is unstable and reacts to make products.

R → I → P

Steps in chain Reaction:

1. 1. Chain Initiation step:

This step consists of slow generation of reactive intermediate species that are initiated thermally or photochemically. Reactive species can be free radicals, atoms, energized molecules, or highly reactive or unstable commonly called as chain carriers.

R → R^⁰

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2. 2. Chain propagation Step:

Chain carriers produced in chain initiation step reacts with reactants and produce products and then regenerate the chain carriers

R → R^⁰

R^⁰+ R → P + n R^⁰

It will then lead to another chain.

n = any number. If

n = 1 then simple chain reaction.

n = many then branched chain reactions.

n = ∞ (millions, Billions) then explosive chain reactions.

3. 3. Chain transfer step:

It is propagation step (part of it). In this step, one type of chain carrier is consumed and another type is produced.

R^⁰+ R → P + n R^⁰/

New chain carriers are produced.

n R^⁰/+ R → R + R^⁰

4. 4. Elimination or Inhibition step:

Final product reacts with chain carrier and yield original reactant or by-product.

R^⁰+ P → R^⁰+ R

There is a decreasing effect on overall rate

5. 5. Chain Termination step:

Chain carriers are destroyed (no move available). Every chain reactions does not include all the five steps. But, initiation, propagation and termination steps are must. Other steps may or may not be present in all.

Characteristics of chain reactions

1. Every chain reaction consists of slow inhibition step, generating reactive inhibition species.

2. The rate of chain runs is very much higher as than expected Collision Theory.

3. The mechanism of chain reaction is highly complex.

4. In non-chain reactions, rate of new is higher in the start and decreases with time, but in chain reactions the rate is slow at start, then increases with time, then reaches to maximum and then decreases as shown in Figure 19.1.

5. Chain reactions have an induction period. Chain runs have zero rate at the beginning, thus it requires certain time to make products. That time lapse is called induction period. It is the time required to detect /measure rate of reaction.

6. Chain reactions are highly affected by impurities (can destroy chain runs).

7. Chain runs are highly influenced by pressure and concentration.

8. Chain runs are mostly fractional order.