Chain Reaction and Their Characteristics
Chain Reaction
A chain reaction is one in which some intermediates are consumed and regenerated in a cycle of reactions before products are formed They usually have following steps-
Chain Initiation: Reactive species (atoms or free radicals) are produced thermally or photochemically.
Chain Propagation: Reactive species formed chain initiation reacts with one or more reactants and products thus formed are again reactive species. The reaction is much faster so that the concentration of reactive species remains low. Most of the products are formed here.
Chain Termination: Reactive species are destroyed and are no longer available for chain propagation.
Chain Transfer: Reactive species may interact with some foreign substances instead of reactants.
Characteristics of Chain Reactions
Important characteristics of chain reactions are given below-
1. The rate of reaction is considerably greater than that expected from the frequency factor and energy of activation calculations.
2. The rate of chain reactions is influenced by the changes in the shape of the container. The rate is reduced by lowering the radius of the container.
3. Abnormal influence of pressure or concentration of reactants takes place on the reactions.
4. Abnormally high quantum yield is observed in photochemical reactions.
5. Explosion takes place in many reactions.
6. The rate is accelerated by inert gases and retarded by decreasing surface dimensions of the reacting vessel.
7. These reactions show fractional orders.
8. These reactions are sensitive to inhibitors.
9. Contrast to non-chain reactions, they begin at zero rate, rise to the maximum and then fall off with time.
Kinetics of chain reaction is discussed in term of chain length which is defined as the number of product molecule produced per chain carrier generated in its initiation step. It also depends on the relative rates of chain propagation and chain termination steps.
Chain length = d[product]/d[initioan step]
Thermal chain reactions are of two types-stationary and branching. In the former, one chain carrier (free radical or atom) is formed for one chain carrier consumed in each propagating step e.g. H2-Br2 reaction, pyrolysis of acetaldehyde, thermal decompositio of N2O5 and thermal decoposition of ozone etc.
In the branching chain reactions, more than one chain carrier is produced for one chain carrier consumed in each propagating step. In such reactions, the number of chain carriers increases as the reaction proceeds. The rate of reaction, therefore keeps on increasing and often results in explosion eg H2O2 reaction. In this reaction, the number of chain carriers get reduced at low pressure by their removal through collisions against the walls of the container. The rate is therefore under control. On increasing pressure, a stage is obtained when the rate at which carriers are formed becomes greater than the rate at which they are destroyed and thus reaction becomes explosive By further increase of pressure, the reaction is more and more explosive and then at a certain stage, it again becomes non-explosive because carriers are destroyed not only by collision with walls but also by collision amongst themselves. At still higher pressures, the reaction again becomes explosive due to the existence of some other chain mechanism in which the rate of formation of carriers is more than the rate of their destruction.