Chain Reaction
Definition and meaning of Chain Reaction in chemistry.
A chain reaction is a sequence of reaction steps in which a reactive intermediate produced in one step, such as a free radical, generates another reactive intermediate that carries the process forward through repeated cycles. Each cycle reproduces the species needed to sustain the next, allowing the overall reaction to proceed rapidly, often explosively, once initiated.
In more detail
A chain reaction proceeds through three kinetically distinct stages: initiation, which generates the first reactive intermediates (often by homolytic bond cleavage from heat, light, or a radical initiator); propagation, in which the intermediate reacts with a stable molecule to form a product and regenerate a new reactive intermediate, repeating many times; and termination, where two reactive intermediates combine and destroy each other, ending that chain. If each propagation step produces more than one chain carrier, the reaction can branch and accelerate uncontrollably, as in explosions and nuclear fission.
Key facts
| Field | Physical Chemistry |
|---|---|
| Key stages | Initiation, propagation, termination |
| Chain carriers | Free radicals (e.g., Cl•, CH3•) |
| Related phenomenon | Branched chain reactions cause explosions |
In the gas-phase chlorination of methane, UV light splits Cl2 into two Cl atoms (initiation); each Cl atom abstracts a hydrogen from CH4 to form HCl and a CH3 radical, which then reacts with another Cl2 to form CH3Cl and a new Cl atom (propagation), and the cycle repeats until two radicals collide (termination).
Frequently asked questions
What is the difference between a chain reaction and a branched chain reaction?
In a simple (unbranched) chain reaction, each propagation step consumes one radical and produces exactly one new radical, so the rate stays roughly constant. In a branched chain reaction, a propagation step produces more than one new radical, so the number of chain carriers grows exponentially, often leading to a rapid explosion.
Why do chain reactions eventually stop?
They stop through termination steps, in which two reactive intermediates (such as two radicals) collide and combine into a stable, non-reactive molecule, removing chain carriers from the system faster than initiation can replace them.