A chain reaction is a reaction in which a reactive intermediate, such as a radical, is regenerated so that it can repeatedly react with more molecules. In IB Chemistry, a radical chain reaction has three stages: initiation, propagation, and termination.
This is part of Reactivity 3.3, Electron sharing reactions, and is studied at both SL and HL.
The Chain-Reaction Mechanism
Consider the radical substitution reaction between methane and chlorine under ultraviolet light:
| Stage | What happens | Example |
|---|---|---|
| Initiation | Radicals form from stable molecules. UV light causes homolytic fission, meaning each chlorine atom receives one electron from the broken bond. |
The chlorine radical regenerated during propagation can react with another methane molecule. One initiation event can therefore lead to many substitution cycles before termination occurs.
At the particle level, UV radiation supplies enough energy to break the chlorinetochlorine bond evenly. The two bonding electrons separate, producing two chlorine radicals. Each propagation cycle converts one methane molecule into chloromethane while regenerating a chlorine radical, so the overall equation is obtained by adding the propagation steps and cancelling the radical intermediates.
Because radicals are highly reactive, termination occurs whenever two radicals collide with a suitable orientation.
A common misconception is that a radical is permanently used up during propagation. In fact, the radical's identity may change between steps, but a radical is regenerated, maintaining the chain.
Exam Technique
For an IB explain question, identify all three stages and state what happens to the radicals. Use radical dots, show UV light for initiation, and ensure termination equations contain two radical reactants but no radical product.