Editing Chain reaction (section)

=== Detailed example: the hydrogen-bromine reaction ===
The reaction H<sub>2</sub> + Br<sub>2</sub> → 2 HBr proceeds by the following mechanism:<ref name=Laidler291>[[Keith J. Laidler|Laidler K.J.]], ''Chemical Kinetics'' (3rd ed., Harper & Row 1987) p.291-4  {{ISBN|0-06-043862-2}}</ref><ref name=Atkins>P. Atkins and J. de Paula ''Physical Chemistry'' (8th ed., W.H. Freeman 2006), p.830-1  {{ISBN|0-7167-8759-8}}</ref>
* Initiation 
: Br<sub>2</sub> → 2 Br• (thermal) or Br<sub>2</sub> + hν → 2 Br• (photochemical)
: each Br atom is a free radical, indicated by the symbol "•" representing an unpaired electron.

* Propagation (here a cycle of two steps)
: Br• + H<sub>2</sub> → HBr + H•
: H• + Br<sub>2</sub> → HBr + Br•
: the sum of these two steps corresponds to the overall reaction H<sub>2</sub> + Br<sub>2</sub> → 2 HBr, with [[catalysis]] by Br• which participates in the first step and is regenerated in the second step.

* Retardation (inhibition)
: H• + HBr → H<sub>2</sub> + Br•
: this step is specific to this example, and corresponds to the first propagation step in reverse.

* Termination 2 Br• → Br<sub>2</sub>
: recombination of two radicals, corresponding in this example to initiation in reverse.

As can be explained using the [[Steady state (chemistry)|steady-state approximation]], the thermal  reaction has an initial rate of [[Order of reaction#Fractional order|fractional order]] (3/2), and a complete rate equation with a two-term denominator ([[Order of reaction#Mixed order|mixed-order kinetics]]).<ref name=Laidler291/><ref name=Atkins/>