Chain Reactions

Consider the reaction between hydrogen and bromine, forming hydrogen bromide. This reaction follows a multi-step mechanism involving bromine and hydrogen radicals. Since radicals are continuously formed and regenerated, this reaction is called a chain reaction.

The reaction begins with initiation. A bromine molecule collides with a third body, M, which provides energy to split it into two bromine radicals.

During propagation, a bromine radical reacts with hydrogen to form hydrogen bromide and a hydrogen radical. The hydrogen radical then reacts with another bromine molecule, continuing the chain.

Inhibition happens when hydrogen bromide reacts with a hydrogen radical. This reduces the concentration of hydrogen radicals and slows product formation.

Finally, termination happens when two bromine radicals recombine to form bromine in the presence of M, which absorbs the energy released during the reaction.

Although radicals drive the reaction, the observed rate law depends only on stable species. The square-root dependence on bromine suggests radical formation, while hydrogen bromide in the denominator indicates product inhibition.