9701 · 11.4
The reactions of chlorine
Chlorine doesn't just react; it can simultaneously oxidise and reduce itself in a process called disproportionation, creating useful substances like bleach. It also reacts with organic molecules under UV light to form halogenoalkanes.
Need to know
What you need to know
- The oxidation state of chlorine changes from 0 in Cl₂ to -1 in hydrochloric acid (HCl) and +1 in chloric(I) acid (HOCl).
- The reaction is reversible, indicated by the ⇌ symbol. In sunlight, the equilibrium is disturbed as HOCl decomposes to HCl and O₂.
- Chloric(I) acid is a weak acid but a powerful oxidising agent, responsible for the bleaching action of chlorine water.
Explanation
Chlorine's Double Life
- When chlorine dissolves in water, it forms a mixture of hydrochloric acid (HCl) and chloric(I) acid (HOCl). This is a reversible disproportionation reaction where chlorine is both oxidised (+1 in HOCl) and reduced (-1 in HCl).
- In cold, dilute sodium hydroxide, chlorine disproportionates to form sodium chloride (Cl⁻) and sodium chlorate(I) (ClO⁻), the active ingredient in household bleach. The reaction goes to completion because the alkali neutralises the acidic products.
- The addition of chlorine to drinking water is a major public health measure. The chloric(I) acid (HOCl) formed is a powerful oxidising agent that kills harmful bacteria and pathogens.
- With alkanes like methane, chlorine undergoes free-radical substitution in the presence of UV light. This reaction breaks the Cl-Cl bond homolytically, initiating a chain reaction that substitutes hydrogen atoms with chlorine atoms.