The reactions of chlorine — common mistakes

Examiners frequently contrast the reaction with cold, dilute NaOH with the reaction with hot, concentrated NaOH. With hot, concentrated alkali, chlorine is oxidised further to the +5 oxidation state, forming sodium chlorate(V), NaClO₃. The equation is: $3Cl_{2}(g) + 6NaOH(aq) \to 5NaCl(aq) + NaClO_{3}(aq) + 3H_{2}O(l)$. Always check the conditions given in the question.

The reaction is temperature-dependent. In cold, dilute conditions, chlorine disproportionates to oxidation states -1 (in Cl⁻) and +1 (in ClO⁻). In hot, concentrated conditions, the higher temperature provides sufficient activation energy for further oxidation of chlorine. The intermediate chlorate(I) ion is unstable at high temperatures and disproportionates further, leading to the more stable chlorate(V) ion (ClO₃⁻), where chlorine has an oxidation state of +5.

Chlorine gas is much easier and cheaper to transport and store in large, pressurised quantities than chloric(I) acid, which is unstable. Adding chlorine gas to water allows the disinfectant to be generated 'in situ' (in place) in a controlled manner. The equilibrium Cl₂(aq) + H₂O(l) ⇌ HCl(aq) + HOCl(aq) also acts as a reservoir, producing more HOCl as it is used up, providing a lasting disinfecting effect throughout the water distribution system.

Not exactly. The term 'bleach' usually refers to a solution, not a pure compound. Household bleach is an aqueous solution of sodium chlorate(I) (NaClO), which is the active ingredient. The solution also contains sodium chloride (NaCl) and leftover sodium hydroxide (NaOH) from its manufacturing process.