9701 · 11.4
The reactions of chlorine — FAQ
Frequently asked questions for 9701 The reactions of chlorine. Direct answers first, then deeper explanation — then practise with marking.
Why does chlorine react differently with cold, dilute NaOH compared to hot, concentrated NaOH?
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.
If chloric(I) acid is the disinfectant, why do we add chlorine gas to water and not just the acid itself?
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.
Is 'bleach' a single chemical?
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.