11.4B - Group 17 (The Halogens)

know how chlorine water is formed and the reactions of chlorine with aqueous sodium hydroxide understand the use of chlorine as a water treatment and understand the balance of risks and benefits in this process

- justifies the use of chlorine for water purification (drinking water, pool water) - considers the formation of substances with chlorinated water and their effect on humans - considers the advantages and disadvantages of chlorination of water - considers the harmful effects of unchlorinated water

Disproportionation o The element chlorine (Cl 2, oxidation number=0) undergoes a type of redox reaction called disproportionation when it reacts with alkali. o Disproportionation can be thought of as a self reduction/oxidation reaction. o When chlorine reacts with dilute alkali some chlorine atoms are reduced and some are oxidised in the same reaction. o The actual reaction that takes place depends on the temperature.

Chlorine in cold alkali (15 °C) Cl 2(aq) + 2NaOH (aq) NaCl (aq) + NaClO (aq) + H 2 O (l) sodium chlorate(I) o The ionic equation for the reaction is: Cl 2(aq) + 2OH (aq) Cl (aq) + ClO (aq) + H 2 O (l) oxidation number of Cl o The ionic equation for this redox reaction can be split into two half-equations, showing the reduction and oxidation. The reduction reaction (in which chlorines oxidation number is reduced is): ½Cl 2 + e Cl 0 1 The oxidation reaction is: ½Cl 2 + 2OH ClO + H 2 O + e 0 +1

Chlorine in hot alkali (70 °C) o When we add chlorine and hot concentrated aqueous sodium hydroxide a different disproportionation reaction takes place:

Chlorination of water o Adding a small amount of chlorine to a water supply will kill bacteria and make the water safer to drink. o The chlorine undergoes disproportionation in water: Cl 2(aq) + H 2 O (l) HCl (aq) + HClO (aq) o HClO is called chloric(I) acid, and it decomposes slowly in solution. o One theory suggests that it produces reactive oxygen atoms that can kill bacteria in water: HClO HCl + [O]

Bleach o Bleach is an equal mixture of sodium chloride (NaCl) and sodium chlorate(I) (NaClO), made from chlorine and cold alkali. o It bleaches colours and stains because oxygen atoms from the chlorate(I) ions oxidise dye and other coloured molecules. o They also kill bacteria when toilets are cleaned with bleach.