Reactions with Group 1 & 2 metals

• The halogens react with some metals to form?ionic compounds?which are?metal?halide?salts
• In all reactions where halogens are reacting with metals, the metals are being oxidised
• Reaction of sodium and chlorine
  • 2Na (s) + Cl2?(g) → 2NaCl (s)
  • Na is being oxidised, the oxidation number is changing from 0 to +1

   

• Calcium is a group 2 metal:
  • Ca (s) + Br2?(l) → CaBr2?(s)
  • Ca is being oxidised, the oxidation number is changing from 0 to +2

   

• Therefore the halogens are acting as oxidising agents

Reactions with Iron(II)

• Chlorine and bromine can oxidise iron(II) to iron(III)

Cl2?(g) + 2Fe2+?(aq) → 2Cl-?(aq) + 2Fe3+?(aq)

Br2?(g) + 2Fe2+?(aq) → 2Br-?(aq) + 2Fe3+?(aq)

• However, iodine is not a strong enough reducing argent to oxidise iron(II) to iron(III)
• Iodine is actually oxidised from iodide ions to iodine by iron(III)

2I-?(aq) + 2Fe3+?(aq) → I2?(aq) + 2Fe2+?(aq)

Disproportionation reaction

• A?disproportionation?reaction?is a reaction in which the same species is both oxidised and reduced
• The reaction of?chlorine?with?dilute alkali?is an example of a disproportionation reaction
• In these reactions, the chlorine gets oxidised and reduced at the same time
• Different reactions take place at different?temperatures?of the dilute alkali

Chlorine in cold alkali (15?oC)

• The reaction that takes place is:

• The ionic equation is:

• The ionic equation shows that the chlorine gets both oxidised and reduced
• Chlorine gets oxidised as there is an increase in ox. no. from 0 to +1 in ClO-(aq)
  • The half-equation for the oxidation reaction is:

   

• Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl-(aq)
  • The half-equation for the reduction reaction is:

   

Chlorine in hot alkali (70?oC)

• The reaction that takes place is:

• The ionic equation is:

• The ionic equation shows that the chlorine gets both oxidised and reduced
• Chlorine gets oxidised as there is an increase in ox. no. from 0 to +5 in ClO3-(aq)
  • The half-equation for the oxidation reaction is:

   

• Chlorine gets reduced as there is a decrease in ox. no. from 0 to -1 in Cl-(aq)
  • The half-equation for the reduction reaction is:

   

Drinking water

• Chlorine can be used to clean water and make it drinkable
• The reaction of chlorine in water is a?disproportionation reaction?in which the chlorine gets both?oxidised?and?reduced

The disproportionation reaction of chlorine with water in which chlorine gets reduced to HCl and oxidised to HClO

• Chloric(I) acid (HClO)?sterilises?water by killing?bacteria
• Chloric acid can further dissociate in water to form ClO-(aq):

HClO (aq) → H+?(aq) + ClO-?(aq)

• ClO-(aq) also acts as a?sterilising agent?cleaning the water