Edexcel A Level Chemistry:复习笔记2.3.3 Halogen Redox Reactions

Halogen Redox Reactions

 

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:

2.3-Group-17-General-Equation

  • The ionic equation is:

2.3-Group-17-Ionic-Equation

  • 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:

     

2.3-Group-17-Oxidation-Half-Equation

  • 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:

     

2.3-Group-17-Reduction-Half-Equation

 

Chlorine in hot alkali (70 oC)

  • The reaction that takes place is:

2.3-Group-17-General-Equation-2

  • The ionic equation is:

2.3-Group-17-Ionic-Equation-2_1

  • 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:

     

2.3-Group-17-Oxidation-Half-Equation-2

  • 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:

     

2.3-Group-17-Reduction-Half-Equation-2

 

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

2.3-Group-17-Chlorine-and-Water-Reaction

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

 

转载自savemyexams

更多Alevel课程
翰林国际教育资讯二维码