Edexcel A Level Chemistry:复习笔记2.3.4 Halide Ion Reactions

Halide Ion Reactions

 

Concentrated sulfuric acid

  • Chloride, bromide and iodide ions react with concentrated sulfuric acid to produce toxic gases
  • These reactions should therefore be carried out in a fume cupboard
  • The general reaction of the halide ions with concentrated sulfuric acid is:

H2SO4(l) + X-(aq) → HX(g) + HSO4-(aq)

(general equation)

Where X- is the halide ion

Reaction of chloride ions with concentrated sulfuric Acid

  • Concentrated sulfuric acid is dropwise added to sodium chloride crystals to produce hydrogen chloride gas

2.3-Group-17-Apparatus-Set-Up

Apparatus set up for the reaction of sodium chloride with concentrated sulfuric acid

 

  • The reaction that takes place is:

H2SO4 (l) + NaCl (s) → HCl (g) + NaHSO4 (s)

  • The HCl gas produces is seen as white fumes

Reaction of bromide ions with concentrated sulfuric acid

  • The thermal stability of the hydrogen halides decreases down the group
  • The reaction of sodium bromide and concentrated sulfuric acid is:

H2SO4 (l) + NaBr (s) → HBr (g) + NaHSO4 (s)

  • The concentrated sulfuric acid oxidises HBr which decomposes into bromine and hydrogen gas and sulfuric acid itself is reduced to sulfur dioxide gas:

2HBr (g) + H2SO4 (l) → Br2 (g) + SO2 (g) + 2H2O (l)

  • The bromine is seen as a reddish-brown gas

Reaction of iodide ions with concentrated sulfuric acid

  • The reaction of sodium iodide and concentrated sulfuric acid is:

H2SO4 (l) + NaI (s) → HI (g) + NaHSO4 (s)

  • Hydrogen iodide decomposes the easiest
  • Sulfuric acid oxidises the hydrogen iodide to several extents:
  • The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur dioxide gas:

2HI (g) + H2SO4 (l) → I2 (g) + SO2 (g) + 2H2O (l)

  • Iodine is seen as a violet/purple vapour
  • The concentrated sulfuric acid oxidises HI and is itself reduced to sulfur:

6HI (g) + H2SO4 (l) → 3I2 (g) + S (s) + 4H2O (l)

  • Sulfur is seen as a yellow solid
  • The concentrated sulfuric acid oxidises HI and is itself reduced to hydrogen sulfide:

8HI (g) + H2SO4 (l) → 4I2 (g) + H2S (s) + 4H2O (l)

  • Hydrogen sulfide has a strong smell of bad eggs

Halide ion reactions with concentrated sulfuric acid table

2.3-Group-17-Table-2_Reactions-of-Halide-Ions

Silver ions & ammonia

  • Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding a silver nitrate solution followed by ammonia solution
  • The halide ions will react with the silver nitrate solution as follows:

AgNO3 (aq) + X- (aq) → AgX (s) + NO3- (aq)

Ag+ (aq) + X- (aq) → AgX (s)

  • X- is the halide ion in both equations
  • If the unknown solution contains halide ions, then a precipitate of the silver halide will be formed (AgX)

 

2.3-Group-17-Halide-Test

A silver halide precipitate is formed upon addition of silver nitrate solution to halide ion solution

 

  • Dilute followed by concentrated ammonia is added to the silver halide solution to identify the halide ion
  • If the precipitate dissolves in dilute ammonia the unknown halide is chloride
  • If the precipitate does not dissolve in dilute but in concentrated ammonia the unknown halide is bromide
  • If the precipitate does not dissolve in dilute nor concentrated ammonia the unknown halide is iodide

2.3-Group-17-Dissolving-of-Silver-Halide

 

Silver chloride and silver bromide precipitates dissolve on addition of ammonia solution whereas silver iodide doesn’t

Reaction of halide ions with silver nitrate & ammonia solutions table

2.3-Group-17-Table-1_Reactions-of-Halide-Ions

Reactions with hydrogen halides

  • When a halogen reacts with hydrogen a hydrogen halide is produced, for example:
    • Cl2 (g) + H2 (g) → 2HCl (g)
  • These hydrogen halides react with ammonia gas to form ammonium halides
    • NH3 (g) + HCl (g) → NH4Cl (s)
  • Hydrogen halides will also react with water
  • For example, hydrogen chloride also dissolves in water to form hydrochloric acid
    • HCl (g) → H+ (aq) + Cl- (aq)

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