AQA A Level Chemistry复习笔记2.3.3 Testing for Halide Ions

Testing for Halides

 

Silver ions & ammonia

  • Halide ions can be identified in an unknown solution by dissolving the solution in nitric acid and then adding silver nitrate solution dropwise
  • The nitric acid is to prevent any false positive results from carbonate ions precipitating out with silver ions
  • The halide ions will react with the silver nitrate solution as follows:

Ag+ (aq) + X- (aq) → AgX (s)

(ionic equation)

Where X- is the halide ion

  • The state symbols are key in this equation
  • If the unknown solution contains halide ions, a precipitate of the silver halide will be formed (AgX)

 

Testing-for-Halides

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

 

  • Silver chloride (AgCl) is a white precipitate
  • Silver bromide (AgBr) is a cream precipitate
  • Silver iodide (AgI) is a yellow precipitate

 

Testing-for-halide-ions-results

The silver halide precipitates are dense and characteristically coloured

 

Adding ammonia

  • Because the white, cream and yellow precipitates could look very similar in colour, ammonia is often used as a follow up test to determine which halide ion is present
  • 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 does dissolve in concentrated ammonia the unknown halide is bromide
  • If the precipitate does not dissolve in dilute or concentrated ammonia, then the unknown halide is iodide

 

AgCl-precipitateAgBr-precipitateAgI-precipitate

Silver chloride and silver bromide precipitates dissolve on addition of ammonia solution whereas silver iodide is insoluble in ammonia

Reaction of Halide Ions with Silver Nitrate & Ammonia Solutions

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

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

 

Sodium-chloride-and-concentrated-sulfuric-acid

Apparatus set up for the preparation of hydrogen chloride gas from sodium chloride with concentrated sulfuric acid

  • The reaction that takes place is:

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

  • The HCl gas produced is seen as white fumes

Reaction of bromide ions with concentrated sulfuric acid

  • 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 readily
  • Sulfuric acid oxidises the hydrogen iodide to form several products:
  • 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

Summary of the Halide Ion Reactions with Concentrated Sulfuric Acid

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

Exam Tip

It gets easier to oxidise the hydrogen halides going down Group 7: the halides become stronger reducing agents.

 

转载自savemyexams

 

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