Testing for Halide Ions (AQA A Level Chemistry): Revision Note

Testing for Halides

Silver ions and 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 used 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 shown in the following ionic equation:

Ag⁺ (aq) + X^- (aq) → AgX (s)

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)

• Silver chloride (AgCl) is a white precipitate

• Silver bromide (AgBr) is a cream precipitate

• Silver iodide (AgI) is a yellow precipitate

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 solution, the unknown halide is chloride

• If the precipitate does not dissolve in dilute ammonia, 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

Reaction of Halide Ions with Silver Nitrate & Ammonia Solutions

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 added dropwise to sodium chloride crystals to produce hydrogen chloride gas
  

• 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 orange-brown fumes/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 (s) + SO₂ (g) + 2H₂O (l)

• Iodine is seen as a brown solution or black solid

• 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 rotten eggs

Summary of the Halide Ion Reactions with Concentrated Sulfuric Acid 

Halide Ion Reactions with Concentrated Sulfuric Acid Table

Related topics

• Chemical Properties of Group 7

• Redox & Disproportionation Reactions