2.3.3 Testing for Halide Ions

Question 1

How can we test for halides?

Answer 1

• Dissolve solution in nitric acid
• Adding silver nitrate solution dropwise (prevents false positive results from carbonate ions precipitating out with silver ions)
• Halide ions react with silver nitrate solution

Ag+ (aq) + X- (aq) → AgX (s) (ionic equation)

If unknown solution contains halide ions a precipitate of the silver halide is formed

Question 2

What colour is AgCl, AgBr and AgI precipitate?

Answer 2

White, cream and yellow

Question 3

Why do we add ammonia to the solution of silver nitrate + halide?

Answer 3

Colours can look similar so a follow up test can be done

Question 4

Halide testing : results for adding silver nitrate + nitric acid, and dilute / concentrated ammonia

Answer 4

Question 5

What happens when Concentrated sulphuric acid is added to halide ions?

Answer 5

• Toxic gas produced

H2SO4(l) + X-(aq) → HX(g) + HSO4-(aq)

Question 6

Reaction of NaCl and concentrated H2SO4

Answer 6

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

HCL gas is observed as white fumes

Question 7

Reaction of NaBr and concentrated H2SO4

Answer 7

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

H2So4 oxidises HBr decomposing into bromine and H2. H2SO4 reduced to sulfur dioxide gas.

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

Bromine appears as reddish brown gas

Question 8

All reactions of NaX with concentrated sulphuric acid

Answer 8