Other Reactions of Halides

Inorganic Chemistry of Group 7

What reagent is used to test for chloride, bromide and iodide ions?

Aqueous silver nitrate solution.

Why is dilute nitric acid added before silver nitrate in the halide test?

To remove interfering ions such as carbonate ions that could also form precipitates with silver ions.

Why is nitric acid used rather than hydrochloric acid before adding silver nitrate?

Hydrochloric acid contains Cl⁻ ions, which would react with Ag⁺ and give a false positive white precipitate.

What is the general ionic equation for the reaction between silver ions and halide ions?

Ag⁺(aq) + X⁻(aq) → AgX(s)

What precipitate forms when silver nitrate is added to chloride ions?

A white precipitate of silver chloride, AgCl.

What is the ionic equation for the test for chloride ions?

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

What precipitate forms when silver nitrate is added to bromide ions?

A cream precipitate of silver bromide, AgBr.

What is the ionic equation for the test for bromide ions?

Ag⁺(aq) + Br⁻(aq) → AgBr(s)

What precipitate forms when silver nitrate is added to iodide ions?

A yellow precipitate of silver iodide, AgI.

What is the ionic equation for the test for iodide ions?

Ag⁺(aq) + I⁻(aq) → AgI(s)

What are the silver halide precipitate colours in the order Cl⁻, Br⁻, I⁻?

White, cream, yellow.

How can silver halide precipitates be distinguished more reliably?

By testing their solubility in dilute and concentrated aqueous ammonia.

What happens to AgCl when dilute aqueous ammonia is added?

It dissolves.

What happens to AgCl when concentrated aqueous ammonia is added?

It dissolves.

What happens to AgBr when dilute aqueous ammonia is added?

It is insoluble.

What happens to AgBr when concentrated aqueous ammonia is added?

It dissolves.

What happens to AgI when dilute aqueous ammonia is added?

It is insoluble.

What happens to AgI when concentrated aqueous ammonia is added?

It remains insoluble.

What is the ammonia solubility pattern for AgCl?

Soluble in both dilute and concentrated aqueous ammonia.

What is the ammonia solubility pattern for AgBr?

Insoluble in dilute ammonia but soluble in concentrated ammonia.

What is the ammonia solubility pattern for AgI?

Insoluble in both dilute and concentrated ammonia.

What equation shows silver chloride dissolving in ammonia?

AgCl(s) + 2NH₃(aq) → [Ag(NH₃)₂]⁺(aq) + Cl⁻(aq)

Why does adding ammonia help identify halide ions?

AgCl, AgBr and AgI have different solubilities in aqueous ammonia.

Can the silver nitrate test be used to detect fluoride ions?

No, because silver fluoride is soluble in water and therefore does not form a precipitate.

What are hydrogen halides?

Compounds of hydrogen with halogens: HF, HCl, HBr and HI.

What physical state are the hydrogen halides at room temperature?

They are colourless gases.

What type of molecules are hydrogen halides?

Polar diatomic molecules.

What happens when hydrogen halides are added to water?

They react with water to form acidic solutions.

What acid is formed when hydrogen fluoride dissolves in water?

Hydrofluoric acid.

What is the equation for HF reacting with water?

HF + H₂O ⇌ H₃O⁺ + F⁻

Why is the reaction of HF with water shown with a reversible arrow?

HF is a weak acid and only partially ionises in water.

What acid is formed when hydrogen chloride dissolves in water?

Hydrochloric acid.

What is the equation for HCl reacting with water?

HCl + H₂O → H₃O⁺ + Cl⁻

What acid is formed when hydrogen bromide dissolves in water?

Hydrobromic acid.

What is the equation for HBr reacting with water?

HBr + H₂O → H₃O⁺ + Br⁻

What acid is formed when hydrogen iodide dissolves in water?

Hydroiodic acid.

What is the equation for HI reacting with water?

HI + H₂O → H₃O⁺ + I⁻

Which hydrogen halide is a weak acid?

HF.

Which hydrogen halides are strong acids?

HCl, HBr and HI.

What happens when hydrogen halides react with ammonia gas?

They form solid ammonium halide salts.

What is the equation for ammonia reacting with hydrogen chloride?

NH₃(g) + HCl(g) → NH₄Cl(s)

What is observed when ammonia gas and hydrogen chloride gas react?

White solid ammonium chloride forms.

Why does ammonium chloride form closer to the hydrogen chloride end in a diffusion experiment?

NH₃ has a lower relative molecular mass and diffuses faster than HCl, so it travels further before the gases meet.

What is the relative molecular mass of ammonia?

17.0.

What is the relative molecular mass of hydrogen chloride?

36.5.

Which diffuses faster, NH₃ or HCl?

NH₃.

Why does NH₃ diffuse faster than HCl?

NH₃ has a lower relative molecular mass.

What product forms when ammonia reacts with hydrogen bromide?

Ammonium bromide, NH₄Br.

What is the equation for ammonia reacting with hydrogen bromide?

NH₃(g) + HBr(g) → NH₄Br(s)

What product forms when ammonia reacts with hydrogen iodide?

Ammonium iodide, NH₄I.

What is the equation for ammonia reacting with hydrogen iodide?

NH₃(g) + HI(g) → NH₄I(s)

Are the reactions in this topic redox reactions?

No.

Why are the reactions of halides in this topic not redox reactions?

The oxidation number of the halogen remains −1.