Halide reactions and predicting halogen chemistryEdexcel International A Level Chemistry: Revision notes

Section 1

Solid halides with concentrated sulfuric acid

Warming a solid Group 1 halide with concentrated sulfuric acid first gives a hydrogen halide by an acid–base (not redox) step:

$NaX + H_2SO_4 \rightarrow NaHSO_4 + HX$

The hydrogen halide appears as steamy fumes because it dissolves in moisture in the air. What happens next depends on how easily the halide ion is oxidised.

Key termsconcentrated sulfuric acidsteamy fumes

Section 2

Chloride, bromide and iodide results

Chloride: only steamy white fumes of HCl. Sulfur stays at +6, so there is no redox.

Bromide: HBr is oxidised to orange-brown $Br_2$ and the acid is reduced to $SO_2$ (+6 to +4), a colourless choking gas:

$2HBr + H_2SO_4 \rightarrow Br_2 + SO_2 + 2H_2O$

Iodide: HI is a much stronger reducing agent, so $I_2$ (purple fumes, black solid) forms and sulfur is reduced to $SO_2$ , to sulfur (yellow solid) and to $H_2S$ (rotten egg smell, +6 to −2):

$8HI + H_2SO_4 \rightarrow 4I_2 + H_2S + 4H_2O$

Key termsreducing agentoxidation number

Exam tip

Use the sulfur product to judge reducing power: SO₂ (+4) is a small change, S (0) larger, H₂S (−2) the largest.

Section 3

The trend in reducing ability

Reducing ability increases $Cl^- < Br^- < I^-$ . Going down the group the ions get larger, the outer electron is further from the nucleus and more shielded, so it is lost more easily. The halide ion is the reducing agent, so iodide is oxidised most readily.

Key termsreducing ability

Common mistake

Do not say chlorine is 'less reactive than iodine' to explain this. The halogens get weaker oxidising agents down the group, while the halide ions get stronger reducing agents.

Section 4

Silver nitrate tests for halide ions

Add dilute nitric acid first to remove carbonate or hydroxide ions, then aqueous silver nitrate. A precipitate forms: $Ag^+ + X^- \rightarrow AgX$ .

$Cl^-$ : white ppt, dissolves in dilute ammonia
$Br^-$ : cream ppt, dissolves in concentrated ammonia
$I^-$ : yellow ppt, insoluble in concentrated ammonia

The precipitates dissolve because ammonia forms the soluble complex $[Ag(NH_3)_2]^+$ .

Key termssilver halide precipitatecomplex ion

Section 5

Hydrogen halides with ammonia and water

With ammonia gas, hydrogen halides give white smoke of an ammonium halide: $NH_3 + HCl \rightarrow NH_4Cl$ . Ammonia acts as a base.

In water, hydrogen halides dissolve and ionise to form acidic solutions: $HX + H_2O \rightarrow H_3O^+ + X^-$ .

Key termsammonium halide

Section 6

Predicting fluorine, astatine and their compounds

Use the trends:

Fluorine: pale yellow gas, strongest oxidising agent, $F^-$ is the weakest reducing agent. NaF + conc. $H_2SO_4$ gives HF only. Silver fluoride is soluble, so no precipitate. HF has the strongest H–X bond.
Astatine: black solid, weakest oxidising agent, $At^-$ the strongest reducing agent, so NaAt would reduce the acid, probably to $H_2S$ . AgAt would be a dark precipitate insoluble in concentrated ammonia. HAt would be the least stable hydrogen halide.

Key termsprediction from trends

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Halide reactions and predicting halogen chemistry

A technician tests three unlabelled solid sodium halides, each known to be sodium chloride, sodium bromide or sodium iodide. In a fume cupboard she adds a few drops of concentrated sulfuric acid to a small sample of each and records what happens.
Explain why iodide ions reduce concentrated sulfuric acid but chloride ions do not.2 marks
A student adds aqueous silver nitrate, acidified with dilute nitric acid, to separate aqueous solutions of potassium chloride, potassium bromide and potassium iodide. She then tests how each precipitate behaves with aqueous ammonia.
Explain why dilute nitric acid is added to the halide solution before the silver nitrate.2 marks
Hydrogen chloride gas is made in the laboratory by warming solid sodium chloride with excess concentrated sulfuric acid. In one experiment the gas is passed into a gas jar of ammonia gas. In another it is bubbled into water. Take the molar volume of a gas at room temperature and pressure as 24 dm³ mol⁻¹.
State what is seen when hydrogen chloride gas meets ammonia gas, write the equation for the reaction and state the role of the ammonia.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).