Halide ions are reducing agents, and the bigger they are the more readily they hand over electrons. Concentrated sulfuric acid reveals the difference dramatically, and silver nitrate with ammonia tells the three ions apart.
Halide ions act as reducing agents, losing electrons to form halogens. Reducing power increases down the group (I− > Br− > Cl−) because the outer electrons of a larger ion are further from the nucleus and more easily lost. With concentrated sulfuric acid, chloride gives only HCl; bromide also reduces the acid to SO2 (S: +6 → +4); iodide reduces it as far as S and H2S (S: +6 → −2). To test for halides, acidify with dilute nitric acid and add AgNO3(aq): AgCl is white, AgBr cream, AgI yellow, distinguished further by their solubility in ammonia.
Run the silver nitrate test on each halide in the simulator, then add dilute and concentrated ammonia to the precipitates.
| Halide | With AgNO3(aq) | Dilute NH3 | Concentrated NH3 |
|---|---|---|---|
| Cl− | white AgCl | dissolves | dissolves |
| Br− | cream AgBr | does not dissolve | dissolves |
| I− | yellow AgI | does not dissolve | does not dissolve |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which halide ion is the strongest reducing agent?
Aqueous silver nitrate is added to an acidified solution and a cream precipitate forms. Which ion is present?
Which precipitate dissolves in dilute aqueous ammonia?
What is seen when concentrated sulfuric acid is added to solid sodium chloride?
Each solid halide first forms the hydrogen halide. What happens next depends on how strong a reducing agent the halide is.
| Solid | Products | Observations | S oxidation number |
|---|---|---|---|
| NaCl | NaCl + H2SO4 → NaHSO4 + HCl | steamy white fumes of HCl | stays +6 |
| NaBr | HBr, then 2HBr + H2SO4 → Br2 + SO2 + 2H2O | steamy fumes, orange-brown Br2 vapour | +6 → +4 |
| NaI | HI, then I2, SO2, S and H2S | purple vapour, black solid, yellow S, smell of bad eggs | +6 → +4, 0, −2 |
Chloride ions are too weak a reducing agent to reduce concentrated sulfuric acid, so no chlorine forms; the only product is HCl. The further the sulfur is reduced, the stronger the reducing agent: iodide takes sulfur all the way to −2 in H2S. In the silver nitrate test, always acidify with nitric acid, never hydrochloric.
Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.
Which sulfur-containing product forms when concentrated sulfuric acid reacts with sodium iodide but NOT with sodium bromide?
What is the change in oxidation number of sulfur when HBr reacts with concentrated H2SO4 to form SO2?
Why is the halide test carried out with dilute nitric acid rather than hydrochloric acid?
A precipitate forms with acidified AgNO3(aq). It does not dissolve in dilute NH3 but dissolves in concentrated NH3. The precipitate is:
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