Reactions of the halide ions
Introduced| English | Meaning |
|---|---|
| halide/ˈhælaɪd/ | halide |
| reducing agent/rɪˈdjuːsɪŋ ˈeɪdʒənt/ | A substance that donates electrons to another substance and is itself oxidised. |
| precipitate/prɪˈsɪpɪteɪt/ | An insoluble solid formed when substances in solution react. |
| silver nitrate/ˈsɪlvə ˈnaɪtreɪt/ | An ionic compound with formula AgNO3, commonly used to test for halide ions. |
Spotting a halide 卤离子
- A halide ion can give away an electron — so it is a reducing agent 还原剂.
- This power increases down the group.
- Two key tests identify which halide is present.
Halide ion test lab
Match halide ion evidence to the ion present.
The reducing power of the halide ions increases down the group because:
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Reducing means losing an electron; bigger ions lose theirs more easily, so reducing power rises down the group.
Silver nitrate solution is used to test for ______ ions.
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Different halides give different coloured precipitates.
Reducing power
- A halide ion ($\text{Cl}^{-}$, $\text{Br}^{-}$, $\text{I}^{-}$) acts as a reducing agent (it loses an electron).
- This power increases down the group, because a larger ion holds its outer electron less tightly.
Silver halide precipitate 沉淀 colours and their solubility in ammonia identify the halide
Match each halide to its silver halide precipitate colour.
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AgCl is white, AgBr cream, AgI yellow; ammonia solubility confirms (Cl dissolves in dilute, I insoluble).
Silver iodide (AgI) is:
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AgI is yellow and does not dissolve in ammonia, distinguishing it from AgCl (white) and AgBr (cream).
Silver nitrate 硝酸银 test
Add silver nitrate, then ammonia, and read the precipitate:
| Halide | Precipitate | In ammonia |
|---|---|---|
| $\text{Cl}^{-}$ | white | dissolves in dilute ammonia |
| $\text{Br}^{-}$ | cream | dissolves in concentrated ammonia |
| $\text{I}^{-}$ | yellow | insoluble |
With concentrated sulfuric acid, iodide gives I₂ plus H₂S because:
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I⁻ is the strongest reducer, so it reduces the sulfuric acid all the way to H₂S (and forms I₂); Cl⁻ only gives HCl.
With concentrated sulfuric acid
- All give the hydrogen halide first; the lower halides are then oxidised (they are stronger reducing agents):
- chloride: only $\text{HCl}$ (no redox).
- bromide: also brown $\text{Br}_2$ and $\text{SO}_2$.
- iodide: $\text{I}_2$ plus smelly $\text{H}_2\text{S}$ and $\text{SO}_2$ ($\text{I}^{-}$ is the strongest reducing agent).
You've got it
- halide ions are reducing agents; power increases down the group (larger ion, electron held loosely)
- silver nitrate test: AgCl white, AgBr cream, AgI yellow (confirm with ammonia solubility)
- conc. H₂SO₄: Cl⁻ → HCl only; Br⁻ → Br₂ + SO₂; I⁻ → I₂ + H₂S + SO₂ (strongest reducer)