Halogen evidence and reaction conditions
Introduced| English | Meaning |
|---|---|
| oxidising power | How strongly a species takes electrons; stronger oxidising agents are reduced more easily. |
| disproportionation/ˌdɪsprəˈpɔːʃəneɪʃn/ | A reaction in which the same element is both oxidised and reduced. |
| aliquot/ˈælɪkwɒt/ | A measured portion of a larger solution sample, often transferred with a calibrated pipette. |
Why can the same halogen form different salts?
- Chlorine with cold dilute alkali forms hypochlorite. Hot concentrated alkali gives chlorate instead.
- Record conditions before balancing or interpreting oxidation numbers.
Predict displacement by electron gain
- Chlorine displaces bromide and iodide; bromine displaces iodide; iodine displaces neither.
- Down the group, greater radius and shielding weaken attraction to an incoming electron, lowering oxidising power 氧化能力.
What happens when bromine is added to aqueous iodide?
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Bromine is the stronger oxidant and accepts electrons from iodide.
Track disproportionation 歧化
- Cold dilute OH⁻: Cl₂ + 2OH⁻ → Cl⁻ + ClO⁻ + H₂O. Chlorine changes from 0 to −1 and +1.
- Hot concentrated OH⁻: 3Cl₂ + 6OH⁻ → 5Cl⁻ + ClO₃⁻ + 3H₂O. Chlorine changes from 0 to −1 and +5.
Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂. Chlorine is reduced and is the oxidising agent; bromide is oxidised and is the reducing agent.
Find chlorine’s oxidation number in ClO₃⁻. Use oxygen = −2.
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x + 3(−2) = −1, so x = +5.
Disproportionation includes both oxidation and reduction of the same element from one species.
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For chlorine, atoms initially at zero become both negative and positive oxidation states.
Combine observations with a test
- Acidify halide solution with dilute HNO₃ before AgNO₃. AgCl is white, AgBr cream and AgI yellow.
- AgCl dissolves in dilute ammonia; AgBr requires concentrated ammonia; AgI remains. Use a separate aliquot 等分试样 for each independent test.
A cream silver halide precipitate is most consistent with which halide ion? Write its name.
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AgBr is cream and dissolves in concentrated rather than dilute ammonia.
Keep colours and hazards in context
- Iodine is brown in water and violet in a suitable non-polar organic solvent. Its ordinary solid is grey-black.
- Chlorine water disinfects but needs controlled dosing. Concentrated-acid halide reactions and hazardous gas tests require the approved supervised method.
“A violet solution proves iodine in any solvent” is false. Solvent affects the observed colour; aqueous iodine is brown.
State the reagent, temperature, concentration and solvent when they change the product or observation.
Which two claims are correct?
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Solvent and alkali conditions determine these observations. HCl contaminates the halide test; acid can release hazardous chlorine from bleach.