📚 Reactions of the Halide Ions | 卤素离子的反应
Halide ions are the negative ions formed when Group 17 halogens gain one electron, giving Cl⁻, Br⁻ and I⁻. In aqueous solution they are usually supplied as sodium or potassium salts. Their chemistry is dominated by precipitation with silver ions, by displacement by more oxidising halogens, and by redox reactions with concentrated sulfuric acid.
卤素离子是第17族卤素原子获得一个电子后形成的阴离子，包括Cl⁻、Br⁻和I⁻。溶液中通常以钠盐或钾盐形式存在。卤素离子的化学性质主要体现在与银离子生成沉淀、被氧化性更强的卤素置换，以及与浓硫酸发生的氧化还原反应。
1. Introduction to Halide Ions | 卤素离子简介
Halide ions, X⁻, carry a single negative charge and are formed when halogen atoms gain one electron. In aqueous solution, common sources are NaCl, NaBr and NaI. The halide ions are colourless in solution, so any observed colour changes in their reactions come from the products formed, especially halogens such as Br₂ and I₂.
卤素离子X⁻带一个负电荷，由卤素原子得到一个电子形成。水溶液中常见的来源是NaCl、NaBr和NaI。卤素离子在溶液中是无色的，因此反应中观察到的颜色变化来自生成的产物，特别是Br₂和I₂等卤素单质。
2. Reducing Power Trend Down Group 17 | 第17族还原性趋势
Down Group 17, the ionic radius increases from Cl⁻ to I⁻, so the outer electrons are further from the nucleus and experience more shielding. This means the electrons are lost more easily. Therefore reducing power increases in the order: Cl⁻ < Br⁻ < I⁻.
沿第17族向下，离子半径从Cl⁻到I⁻逐渐增大，外层电子离核更远且屏蔽效应增强，因此电子更容易失去。所以还原性依次增强：Cl⁻ < Br⁻ < I⁻。
This trend is critical for understanding the different reactions of solid sodium halides with concentrated sulfuric acid. Chloride ions are too weak a reducing agent to reduce sulfur in H₂SO₄, bromide ions can reduce it to SO₂, and iodide ions can reduce it further to S and H₂S.
这一趋势对理解固体卤化钠与浓硫酸的不同反应至关重要。氯离子还原性太弱，不能还原H₂SO₄中的硫；溴离子可将其还原为SO₂；碘离子则可进一步将其还原为S和H₂S。
3. Reaction of Sodium Chloride with Concentrated Sulfuric Acid | 氯化钠与浓硫酸的反应
Solid sodium chloride reacts with concentrated sulfuric acid in an acid–base reaction to form hydrogen chloride gas. The chloride ion is too weak a reducing agent to reduce sulfur in H₂SO₄, so no redox products such as SO₂, S or H₂S are formed.
固体氯化钠与浓硫酸发生酸碱反应，生成氯化氢气体。氯离子还原性太弱，不能还原H₂SO₄中的硫，因此不会生成SO₂、S或H₂S等氧化还原产物。
NaCl + H₂SO₄ → NaHSO₄ + HCl
The HCl appears as white misty fumes because it dissolves readily in moisture in the air. No coloured gases or solids are observed in this reaction.
HCl在潮湿空气中溶解形成白色酸雾。该反应中观察不到有色气体或固体。
4. Reaction of Sodium Bromide with Concentrated Sulfuric Acid | 溴化钠与浓硫酸的反应
Sodium bromide first reacts with concentrated sulfuric acid to produce hydrogen bromide gas. However, Br⁻ is a stronger reducing agent than Cl⁻, so the HBr formed reduces H₂SO₄ to sulfur dioxide.
溴化钠首先与浓硫酸反应生成溴化氢气体。但Br⁻的还原性比Cl⁻强，因此生成的HBr会将H₂SO₄还原为二氧化硫。
NaBr + H₂SO₄ → NaHSO₄ + HBr
2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O
The overall reaction can be written as:
总反应可写为：
2NaBr + 3H₂SO₄ → 2NaHSO₄ + Br₂ + SO₂ + 2H₂O
The observed products include orange-brown bromine vapour and colourless sulfur dioxide gas, which has a choking smell. The reaction mixture may also produce white fumes of hydrogen bromide before it is oxidised.
观察到的产物包括橙棕色溴蒸气和无色有刺激性气味的二氧化硫气体。反应混合物中还可能先出现溴化氢的白色酸雾，然后被氧化。
5. Reaction of Sodium Iodide with Concentrated Sulfuric Acid | 碘化钠与浓硫酸的反应
Sodium iodide first produces hydrogen iodide gas. I⁻ is the strongest reducing agent of the three halide ions, so HI reduces H₂SO₄ to a mixture of sulfur-containing products, including SO₂, S and H₂S. The exact mixture depends on reaction conditions and the strength of the reducing agent.
碘化钠首先生成碘化氢气体。I⁻是三种卤素离子中最强的还原剂，因此HI能将H₂SO₄还原为含硫产物的混合物，包括SO₂、S和H₂S。具体产物取决于反应条件和还原剂的强弱。
NaI + H₂SO₄ → NaHSO₄ + HI
2HI + H₂SO₄ → I₂ + SO₂ + 2H₂O
6HI + H₂SO₄ → 3I₂ + S + 4H₂O
8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O
The main observations are purple iodine vapour, a yellow-black solid of sulfur, and the rotten-egg smell of hydrogen sulfide. Sulfur dioxide may also be present as a colourless choking gas.
主要观察现象有紫色碘蒸气、黄黑色固体硫，以及硫化氢的臭鸡蛋气味。还可能存在无色刺激性气体二氧化硫。
6. Preparing Hydrogen Halides with Phosphoric Acid | 用磷酸制备卤化氢
To prepare HBr and HI without oxidation by concentrated sulfuric acid, concentrated phosphoric acid is used instead. Phosphoric acid is not a strong oxidising agent, so it simply protonates the halide ion and releases the hydrogen halide.
为避免浓硫酸氧化HBr和HI，制备时使用浓磷酸代替。磷酸不是强氧化剂，只与卤素离子发生质子化反应，释放出卤化氢。
NaX + H₃PO₄ → NaH₂PO₄ + HX
This method is especially useful for generating HBr and HI in the laboratory without contamination by Br₂ or I₂.
该方法特别适用于实验室制备HBr和HI，避免产物中混入Br₂或I₂。
7. Halogen Displacement Reactions | 卤素置换反应
A halogen higher in Group 17 is a stronger oxidising agent and can displace a halide ion lower down from its salt solution. For example, chlorine oxidises bromide ions and iodide ions, while bromine oxidises iodide ions but not chloride ions.
第17族中上方的卤素氧化性更强，可以从下方卤素离子的盐溶液中置换出卤素单质。例如，氯可氧化溴离子和碘离子；溴可氧化碘离子但不能氧化氯离子。
Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
Cl₂ + 2I⁻ → 2Cl⁻ + I₂
Br₂ + 2I⁻ → 2Br⁻ + I₂
In aqueous solution, the displaced halogen gives a characteristic colour. When extracted into an organic solvent such as cyclohexane, chlorine appears pale yellow-green, bromine appears orange-brown, and iodine appears violet. This colour test can be used to identify which halogen has been formed.
在水溶液中，被置换出的卤素呈现特征颜色。用环己烷等有机溶剂萃取后，氯呈淡黄绿色，溴呈橙棕色，碘呈紫色。该颜色检验可用于鉴别生成的卤素单质。
8. Precipitation of Silver Halides | 卤化银的沉淀
Silver ions react with halide ions in aqueous solution to form insoluble silver halide precipitates. The test is carried out by adding dilute nitric acid first, followed by aqueous silver nitrate.
银离子与溶液中的卤素离子反应生成不溶性卤化银沉淀。检验时先加稀硝酸，然后加入硝酸银溶液。
Ag⁺ + Cl⁻ → AgCl(s)
Ag⁺ + Br⁻ → AgBr(s)
Ag⁺ + I⁻ → AgI(s)
Dilute nitric acid is added to remove carbonate ions and sulfite ions, which could also form precipitates with silver nitrate and interfere with the test.
加入稀硝酸是为了除去碳酸根离子和亚硫酸根离子，因为它们也能与硝酸银生成沉淀，干扰检验。
The colours of the precipitates are white for AgCl, cream for AgBr, and pale yellow for AgI. Silver chloride slowly darkens in sunlight due to decomposition to silver.
沉淀的颜色分别是：AgCl白色，AgBr奶油色，AgI淡黄色。氯化银在阳光下会因分解出银而逐渐变黑。
9. Distinguishing Silver Halides with Ammonia | 用氨水区分卤化银
The silver halides can be distinguished by their different solubilities in aqueous ammonia. AgCl dissolves in dilute ammonia solution, AgBr dissolves only in concentrated ammonia solution, and AgI is insoluble even in concentrated ammonia.
卤化银可通过在氨水中溶解性的不同来区分。AgCl溶于稀氨水，AgBr只溶于浓氨水，AgI即使在浓氨水中也不溶解。
The dissolving process occurs because ammonia molecules act as ligands and form the soluble complex ion [Ag(NH₃)₂]⁺
Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
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