📚 Reactions of Group 17 Elements | 第17族元素的反应
Group 17 elements, known as the halogens, display a clear reactivity trend that is central to A-Level Chemistry. This article reviews their reactions with hydrogen, metals, alkalis, water, halide salts and concentrated sulfuric acid, linking observations to oxidation states and electrode potentials.
第17族元素又称卤素，表现出清晰的反应性递变规律，这是 A-Level 化学的核心内容。本文梳理卤素与氢气、金属、碱、水、卤化物盐和浓硫酸的反应，并将实验现象与氧化态和电极电势联系起来。
1. Overview and Physical Trends | 概述与物理性质趋势
The halogens are fluorine, chlorine, bromine, iodine and astatine. They exist as diatomic molecules F₂, Cl₂, Br₂ and I₂. Down the group, atomic radius increases, electronegativity decreases, boiling point increases and colour darkens.
卤素包括氟、氯、溴、碘和砹。它们以双原子分子 F₂、Cl₂、Br₂、I₂ 存在。沿族向下，原子半径增大，电负性减小，沸点升高，颜色逐渐加深。
Each halogen atom has seven valence electrons (ns² np⁵). To achieve a stable noble-gas configuration, the atom tends to gain one electron and form a halide ion X⁻. Therefore halogens act as oxidising agents.
每个卤素原子有七个价电子（ns² np⁵）。为达到稳定的稀有气体结构，原子倾向于获得一个电子形成卤素离子 X⁻。因此卤素常作为氧化剂。
F₂ + 2e⁻ → 2F⁻
Cl₂ + 2e⁻ → 2Cl⁻
The table below summarises the key physical trends.
下表总结了关键的物理性质递变规律。
| Property | F₂ | Cl₂ | Br₂ | I₂ |
|---|---|---|---|---|
| Colour | pale yellow | yellow-green | red-brown liquid | grey-black solid / violet vapour |
| Boiling point / °C | −188 | −34 | 59 | 184 |
| Electronegativity | 4.0 | 3.0 | 2.8 | 2.5 |
2. Oxidising Ability and Standard Electrode Potentials | 氧化能力与标准电极电势
Halogens become weaker oxidising agents down the group. Standard electrode potentials for the reduction X₂ + 2e⁻ → 2X⁻ are F₂ +2.87 V, Cl₂ +1.36 V, Br₂ +1.07 V and I₂ +0.54 V.
卤素沿族向下氧化能力逐渐减弱。还原反应 X₂ + 2e⁻ → 2X⁻ 的标准电极电势分别为：F₂ +2.87 V、Cl₂ +1.36 V、Br₂ +1.07 V、I₂ +0.54 V。
The more positive the electrode potential, the stronger the oxidising agent. Therefore F₂ can oxidise chloride, bromide and iodide ions. Cl₂ can oxidise Br⁻ and I⁻, while Br₂ can oxidise only I⁻.
电极电势越正，氧化剂越强。因此 F₂ 可以氧化 Cl⁻、Br⁻ 和 I⁻；Cl₂ 能氧化 Br⁻ 和 I⁻；而 Br₂ 只能氧化 I⁻。
This trend is explained by the increasing atomic radius and shielding down the group, which reduce the ability of the nucleus to attract an added electron. Hydration enthalpy of the halide ion also becomes less exothermic as ion size increases.
这一趋势可解释为：沿族向下原子半径增大、屏蔽效应增强，使原子核吸引外来电子的能力下降。同时随离子半径增大，卤素离子的水合焓放热减少。
3. Reactions with Hydrogen: Hydrogen Halides | 与氢气的反应：卤化氢
All halogens react with hydrogen to form hydrogen halides: H₂ + X₂ → 2HX. Reactivity decreases sharply down the group.
所有卤素都能与氢气反应生成卤化氢：H₂ + X₂ → 2HX。反应活性沿族向下急剧下降。
H₂ + F₂ → 2HF
H₂ + Cl₂ → 2HCl
Fluorine explodes with hydrogen even in the dark and cold. Chlorine reacts slowly in the dark but explosively in sunlight. Bromine requires heating and a platinum catalyst, while iodine requires continuous heating and the reaction is reversible.
氟与氢气即使在黑暗和低温下也会爆炸。氯在黑暗中反应缓慢，但在阳光下会爆炸。溴需要加热和铂催化剂，而碘需要持续加热且反应可逆。
Thermal stability of hydrogen halides decreases in the order HF > HCl > HBr > HI. This is because the H–X bond enthalpy decreases as the halogen atom becomes larger. In aqueous solution, HCl, HBr and HI are strong acids, but HF is a weak acid due to the strong H–F bond and hydrogen bonding.
卤化氢的热稳定性顺序为 HF > HCl > HBr > HI，原因是随卤原子变大，H–X 键能降低。在水溶液中 HCl、HBr、HI 是强酸，而 HF 因 H–F 键很强并存在氢键，是弱酸。
4. Reactions with Metals: Formation of Halides | 与金属的反应：卤化物的生成
Halogens oxidise metals to form ionic metal halides. Sodium burns in chlorine with a bright yellow flame.
卤素氧化金属，生成离子型金属卤化物。钠在氯气中燃烧，产生明亮的黄色火焰。
2Na + Cl₂ → 2NaCl
2Fe + 3Cl₂ → 2FeCl₃
Chlorine and bromine usually oxidise iron to the +3 state, but iodine only gives iron(II) iodide because iodine is a weaker oxidising agent.
氯和溴通常将铁氧化为 +3 价，但碘只能生成碘化亚铁（FeI₂），因为碘是较弱的氧化剂。
Fe + I₂ → FeI₂
Chlorine also reacts with non-metals such as phosphorus. Excess chlorine gives phosphorus(V) chloride.
氯还能与非金属如磷反应。过量氯气可生成五氯化磷。
2P + 3Cl₂ → 2PCl₃
PCl₃ + Cl₂ → PCl₅
5. Displacement Reactions between Halogens and Halides | 卤素与卤化物之间的置换反应
A more reactive halogen displaces a less reactive halogen from an aqueous solution of its halide salt. These are redox reactions in which the halide ion is oxidised.
较活泼的卤素能从较不活泼卤素的卤化物水溶液中置换出后者。这些是氧化还原反应，卤素离子被氧化。
Cl₂ + 2KBr → 2KCl + Br₂
Cl₂ + 2KI → 2KCl + I₂
Br₂ + 2KI → 2KBr + I₂
Chlorine cannot displace fluorine from fluoride ions, and bromine cannot displace chlorine from chloride ions. This provides direct evidence for the decreasing oxidising power F₂ > Cl₂ > Br₂ > I₂.
氯不能从氟离子中置换出氟，溴也不能从氯离子中置换出氯。这直接证明氧化能力顺序为 F₂ > Cl₂ > Br₂ > I₂。
After displacement, the halogen can be extracted into an organic solvent such as hexane. A chlorine water and potassium bromide mixture gives an orange hexane layer of Br₂; with potassium iodide it gives a violet layer of I₂.
置换出的卤素可用己烷等有机溶剂萃取。氯水和溴化钾混合物产生橙色 Br₂ 己烷层；与碘化钾作用则产生紫色 I₂ 层。
6. Reactions with Alkalis: Disproportionation | 与碱的反应：歧化反应
Chlorine, bromine and iodine react with cold dilute sodium hydroxide. Chlorine undergoes disproportionation: the same element is both oxidised and reduced.
氯、溴和碘与冷稀氢氧化钠反应。氯发生歧化反应：同一种元素既被氧化又被还原。
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
In this reaction, chlorine changes from oxidation state 0 in Cl₂ to −1 in NaCl and +1 in NaClO. Sodium chlorate(I), NaClO, is the active ingredient in household bleach.
在此反应中，氯的氧化态从 Cl₂ 的 0 变为 NaCl 中的 −1 和 NaClO 中的 +1。次氯酸钠 NaClO 是家用漂白剂的有效成分。
With hot concentrated sodium hydroxide, chlorine forms sodium chlorate(V), where chlorine reaches the +5 oxidation state.
与热的浓氢氧化钠反应时，氯生成氯酸钠 NaClO₃，其中氯的氧化态达到 +5。
3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂OPublished by TutorHao | A-Level Chemistry Revision Series | aleveler.com
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