A-Level Chemistry: Chemical Reactions of Group 7 Elements | A-Level化学：第七主族元素的化学反应

📚 A-Level Chemistry: Chemical Reactions of Group 7 Elements | A-Level化学：第七主族元素的化学反应

The Group 7 elements, also known as the halogens, include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements are highly reactive non-metals that display clear trends in their physical and chemical properties. This article focuses on their characteristic chemical reactions — with hydrogen, water, and alkali, as well as halide displacement reactions and tests — all essential for CIE A-Level Chemistry.

第七主族元素又称卤素，包括氟（F）、氯（Cl）、溴（Br）、碘（I）和砹（At）。这些元素是活泼的非金属，其物理性质和化学性质呈现明显的递变规律。本文聚焦于它们的特征化学反应——与氢气、水和碱的反应，以及卤化物置换反应和检验方法——这些都是CIE A-Level化学的核心考点。

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1. Reactivity Trend and Electron Configuration | 反应活性趋势与电子排布

The halogens all have the general outer-shell configuration ns²np⁵, meaning they require one more electron to achieve a stable noble-gas configuration. This drives their strong tendency to gain an electron and act as oxidising agents.

卤素原子的最外层电子排布通式为 ns²np⁵，即它们只需再获得一个电子便能达到稳定的稀有气体电子构型。这使它们具有很强的得电子倾向，表现为强氧化剂。

Going down the group, the atomic radius increases, shielding increases, and the attraction between the nucleus and an incoming electron decreases. Consequently, the ease of gaining an electron — oxidising ability — decreases down the group:

从氟到碘，原子半径增大、内层电子屏蔽效应增强，原子核对 incoming 电子的吸引力减弱。因此，得电子能力（即氧化性）随原子序数增大而减弱：

Halogen | 卤素	Oxidising ability | 氧化性	Electronegativity | 电负性
F₂	Strongest | 最强	4.0 (highest)
Cl₂	Strong | 强	3.0
Br₂	Moderate | 中等	2.8
I₂	Weakest | 最弱	2.5

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2. Reaction with Hydrogen | 与氢气的反应

All halogens react with hydrogen to form hydrogen halides, HX, but the vigour of the reaction decreases down the group. Fluorine reacts explosively even in the dark at very low temperatures; chlorine reacts readily in sunlight; bromine reacts slowly under mild heating; iodine reacts only partially and reversibly at high temperatures.

所有卤素都能与氢气反应生成卤化氢（HX），但反应的剧烈程度随原子序数增大而减弱。氟即使在暗处和极低温度下也会与氢气发生爆炸性反应；氯在光照下迅速反应；溴在温和加热时缓慢反应；碘则需在高温下才能进行，且为可逆反应。

H₂ + X₂ → 2HX

• Fluorine: H₂ + F₂ → 2HF, explosive at low temperature in the dark.

  氟：H₂ + F₂ → 2HF，在暗处低温即可爆炸反应。

• Chlorine: H₂ + Cl₂ → 2HCl, explosive in bright sunlight.

  氯：H₂ + Cl₂ → 2HCl，在强光下爆炸反应。

• Bromine: H₂ + Br₂ → 2HBr, slow reaction with mild heating.

  溴：H₂ + Br₂ → 2HBr，温和加热下缓慢反应。

• Iodine: H₂ + I₂ ⇌ 2HI, incomplete, reversible, endothermic.

  碘：H₂ + I₂ ⇌ 2HI，反应不完全、可逆且吸热。

The thermal stability of hydrogen halides also decreases down the group. HF is the most stable, while HI decomposes most readily on heating. This follows the trend in bond energy: the H–X bond length increases down the group, so the bond becomes weaker.

卤化氢的热稳定性也随原子序数增大而降低。HF 最稳定，HI 受热最容易分解。这符合键能趋势：H–X 键长随原子序数增大而增大，键能因此减弱。

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3. Reaction with Water | 与水的反应

Chlorine, bromine, and iodine dissolve in water to form an acidic solution. Chlorine reacts with water in a disproportionation reaction, where one chlorine atom is oxidised and another is reduced:

氯、溴和碘都能溶于水生成酸性溶液。氯与水发生歧化反应，一个氯原子被氧化，另一个被还原：

Cl₂ + H₂O ⇌ HCl + HClO

In this reaction, the oxidation state of chlorine changes from 0 in Cl₂ to −1 in HCl (reduction) and +1 in HClO (oxidation). The HClO (chloric(I) acid) is an oxidising agent and bleach. The position of equilibrium lies to the left, so only a small proportion of chlorine reacts.

在此反应中，氯元素的氧化态从 Cl₂ 中的 0 变为 HCl 中的 −1（被还原）和 HClO 中的 +1（被氧化）。HClO（次氯酸）具有氧化性和漂白性。该平衡强烈偏向左，因此实际与水反应的氯气很少。

Bromine and iodine show the same type of behaviour but the reaction becomes progressively less extensive. Iodine reacts only very slightly with water. Fluorine, however, reacts violently with water, oxidising the oxygen in water to give oxygen gas:

溴和碘也发生类似反应，但反应程度依次减小。碘与水几乎不反应。而氟与水剧烈反应，将水中的氧氧化为氧气：

2F₂ + 2H₂O → 4HF + O₂

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4. Reaction with Alkali: Disproportionation | 与碱的反应：歧化反应

Chlorine reacts with cold, dilute sodium hydroxide to form a mixture of chloride and chlorate(I) ions:

氯气与冷、稀的氢氧化钠溶液反应生成氯化物和次氯酸盐的混合物：

Cl₂ + 2NaOH → NaCl + NaClO + H₂O

This is a disproportionation reaction in which chlorine is simultaneously oxidised from 0 to +1 (in NaClO) and reduced from 0 to −1 (in NaCl). The product NaClO is the active ingredient in household bleach.

这是一个歧化反应：氯同时从 0 价被氧化为 +1 价（生成 NaClO）和被还原为 −1 价（生成 NaCl）。产物 NaClO 是家用漂白剂的有效成分。

With hot, concentrated alkali, chlorine undergoes a more extensive disproportionation, forming chloride and chlorate(V) ions:

与热、浓碱反应时，氯的歧化更充分，生成氯化物和氯酸盐(V)：

3Cl₂ + 6NaOH → 5NaCl + NaClO₃ + 3H₂O

In this case, five-sixths of the chlorine is reduced to −1 and one-sixth is oxidised to +5 (in ClO₃⁻). The same pattern applies to bromine and iodine: cold dilute alkali gives X⁻ and XO⁻; hot concentrated alkali gives X⁻ and XO₃⁻.

在该反应中，六分之五的氯被还原为 −1 价，六分之一被氧化为 +5 价（ClO₃⁻ 中）。溴和碘遵循相同的规律：冷稀碱生成 X⁻ 和 XO⁻；热浓碱生成 X⁻ 和 XO₃⁻。

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5. Halide Displacement Reactions | 卤素间的置换反应

Because oxidising ability decreases down the group, a more reactive halogen can displace a less reactive halogen from its halide salt solution. The more reactive halogen gains electrons and is reduced, while the halide ion loses electrons and is oxidised.

由于氧化性自上而下减弱，较活泼的卤素可将较不活泼的卤素从其卤化物溶液中置换出来。较活泼的卤素得电子被还原，而卤离子失电子被氧化。

• Chlorine water + potassium bromide solution: Cl₂ + 2KBr → 2KCl + Br₂. The solution turns orange-brown as bromine is formed.

  氯水 + 溴化钾溶液：Cl₂ + 2KBr → 2KCl + Br₂。溶液变为橙棕色，说明生成了溴。

• Chlorine water + potassium iodide solution: Cl₂ + 2KI → 2KCl + I₂. The solution turns brown, and with starch, gives a blue-black colour.

  氯水 + 碘化钾溶液：Cl₂ + 2KI → 2KCl + I₂。溶液变为棕色；加入淀粉后呈蓝黑色。

• Bromine water + potassium iodide solution: Br₂ + 2KI → 2KBr + I₂. The solution turns brown.

  溴水 + 碘化钾溶液：Br₂ + 2KI → 2KBr + I₂。溶液变为棕色。

In contrast, iodine cannot displace chlorine or bromine from their salts because iodine is a weaker oxidising agent. These displacement reactions confirm the trend in oxidising ability: Cl₂ > Br₂ > I₂.

相反，碘不能从氯化物或溴化物中置换出氯或溴，因为碘的氧化性更弱。这些置换反应确证了氧化性强弱的顺序：Cl₂ > Br₂ > I₂。

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6. Halide Ion Tests | 卤离子的检验

Halide ions in aqueous solution can be identified by adding silver nitrate solution followed by ammonia. The formation of a precipitate with characteristic colour confirms the presence of a particular halide.

水溶液中的卤离子可通过加入硝酸银溶液，再加入氨水来鉴别。生成沉淀的特征颜色可确认相应卤离子的存在。

Ag⁺(aq) + X⁻(aq) → AgX(s)

Halide ion | 卤离子	Precipitate colour | 沉淀颜色	Effect of dilute NH₃ | 稀氨水作用	Effect of concentrated NH₃ | 浓氨水作用
Cl⁻	White (AgCl) | 白色	Dissolves | 溶解	Dissolves | 溶解
Br⁻	Cream (AgBr) | 奶油色	Insoluble | 不溶	Dissolves | 溶解
I⁻	Yellow (AgI) | 黄色	Insoluble | 不溶	Insoluble | 不溶

Nitric acid must be added first to remove interfering ions such as carbonate or sulfite, which would also form precipitates with silver ions. The silver halide precipitates confirm the identity and relative solubility trend: AgCl is the most soluble and AgI the least.

必须先加入硝酸以排除碳酸根、亚硫酸根等干扰离子，因为它们也会与银离子生成沉淀。卤化银沉淀同时说明了溶解度趋势：AgCl 溶解度最大，AgI 最小。

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7. Acid–Base Character of Hydrogen Halides | 卤化氢的酸碱性

When hydrogen halides dissolve in water, they form acidic solutions. The acidity of the hydrohalic acids increases down the group: HF is a weak acid, while HCl, HBr, and HI are strong acids in water.

卤化氢溶于水后形成酸性溶液。氢卤酸的酸性随原子序数增大而增强：HF 是弱酸，而 HCl、HBr 和 HI 在水中都是强酸。

The increasing acid strength is explained by the decreasing H–X bond energy. As the bond becomes easier to break, the release of H⁺ becomes more favourable. The trend is therefore:

酸性增强的原因在于 H–X 键能逐渐降低。键越容易断裂，释放 H⁺ 的趋势越大。因此酸性顺序为：

HF < HCl < HBr < HI

In the gas phase, all hydrogen halides are covalent molecules. In aqueous solution, HCl, HBr, and HI ionise completely, whereas HF only partially ionises due to its strong H–F bond and hydrogen bonding in solution.

在气态时，所有卤化氢都是共价分子。在水溶液中，HCl、HBr 和 HI 完全电离，而 HF 仅部分电离，这是因为 H–F 键强且溶液中存在氢键作用。

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8. Uses of Halogens and Their Compounds | 卤素及其化合物的用途

Halogens and their compounds have widespread industrial and everyday applications that relate directly to their chemical reactivity.

卤素及其化合物在工业生产和日常生活中有着广泛用途，这些用途与其化学活性直接相关。

• Chlorine is used in water treatment to kill bacteria and to manufacture bleach (NaClO), PVC, and hydrochloric acid.

  氯气用于饮用水消毒、制造漂白剂（NaClO）、聚氯乙烯（PVC）和盐酸。

• Fluorine compounds, such as NaF and SnF₂, are added to toothpaste to prevent tooth decay. Teflon (PTFE) is a fluoropolymer with high thermal stability.

  氟化物（如 NaF 和 SnF₂）被添加至牙膏中以防蛀牙。特氟龙（PTFE）是一种热稳定性极高的含氟聚合物。

• Bromine is used in the production of flame retardants and in silver bromide photography (AgBr is light-sensitive).

  溴用于生产阻燃剂，以及溴化银感光材料（AgBr 具有光敏性）。

• Iodine is an essential element for thyroid function. Iodine in the form of KI is added to table salt as an iodine supplement, and iodine solution is used as an antiseptic.

  碘是甲状腺功能必需的微量元素。食盐中添加 KI 作为补碘剂，碘溶液可用作消毒剂。

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9. Trend Summary: Oxidising Ability, Reducing Ability, and Thermal Stability | 趋势总结：氧化性、还原性和热稳定性

The key periodic trends in Group 7 are summarised below. These are frequently assessed in CIE A-Level examinations, both in structured questions and data-analysis tasks.

第七主族的关键周期趋势总结如下。这些内容在CIE A-Level考试中经常出现，既出现在结构化问题中，也出现在数据分析题中。

Property | 性质	Trend down the group | 由上至下趋势	Reason | 原因
Atomic radius | 原子半径	Increases | 增大	More electron shells | 电子层数增加
First ionisation energy | 第一电离能	Decreases | 减小	More shielding, larger radius | 屏蔽效应增强、半径增大
Electronegativity | 电负性	Decreases | 减小	Weaker attraction for bonding electrons | 对成键电子吸引力减弱
Oxidising ability | 氧化性	Decreases | 减弱	Harder to gain an electron | 得电子更难
Reducing ability of X⁻ | X⁻的还原性	Increases | 增强	Larger ion loses electron more easily | 离子越大越易失电子
Thermal stability of HX | HX的热稳定性	Decreases | 减弱	H–X bond energy decreases | H–X 键能减弱

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10. Exam-Style Problem: Applying the Trends | 考试真题演练：运用趋势规律

Consider the following reaction scheme. Chlorine gas is bubbled into an aqueous solution of potassium iodide, and the resulting mixture is shaken with an organic solvent such as hexane.

考虑以下反应设计：将氯气通入碘化钾水溶液中，所得混合液与己烷等有机溶剂振荡。

Question 1 | 问题一: Write the ionic equation for the reaction that occurs.

Question 1 | 问题一: 写出发生反应的离子方程式。

Cl₂(g) + 2I⁻(aq) → 2Cl⁻(aq) + I₂(aq)

Question 2 | 问题二: State what would be observed in the aqueous layer and in the organic layer.

Question 2 | 问题二: 说明水层和有机层中的观察现象。

The aqueous layer would turn brown, and the organic layer would appear purple/violet, indicating the presence of iodine in the non-polar solvent.

水层变为棕色，有机层呈紫色/紫红色，说明碘溶解在非极性溶剂中。

Question 3 | 问题三: Explain, in terms of electron transfer, why this displacement occurs.

Question 3 | 问题三: 从电子转移的角度解释为何能发生该置换反应。

Chlorine is a stronger oxidising agent than iodine. Each chlorine atom gains an electron and is reduced to Cl⁻, while each iodide ion loses an electron and is oxidised to I₂.

氯的氧化性强于碘。每个氯原子获得一个电子被还原为 Cl⁻，而每个碘离子失去一个电子被氧化为 I₂。

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11. Common Misconceptions and Exam Advice | 常见误区与备考建议

Many students lose marks in Group 7 questions due to a few repeated mistakes. Here are the most important points to remember.

很多学生在第七主族题目中失分是因为反复出现几个典型错误。以下是需要重点记住的要点。

• Do not say ‘the halogen gains electrons because it is more reactive.’ Instead, link reactivity to oxidising ability and electron affinity trends.

  不要说“因为更活泼所以卤素得电子”。应将活泼性与氧化能力及得电子趋势关联。

• In disproportionation reactions, identify the oxidation states clearly. Chlorine is 0 in Cl₂, −1 in Cl⁻, +1 in ClO⁻, and +5 in ClO₃⁻.

  在歧化反应中要清楚标明氧化态。Cl₂ 中氯为 0 价，Cl⁻ 中为 −1 价，ClO⁻ 中为 +1 价，ClO₃⁻ 中为 +5 价。

• When writing equations for the reaction with alkali, check whether the alkali is cold or hot, dilute or concentrated — this determines whether ClO⁻ or ClO₃⁻ is formed.

  书写与碱反应的方程式时，需注意碱是冷是热、是稀是浓——这决定了生成 ClO⁻ 还是 ClO₃⁻。

• For halide ion tests, always remember to add nitric acid first, before silver nitrate, and record the colour of the precipitate and its solubility in ammonia.

  卤离子检验中，务必记住在加入硝酸银之前先加硝酸，并记录沉淀颜色及其在氨水中的溶解性。

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