📚 A-Level Chemistry: Sulphur and Its Oxides | A-Level化学：硫及其氧化物的性质
Sulphur is a versatile non-metal in Group 16 of the Periodic Table, with the electron configuration [Ne] 3s² 3p⁴. It exists in several allotropic forms and exhibits oxidation states of −2, +4 and +6 in its common compounds. For CIE A-Level Chemistry, the key focus is on the physical properties of sulphur, the acid–base and redox behaviour of sulphur dioxide, the industrial Contact Process for sulphur trioxide, and the environmental consequences of sulphur oxide emissions.
硫位于元素周期表第16族，是一种性质多变且重要的非金属元素，其电子构型为 [Ne] 3s² 3p⁴。硫存在多种同素异形体，在常见化合物中表现出 −2、+4 和 +6 三种主要氧化态。在 CIE A-Level 化学考试中，重点考查硫的物理性质、二氧化硫的酸碱性与氧化还原行为、三氧化硫的工业接触法制备，以及硫氧化物排放带来的环境问题。
1. Physical Properties and Allotropes of Sulphur | 硫的物理性质与同素异形体
Sulphur is a yellow, brittle solid at room temperature. Its most stable allotrope is rhombic sulphur, which consists of crown-shaped S₈ molecules packed in an orthorhombic crystal lattice. When heated above 95.6 °C, rhombic sulphur slowly changes into monoclinic sulphur, another allotrope that is also composed of S₈ molecules but with a different crystal arrangement. Both forms are insoluble in water but dissolve in organic solvents such as carbon disulfide.
硫在室温下是一种黄色脆性固体。其最稳定的同素异形体是正交硫，由皇冠形的 S₈ 分子按正交晶格排列而成。当加热至 95.6 °C 以上时，正交硫会缓慢转变为单斜硫；单斜硫同样由 S₈ 分子构成，但晶体排列方式不同。这两种同素异形体均不溶于水，但可溶于二硫化碳等有机溶剂。
The melting point of sulphur is about 119 °C, and its boiling point is approximately 444 °C. In the solid state, S₈ rings are held together by weak van der Waals forces, which explains the low melting point despite the strong covalent bonds within each ring.
硫的熔点约为 119 °C，沸点约为 444 °C。在固态时，S₈ 环之间仅靠微弱的范德华力结合在一起，因此虽然环内共价键很强，但硫的熔点并不高。
2. The Unusual Viscosity of Molten Sulphur | 熔融硫的异常黏度变化
The behaviour of sulphur when heated is unusual and is a favourite examination topic. When solid sulphur melts at 119 °C, it forms an amber, free-flowing liquid. As the temperature rises to about 160 °C, the liquid becomes darker and increasingly viscous, reaching a maximum viscosity near 190 °C. Above 190 °C, the liquid becomes thinner again until it boils at 444 °C.
硫在加热过程中表现出反常的黏度变化，这也是考试中非常喜欢考查的内容。当固态硫在 119 °C 熔化时，得到琥珀色的易流动液体。随着温度升高至约 160 °C，液体颜色加深，黏度显著增大，在约 190 °C 时黏度达到最大值。超过 190 °C 后，液体又逐渐变稀，直至 444 °C 沸腾。
This behaviour is explained in terms of ring-opening and polymerisation. The S₈ rings open when heated and join end-to-end to form long chains containing many hundreds of sulphur atoms. These long chains become entangled, raising the viscosity. At still higher temperatures, the long chains break into shorter fragments, allowing the liquid to flow more freely again.
这一现象可以用开环与聚合来解释。加热时 S₈ 环打开，分子首尾相连形成包含数百个硫原子的长链。这些长链互相缠绕，使黏度增大。温度进一步升高时，长链断裂为较短的链段，液体因此重新变得容易流动。
3. Chemical Properties: Reaction with Metals and Non-metals | 硫的化学性质：与金属和非金属的反应
Sulphur reacts directly with many metals to form metal sulphides. For example, iron filings heated with sulphur produce iron(II) sulphide, and sodium reacts vigorously to form sodium sulphide. In these reactions sulphur acts as an oxidising agent, accepting electrons and adopting the −2 oxidation state.
硫能与许多金属直接化合生成金属硫化物。例如，铁粉与硫共热生成硫化亚铁，钠与硫剧烈反应生成硫化钠。在这些反应中，硫作为氧化剂获得电子，达到 −2 氧化态。
Fe(s) + S(s) → FeS(s)
2Na(s) + S(s) → Na₂S(s)
Sulphur also burns in oxygen to form sulphur dioxide, a colourless, pungent gas. This combustion reaction is strongly exothermic and is used industrially to generate SO₂ for the manufacture of sulphuric acid.
硫也能在氧气中燃烧生成二氧化硫——一种无色、具有刺激性气味的气体。该燃烧反应强烈放热，工业上正是利用这个反应来制备制造硫酸所需的 SO₂。
S(s) + O₂(g) → SO₂(g)
The important oxidation states of sulphur are −2 in hydrogen sulphide and metal sulphides, +4 in SO₂ and sulphites, and +6 in SO₃ and sulphates. These oxidation states govern the redox behaviour examined in the syllabus.
硫的重要氧化态包括：在硫化氢和金属硫化物中的 −2 态，在 SO₂ 和亚硫酸盐中的 +4 态，以及在 SO₃ 和硫酸盐中的 +6 态。这些氧化态决定了课程大纲中重点考查的氧化还原性质。
4. Laboratory Preparation of Sulphur Dioxide | 二氧化硫的实验室制备
Sulphur dioxide can be prepared in the laboratory by the action of a dilute acid on a sulphite salt. A typical method involves adding dilute hydrochloric acid to solid sodium sulphite; the SO₂ gas is evolved and can be collected by upward delivery or over warm water, since it is denser than air and quite soluble in cold water.
实验室中通常用稀酸与亚硫酸盐反应来制备二氧化硫。典型方法是将稀盐酸滴加到固体亚硫酸钠上，产生的 SO₂ 气体因密度大于空气且易溶于冷水，可用向上排空气法或温水上方收集。
Na₂SO₃(s) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + H₂O(l)
This reaction is a simple acid–base metathesis in which the unstable sulphurous acid, H₂SO₃, decomposes immediately into SO₂ and water. The gas is collected and identified by its characteristic choking smell and by its ability to turn damp blue litmus paper red, showing its acidic nature.
该反应属于酸碱复分解反应：不稳定的亚硫酸 H₂SO₃ 迅速分解为 SO₂ 和水。收集到的气体可通过其特有的刺激性气味以及使湿润蓝色石蕊试纸变红的现象来鉴别，这体现出 SO₂ 的酸性特征。
5. Sulphur Dioxide as an Acidic Oxide | 二氧化硫的酸性氧化物性质
Sulphur dioxide dissolves in water to form sulphurous acid, H₂SO₃. This is a weak dibasic acid that exists only in solution. The equilibrium shown below explains why solutions of SO₂ are acidic and why the gas turns damp litmus paper red.
二氧化硫溶于水生成亚硫酸 H₂SO₃。亚硫酸是一种只存在于溶液中的弱二元酸。下面的平衡式说明了为什么 SO₂ 溶液呈酸性，也解释了为什么 SO₂ 能使湿润的石蕊试纸变红。
SO₂(g) + H₂O(l) ⇌ H₂SO₃(aq)
As an acidic oxide, SO₂ reacts with bases to form salts. With an excess of sodium hydroxide, sodium sulphite is produced. With a limited amount of alkali, the hydrogen sulphite (bisulphite) salt is formed instead.
作为酸性氧化物，SO₂ 与碱反应生成盐。当氢氧化钠过量时，产物为亚硫酸钠；当碱量不足时，则生成亚硫酸氢盐。
SO₂(g) + 2NaOH(aq) → Na₂SO₃(aq) + H₂O(l)
SO₂(g) + NaOH(aq) → NaHSO₃(aq)
These reactions are analogous to those of carbon dioxide with alkalis, and the same care is needed when writing balanced ionic equations. In aqueous solution, sulphite ions, SO₃²⁻, act as weak bases and accept protons from water.
这些反应与二氧化碳和碱的反应类似，书写离子方程式时也需要注意类似的细节。在水溶液中，亚硫酸根离子 SO₃²⁻ 作为弱碱，可以从水中接受质子。
6. Sulphur Dioxide as a Reducing Agent | 二氧化硫的还原性
The most important redox property of SO₂ is its ability to act as a reducing agent. In SO₂ the sulphur atom is in the +4 oxidation state; it can be oxidised further to the +6 state in sulphate ions, SO₄²⁻. Thus SO₂ is readily oxidised by strong oxidising agents.
SO₂ 最重要的氧化还原性质是它的还原性。在 SO₂ 中硫为 +4 氧化态，可进一步被氧化到硫酸根 SO₄²⁻ 中的 +6 态。因此，SO₂ 很容易被强氧化剂氧化。
When acidified potassium manganate(VII) solution is added to SO₂, the purple MnO₄⁻ ion is reduced to colourless Mn²⁺. The SO₂ is simultaneously oxidised to sulphate. This reaction decolorises the purple solution and is a simple test for SO₂.
当酸性高锰酸钾溶液与 SO₂ 反应时，紫色的 MnO₄⁻ 被还原为无色的 Mn²⁺，同时 SO₂ 被氧化为硫酸根。该反应使紫色溶液褪色，是检验 SO₂ 的简单方法。
5SO₂(g) + 2MnO₄⁻(aq) + 2H₂O(l) → 5SO₄²⁻(aq) + 2Mn²⁺(aq) + 4H⁺(aq)
Sulphur dioxide also reduces iron(III) ions to iron(II) ions, and bromine water to bromide ions. In each case SO₂ is oxidised to sulphate, and these reactions are useful examples in both ionic equation and oxidation-state questions.
二氧化硫还能将铁(III)离子还原为铁(II)离子，将溴水还原为溴离子。在这些反应中 SO₂ 都被氧化为硫酸根。这些反应是离子方程式和氧化态考查题中的经典例子。
2Fe³⁺(aq) + SO₂(g) + 2H₂O(l) → 2Fe²⁺(aq) + SO₄²⁻(aq) + 4H⁺(aq)
Br₂(aq) + SO₂(g) + 2H₂O(l) → 2Br⁻(aq) + SO₄²⁻(aq) + 4H⁺(aq)
Another application of the reducing property is the bleaching action of SO₂. It combines with coloured substances and reduces them to colourless compounds. This bleaching is usually reversible: the original colour may return when the material is exposed to air or heated, in contrast to the permanent bleaching achieved by chlorine.
SO₂ 的还原性还体现在其漂白作用中。SO₂ 与有色物质结合并将它们还原为无色化合物。这种漂白通常是可逆的：当材料暴露在空气中或受热时，颜色可能恢复；这与氯气的永久性漂白形成鲜明对比。
7. Sulphur Dioxide as an Oxidising Agent | 二氧化硫的氧化性
Although SO₂ usually acts as a reducing agent, it can also behave as an oxidising agent when it reacts with a stronger reducing agent. The classic example is the reaction between SO₂ and hydrogen sulphide, H₂S. In this reaction, sulphur in SO₂ is reduced from +4 to 0, while sulphur in H₂S is oxidised from −2 to 0.
尽管 SO₂ 通常表现为还原剂，但当它遇到更强的还原剂时，也能表现出氧化性。最经典的例子是 SO₂ 与硫化氢 H₂S 的反应。在该反应中，SO₂ 中的硫从 +4 被还原为 0，而 H₂S 中的硫从 −2 被氧化为 0。
2H₂S(g) + SO₂(g) → 3S(s) + 2H₂O(l)
This reaction produces a yellow precipitate of sulphur and is sometimes called the Claus reaction, used in industry to recover sulphur from hydrogen sulphide in natural gas. In the laboratory, mixing the two gases immediately produces a yellow deposit on the walls of the container, a clear demonstration that SO₂ can act as an oxidising agent.
这个反应生成黄色硫沉淀，工业上称为克劳斯反应，用于从天然气中的硫化氢回收硫。在实验室中，将两种气体混合后，容器壁上会迅速出现黄色沉积物，清楚证明 SO₂ 可以充当氧化剂。
In terms of electron transfer, the SO₂ molecule gains four electrons overall (2 × 2) as two sulphur atoms in H₂S each lose two electrons. The stoichiometry is important: one mole of SO₂ reacts with two moles of H₂S to give three moles of sulphur atoms.
从电子转移角度看，SO₂ 分子总共获得四个电子，而 H₂S 中两个硫原子各失去两个电子。反应计量关系也很重要：1 mol SO₂ 与 2 mol H₂S 反应生成 3 mol 硫原子。
8. Sulphur Trioxide and the Contact Process | 三氧化硫与接触法
Sulphur trioxide, SO₃, is the anhydride of sulphuric acid. It is produced industrially by the oxidation of SO₂ in the Contact Process. The reaction is reversible and exothermic:
三氧化硫 SO₃ 是硫酸的酸酐。工业上通过接触法将 SO₂ 氧化来制备 SO₃。该反应是可逆的放热反应：
2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −196 kJ mol⁻¹
The industrial conditions are a temperature of about 450 °C, a pressure of about 2 atm, and the use of vanadium(V) oxide, V₂O₅, as a catalyst. The catalyst provides an alternative reaction pathway with lower activation energy, allowing equilibrium to be reached more quickly at a moderate temperature.
工业条件为：约 450 °C 的温度、约 2 atm 的压力，并使用五氧化二钒 V₂O₅ 作为催化剂。催化剂提供了活化能更低的反应路径，使体系能在中等温度下更快达到平衡。
A low temperature favours the forward exothermic reaction and improves the equilibrium yield, but it makes the rate too slow. A compromise temperature of 450 °C is therefore chosen. High pressure favours the side with fewer gas molecules, which is the product side, but the yield at 2 atm is already over 95%, so very high pressures are unnecessary and uneconomic.
低温有利于放热的正反应方向，能提高平衡产率，但会使反应速率过慢。因此选择折中温度 450 °C。高压有利于气体分子数更少的一侧，即产物一侧，但在 2 atm 下转化率已超过 95%，所以不需要也不值得采用过高的压力。
The SO₃ formed is not absorbed directly in water because the reaction is violently exothermic and produces a corrosive mist. Instead, SO₃ is first dissolved in concentrated H₂SO₄ to form oleum, H₂S₂O₇, which is then carefully diluted with water to give concentrated sulphuric acid.
生成的 SO₃ 不能直接用水吸收，因为该反应剧烈放热并会产生腐蚀性酸雾。实际生产中，先将 SO₃ 溶解在浓硫酸中生成发烟硫酸 H₂S₂O₇，再小心加水稀释得到浓硫酸。
SO₃(g) + H₂O(l) → H₂SO₄(aq)
Sulphur trioxide itself is a colourless, fuming liquid or solid at room temperature. As an acidic oxide, it reacts vigorously with water to form sulphuric acid and with basic oxides to form sulphates.
三氧化硫在室温下是无色发烟液体或固体。作为一种酸性氧化物，它与水剧烈反应生成硫酸，与碱性氧化物反应生成硫酸盐。
9. Environmental Impact: Acid Rain | 环境影响：酸雨
When coal and other fossil fuels containing sulphur impurities are burned, sulphur dioxide is released into the atmosphere. In the air, SO₂ can dissolve directly in water droplets to form sulphurous acid, and it can also be oxidised slowly to SO₃, which then dissolves in water to form sulphuric acid.
当含硫杂质的煤和其他化石燃料燃烧时，二氧化硫
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