📚 Sulfuric Acid | 硫酸
Sulfuric acid, H₂SO₄, is one of the most important industrial chemicals in the world. Its annual production is often used as an indicator of a country’s industrial development. In the Cambridge International A Level Chemistry syllabus, sulfuric acid appears in topics covering the Contact process, acid-base behaviour, redox reactions, dehydration, and qualitative analysis of sulfate ions.
硫酸（H₂SO₄）是世界上最重要的工业化学品之一，其年产量常被用来衡量一个国家的工业发展水平。在剑桥国际 A Level 化学大纲中，硫酸涉及接触法、酸碱行为、氧化还原反应、脱水作用以及硫酸根离子的定性检验等内容。
1. Importance and Industrial Role | 工业重要性与角色
Sulfuric acid is a strong diprotic acid with the formula H₂SO₄. It is widely used in fertiliser production, petroleum refining, metal processing, and chemical synthesis. Because it is consumed in such large quantities, its production is closely linked to economic activity.
硫酸是一种强二元酸，化学式为 H₂SO₄。它广泛用于化肥生产、石油精炼、金属加工和化学合成。由于用量巨大，硫酸的产量与经济活动的联系非常紧密。
The acid is manufactured industrially by the Contact process, which converts sulfur or sulfide ores into sulfur trioxide and then into sulfuric acid. The process is preferred because it gives a high yield and produces concentrated acid of high purity.
工业上通过接触法生产硫酸，将硫或硫化物矿石转化为三氧化硫，再制成硫酸。该方法产率高，并且能生产高纯度的浓硫酸，因此被广泛采用。
2. Raw Materials and Production of Sulfur Dioxide | 原料与二氧化硫的生产
The first stage of the Contact process is the production of sulfur dioxide, SO₂. This can be achieved by burning sulfur in air or by roasting sulfide ores such as iron pyrite, FeS₂.
接触法的第一阶段是制取二氧化硫（SO₂）。可以通过在空气中燃烧硫，或焙烧黄铁矿（FeS₂）等硫化物矿石来实现。
S(s) + O₂(g) → SO₂(g)
4FeS₂(s) + 11O₂(g) → 2Fe₂O₃(s) + 8SO₂(g)
The sulfur dioxide gas must be purified before the next stage. Dust, arsenic compounds, and water vapour are removed because they can poison the catalyst used in the conversion step.
二氧化硫气体在进入下一阶段前必须净化。粉尘、砷化合物和水蒸气都要除去，因为它们会使转化步骤中使用的催化剂中毒。
3. Catalytic Oxidation of Sulfur Dioxide | 二氧化硫的催化氧化
Sulfur dioxide is oxidised to sulfur trioxide, SO₃, in a reversible exothermic reaction. The equation and enthalpy change are shown below.
二氧化硫被氧化为三氧化硫（SO₃），这是一个可逆的放热反应。反应方程式和焓变如下所示。
2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −197 kJ mol⁻¹
The industrial conditions are a vanadium(V) oxide catalyst, V₂O₅, a temperature of about 450 °C, and a pressure of 1–2 atm. These conditions give approximately 98% conversion of SO₂ to SO₃.
工业条件为：使用五氧化二钒（V₂O₅）催化剂，温度约为 450 °C，压强为 1–2 atm。这些条件可使 SO₂ 到 SO₃ 的转化率达到约 98%。
Because the forward reaction is exothermic, a lower temperature would favour a higher equilibrium yield. However, a very low temperature makes the reaction too slow. The catalyst allows a moderate temperature to be used while still achieving a fast rate.
由于正反应放热，较低温度有利于提高平衡产率。但温度过低会使反应速率太慢。催化剂的使用使反应可以在中等温度下快速进行。
In terms of pressure, the reaction goes from three gas molecules to two, so high pressure would favour the product. In practice, the equilibrium position is already far to the right at 1–2 atm, so higher pressures are not economically justified.
从压强角度看，该反应由三个气体分子变为两个气体分子，因此高压有利于产物生成。但实际上在 1–2 atm 时平衡位置已经大幅偏向产物，因此没有必要使用更高的压强。
4. Absorption of Sulfur Trioxide and Formation of Oleum | 三氧化硫的吸收与发烟硫酸的形成
Sulfur trioxide is not dissolved directly in water because the reaction is highly exothermic and produces a fine mist of sulfuric acid droplets. This mist is difficult to condense and would escape from the absorption tower.
三氧化硫不能直接溶于水，因为该反应大量放热，并会形成细小的硫酸酸雾。这种酸雾难以冷凝，会从吸收塔中逸出。
Instead, SO₃ is absorbed in concentrated sulfuric acid to form oleum, H₂S₂O₇, which is also called fuming sulfuric acid.
因此，SO₃ 被浓硫酸吸收，生成发烟硫酸（oleum，H₂S₂O₇）。
SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l)
Water is then added carefully to oleum to produce concentrated sulfuric acid. This method avoids direct contact between SO₃ and water and prevents acid mist formation.
然后小心地向发烟硫酸中加水，生成浓硫酸。这种方法避免了 SO₃ 与水的直接接触，从而防止酸雾形成。
H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)
5. Physical Properties and Acid-Base Behaviour | 物理性质与酸碱行为
Concentrated sulfuric acid is a colourless, viscous, oily liquid with a density of about 1.84 g cm⁻³. It has a high boiling point of around 337 °C and is hygroscopic, meaning it readily absorbs moisture from the air.
浓硫酸是一种无色、粘稠的油状液体，密度约为 1.84 g cm⁻³。它的沸点很高，约为 337 °C，并且具有吸湿性，容易吸收空气中的水分。
In aqueous solution, sulfuric acid ionises in two steps. The first ionisation is essentially complete, while the second is partial.
在水溶液中，硫酸分两步电离。第一步电离基本完全，第二步电离则是部分进行。
H₂SO₄(aq) → H⁺(aq) + HSO₄⁻(aq)
HSO₄⁻(aq) ⇌ H⁺(aq) + SO₄²⁻(aq)
As a strong acid, dilute sulfuric acid reacts with bases, carbonates, and reactive metals. For example, zinc reacts with dilute sulfuric acid to produce hydrogen gas and zinc sulfate.
作为一种强酸，稀硫酸与碱、碳酸盐和活泼金属反应。例如，锌与稀硫酸反应生成氢气和硫酸锌。
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)
6. Dehydration Reactions of Concentrated Sulfuric Acid | 浓硫酸的脱水反应
Concentrated sulfuric acid has a strong dehydrating action. It removes hydrogen and oxygen atoms from organic compounds in the ratio of 2:1, which is the same ratio as in water.
浓硫酸具有很强的脱水性。它能将有机物中的氢原子和氧原子按 2:1 的比例脱去，这一比例与水中氢氧原子比相同。
A classic demonstration is the dehydration of sucrose, C₁₂H₂₂O₁₁. The sugar turns black as carbon is produced, and a large column of carbon rises from the beaker.
经典的演示实验是蔗糖（C₁₂H₂₂O₁₁）的脱水。蔗糖变黑生成碳，并从烧杯中膨胀出大量碳柱。
C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(l)
Concentrated sulfuric acid also removes water of crystallisation from hydrated salts. For example, blue hydrated copper(II) sulfate becomes white anhydrous copper(II) sulfate.
浓硫酸还能脱去水合盐中的结晶水。例如，蓝色的五水合硫酸铜（CuSO₄·5H₂O）会变为白色的无水硫酸铜（CuSO₄）。
CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(l)
7. Oxidising Action of Hot Concentrated Sulfuric Acid | 热浓硫酸的氧化作用
Hot concentrated sulfuric acid can act as an oxidising agent. The sulfur in H₂SO₄ has an oxidation state of +6, and it is reduced to +4 in sulfur dioxide, SO₂.
热的浓硫酸可以充当氧化剂。H₂SO₄ 中硫的氧化态为 +6，在反应中被还原为 +4，生成二氧化硫（SO₂）。
Copper does not react with dilute sulfuric acid because copper is below hydrogen in the reactivity series. However, copper reacts with hot concentrated sulfuric acid to produce copper(II) sulfate, sulfur dioxide, and water.
铜不与稀硫酸反应，因为铜在金属活动性顺序中位于氢之后。但铜可以与热的浓硫酸反应，生成硫酸铜、二氧化硫和水。
Cu(s) + 2H₂SO₄(l) → CuSO₄(aq) + SO₂(g) + 2H₂O(l)
The reaction mixture turns blue due to aqueous Cu²⁺ ions, and a colourless gas with a choking smell is produced. This gas can be identified as sulfur dioxide because it turns acidified potassium dichromate(VI) from orange to green.
反应后溶液因 Cu²⁺ 离子而呈蓝色，并产生有刺激性气味的无色气体。该气体可通过使酸化重铬酸钾溶液由橙色变为绿色来鉴定为二氧化硫。
Carbon can also be oxidised by hot concentrated sulfuric acid, producing carbon dioxide and sulfur dioxide.
碳也能被热浓硫酸氧化，生成二氧化碳和二氧化硫。
C(s) + 2H₂SO₄(l) → CO₂(g) + 2SO₂(g) + 2H₂O(l)
8. Reactions with Halides and Other Salts | 与卤化物及其他盐的反应
Concentrated sulfuric acid reacts with solid sodium chloride to produce hydrogen chloride gas. This is an acid-base reaction, and the HCl appears as misty fumes in moist air.
浓硫酸与固体氯化钠反应生成氯化氢气体。这是一个酸碱反应，HCl 在潮湿空气中形成白雾。
NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g)
With sodium bromide, the first step also produces hydrogen bromide. However, HBr is a stronger reducing agent than HCl, so hot concentrated sulfuric acid oxidises it to bromine.
与溴化钠反应时，第一步同样生成溴化氢。但 HBr 的还原性比 HCl 强，因此热浓硫酸会将其氧化为溴。
NaBr(s) + H₂SO₄(l) → NaHSO₄(s) + HBr(g)
2HBr(g) + H₂SO₄(l) → Br₂(g) + SO₂(g) + 2H₂O(l)
With sodium iodide, the iodide ion is an even stronger reducing agent. It is oxidised to iodine, and some sulfuric acid is reduced all the way to hydrogen sulfide, H₂S.
与碘化钠反应时，碘离子的还原性更强。它被氧化为碘，同时部分硫酸被还原成硫化氢（H₂S）。
8NaI(s) + 9H₂SO₄(l) → 8NaHSO₄(s) + 4I₂(s) + H₂S(g) + 4H₂O(l)
This trend shows the increasing reducing power of the halide ions down Group 17: Cl⁻ < Br⁻ < I⁻. The products observed are misty fumes for HCl, orange-brown bromine vapour for NaBr, and purple iodine vapour together with a rotten-egg smell from H₂S for NaI.
这一趋势说明卤素离子的还原性沿第 17 族向下增强：Cl⁻ < Br⁻ < I⁻。观察到的产物分别是：氯化钠产生白雾 HCl，溴化钠产生橙棕色溴蒸气，碘化钠产生紫色碘蒸气以及来自 H₂S 的臭鸡蛋气味。
9. Test for Sulfate Ions | 硫酸根离子的检验
The standard test for sulfate ions, SO₄²⁻, is to add dilute hydrochloric acid followed by aqueous barium chloride solution. A white precipitate of barium sulfate is formed.
检验硫酸根离子（SO₄²⁻）的标准方法是：先加入稀盐酸，再加入氯化钡溶液，会生成白色的硫酸钡沉淀。
Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
The white precipitate is insoluble in dilute hydrochloric acid. The acid is added first to remove carbonate or sulfite ions, which would otherwise also give white precipitates with barium chloride.
该白色沉淀不溶于稀盐酸。先加酸是为了除去碳酸根或亚硫酸根离子，否则它们也会与氯化钡生成白色沉淀。
This test is commonly used in qualitative analysis to distinguish sulfate ions from other anions such as carbonate, CO₃²⁻, and sulfite, SO₃²⁻, because their barium salts dissolve in dilute acid whereas barium sulfate does not.
该检验常用于定性分析中，以区分硫酸根与其他阴离子，如碳酸根 CO₃²⁻ 和亚硫酸根 SO₃²⁻，因为它们的钡盐溶于稀酸，而硫酸钡不溶。
10. Major Uses of Sulfuric Acid | 硫酸的主要用途
The largest use of sulfuric acid is in the manufacture of phosphate fertilisers. It is also used to produce ammonium sulfate, a common nitrogen fertiliser.
硫酸最大的用途是制造磷肥。它也用于生产硫酸铵，一种常用的氮肥。
Other important applications include the production of detergents, pigments such as titanium dioxide, synthetic fibres, and dyes. In petroleum refining, sulfuric acid is used as a catalyst and to remove impurities.
其他重要用途包括生产洗涤剂、颜料（如二氧化钛）、合成纤维和染料。在石油精炼中，硫酸用作催化剂并用于去除杂质。
In the laboratory, concentrated sulfuric acid is used as a drying agent for gases that do not react with it, and as a dehydrating agent in organic reactions such as esterification.
在实验室中，浓硫酸用作不与它反应的气体的干燥剂，并在酯化等有机反应中用作脱水剂。
Dilute sulfuric acid is the electrolyte in lead-acid car batteries, where it reacts with lead and lead(IV) oxide during discharge.
稀硫酸是铅酸汽车电池中的电解质，在放电过程中与铅和二氧化铅发生反应。
11. Environmental and Safety Considerations | 环境与安全考量
Sulfur dioxide released during the Contact process contributes to acid rain if it escapes into the atmosphere. Acid rain damages buildings, aquatic life, and vegetation, so modern plants remove SO₂ from waste gases.
接触法中逸出的二氧化硫如果进入大气，会导致酸雨。酸雨会破坏建筑物、水生生物和植被，因此现代化工厂会从废气中除去 SO₂。
One common method is to pass the waste gases through an alkaline slurry of calcium oxide or calcium carbonate. Sulfur dioxide reacts to form calcium sulfite, which can be further oxidised to calcium sulfate.
常用方法之一是让废气通过氧化钙或碳酸钙的碱性浆液。二氧化硫与其反应生成亚硫酸钙，亚硫酸钙可进一步氧化为硫酸钙。
Concentrated sulfuric acid is highly corrosive and causes severe burns. When diluting it, the acid must always be added slowly to water with stirring, never water to acid, because the dilution is highly exothermic and adding water to acid can cause violent boiling and splashing.
浓硫酸具有强腐蚀性，会造成严重灼伤。稀释时必须将酸缓慢加入水中并不断搅拌，绝不能将水加入酸中，因为稀释过程大量放热，将水加入酸中会引起剧烈沸腾和飞溅。
Storage and transport require acid-resistant containers, and spillages are neutralised with bases such as calcium hydroxide or sodium carbonate before disposal.
储存和运输需要使用耐酸容器，泄漏的硫酸应先用氢氧化钙或碳酸钠等碱中和，再进行处置。
12. Summary and Exam Focus | 总结与考试重点
For Cambridge A Level Chemistry, students should be able to describe the Contact process and explain the choice of conditions using equilibrium and rate principles. The key equations include the combustion of sulfur, the reversible oxidation of SO₂, and the formation and dilution of oleum.
在剑桥 A Level 化学中，学生应能描述接触法，并运用平衡和速率原理解释工艺条件的选择。关键方程式包括硫的燃烧、SO₂ 的可逆氧化以及发烟硫酸的生成与稀释。
It is also important to distinguish between the acid-base behaviour of dilute sulfuric acid and the oxidising and dehydrating behaviour of concentrated sulfuric acid. Students should be able to write equations for reactions with copper, carbon, halides, and sugar, and to describe the barium chloride test for sulfate ions.
同样重要的是，要区分稀硫酸的酸碱行为与浓硫酸的氧化性和脱水性。学生应能写出浓硫酸与铜、碳、卤化物和蔗糖反应的方程式，并能描述检验硫酸根离子的氯化钡实验。
Concentrated H₂SO₄ = dehydrating agent + oxidising agent; dilute H₂SO₄ = typical strong acid
浓 H₂SO₄ = 脱水剂 + 氧化剂；稀 H₂SO₄ = 典型的强酸
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