Sulfuric Acid: Properties and Uses — 硫酸的性质与用途

1. The Contact Process: How Sulfuric Acid Is Made | 接触法：硫酸是如何生产的

硫酸是世界上产量最大的化工产品之一，年产量超过两亿吨。在A-Level化学中，CIE考试局要求你掌握它的工业制备方法，即接触法（Contact Process）。理解这个流程不仅是考试的重点，也是理解后续性质与用途的基础，因为工业制备的细节直接决定了产品的纯度和浓度。

Sulfuric acid is one of the most-produced chemicals in the world, with an annual output of over 200 million tonnes. In A-Level Chemistry, the CIE syllabus requires you to master its industrial manufacture, the Contact Process. Understanding this flow is not only a key exam focus but also the foundation for understanding later properties and uses, because the details of industrial manufacture directly determine the purity and concentration of the product.

接触法主要分为三个阶段：第一步，燃烧硫磺或焙烧金属硫化物矿石来制取二氧化硫；第二步，二氧化硫在催化剂作用下与氧气反应生成三氧化硫；第三步，三氧化硫溶解在浓硫酸中形成发烟硫酸，再用水稀释得到所需浓度的硫酸。这三个阶段环环相扣，任何一个环节的条件控制都会影响最终收率。

The Contact Process consists of three main stages. First, sulfur is burned or metal sulfide ores are roasted to produce sulfur dioxide. Second, sulfur dioxide reacts with oxygen in the presence of a catalyst to form sulfur trioxide. Third, sulfur trioxide dissolves in concentrated sulfuric acid to form oleum, which is then diluted with water to obtain sulfuric acid of the required concentration. These three stages are closely linked, and the control of conditions in any one stage affects the final yield.

2. Making Sulfur Dioxide: Burning Sulfur or Roasting Sulfide Ores | 制备二氧化硫：燃烧硫磺或焙烧硫化物矿石

接触法的原料之一是二氧化硫。工业上最常见的做法是直接燃烧硫磺，反应方程式为S + O2 → SO2。硫磺燃烧时产生明亮的蓝色火焰，反应放出大量热，生成的气体经过净化后直接进入下一阶段。另一种常见来源是焙烧硫化物矿石，例如闪锌矿（ZnS）和黄铁矿（FeS2），这在一些没有天然硫磺资源的地区尤为重要。

One of the raw materials of the Contact Process is sulfur dioxide. Industrially, the most common method is to burn elemental sulfur directly, with the equation S + O2 → SO2. Sulfur burns with a bright blue flame, releasing a large amount of heat, and the gas produced is purified before entering the next stage. Another common source is roasting sulfide ores such as sphalerite (ZnS) and pyrite (FeS2), which is especially important in regions without natural sulfur deposits.

为什么必须净化气体？因为矿石焙烧产生的气体中可能含有砷的化合物和粉尘，这些杂质会使催化剂”中毒”而失效。催化剂中毒是工业催化中的经典问题：少量杂质就能让昂贵的催化剂永久失活。因此，气体进入催化转化器之前必须经过除尘、洗涤和干燥处理。

Why must the gas be purified? Gas from ore roasting may contain arsenic compounds and dust, which can poison and deactivate the catalyst. Catalyst poisoning is a classic problem in industrial catalysis: even small amounts of impurities can permanently deactivate an expensive catalyst. Therefore, before entering the catalytic converter, the gas must be cleaned, washed and dried.

3. The Catalytic Oxidation of Sulfur Dioxide: Why Vanadium(V) Oxide | 二氧化硫的催化氧化：为什么选用五氧化二钒

核心反应是二氧化硫与氧气生成三氧化硫：2SO2 + O2 ⇌ 2SO3，这是一个放热、体积减小的可逆反应。根据勒夏特列原理（Le Chatelier’s principle），低温高压有利于提高三氧化硫的平衡产率，但温度太低反应速率过慢。工业上需要在速率与产率之间取得平衡。

The core reaction is the oxidation of sulfur dioxide to sulfur trioxide: 2SO2 + O2 ⇌ 2SO3, which is exothermic and involves a decrease in volume. According to Le Chatelier’s principle, low temperature and high pressure favour a higher equilibrium yield of sulfur trioxide, but too low a temperature makes the reaction too slow. Industry must strike a balance between rate and yield.

工业上选择的条件是：温度约450°C，压力约1-2个大气压（常压稍加压），催化剂为五氧化二钒（V2O5）。在450°C下，转化率可达到约97%，已经足够经济。为什么不追求更高的转化率？因为进一步提高压力会大幅增加设备成本，而97%的转化率已经使未反应的二氧化硫量很小，循环利用即可。

The industrial conditions chosen are: a temperature of about 450°C, a pressure of about 1-2 atmospheres (around atmospheric pressure), and vanadium(V) oxide (V2O5) as the catalyst. At 450°C the conversion reaches about 97%, which is economical enough. Why not aim for higher conversion? Because higher pressure greatly increases equipment costs, and at 97% conversion the amount of unreacted sulfur dioxide is already small; the unreacted gas is simply recycled.

五氧化二钒如何起催化作用？它的机理涉及钒的价态变化：V2O5先被SO2还原为V2O4（或VO2），然后V2O4再被O2重新氧化回V2O5。这个氧化还原循环使催化剂能够反复使用。考试中常要求你解释催化剂的作用机理，记住”催化剂通过改变价态循环参与反应”这个要点非常关键。

How does vanadium(V) oxide catalyse the reaction? The mechanism involves a change in the oxidation state of vanadium: V2O5 is first reduced by SO2 to V2O4 (or VO2), then V2O4 is re-oxidised back to V2O5 by O2. This redox cycle allows the catalyst to be reused indefinitely. Exams often ask you to explain the catalytic mechanism; remembering that “the catalyst participates in the reaction through a cycle of oxidation state changes” is a key point.

4. Absorption in the Tower: Oleum and Controlled Dilution | 吸收塔中的反应：发烟硫酸与受控稀释

三氧化硫不能直接用水吸收，因为SO3与水反应极为剧烈，会生成硫酸酸雾（mist），这些细小的酸雾难以收集，造成产品损失和严重污染。因此工业上把SO3溶解在98%的浓硫酸中，生成发烟硫酸（oleum，化学式H2S2O7，又称焦硫酸）。

Sulfur trioxide cannot be absorbed directly in water, because the reaction between SO3 and water is extremely vigorous and produces a sulfuric acid mist. These fine droplets are hard to collect, causing product loss and serious pollution. Therefore industry dissolves SO3 in 98% concentrated sulfuric acid to form oleum (H2S2O7, also called pyrosulfuric acid or fuming sulfuric acid).

发烟硫酸随后被小心地用水稀释，得到浓度合适的成品硫酸。稀释过程必须缓慢进行，因为硫酸与水混合会放出大量热 – 这既是工业上的注意事项，也是实验室安全规则：稀释浓硫酸时，必须”酸入水”（将酸缓慢加入水中并不断搅拌），而不是”水入酸”。这个考点几乎每年都会出现在安全类题目中。

The oleum is then carefully diluted with water to obtain product sulfuric acid of the desired concentration. The dilution must be done slowly because mixing sulfuric acid with water releases a large amount of heat. This is both an industrial precaution and a laboratory safety rule: when diluting concentrated sulfuric acid, always “add acid to water” slowly with constant stirring, never water to acid. This point appears in safety questions almost every year.

5. Physical Properties: A Dense, High-Boiling, Hygroscopic Liquid | 物理性质：高密度、高沸点、吸湿性液体

纯硫酸是无色、油状、黏稠的液体，密度约1.84 g/cm³，远大于水。它的沸点高达337°C，远高于水，这是因为硫酸分子之间存在强烈的氢键网络。高沸点使浓硫酸成为制备挥发性酸（如HCl、HNO3）的理想试剂：利用”难挥发性酸制易挥发性酸”的原理，浓硫酸与氯化钠或硝酸盐反应可以置换出相应挥发性酸。

Pure sulfuric acid is a colourless, oily, viscous liquid with a density of about 1.84 g/cm³, much greater than water. Its boiling point is as high as 337°C, far above water, because of the strong hydrogen-bonding network between molecules. This high boiling point makes concentrated sulfuric acid an ideal reagent for preparing volatile acids such as HCl and HNO3: using the principle that a less volatile acid displaces a more volatile one, concentrated sulfuric acid reacts with sodium chloride or nitrates to release the corresponding volatile acid.

浓硫酸还具有强烈的吸水性（hygroscopic）和脱水性（dehydrating），这两个概念考试中经常被混淆。吸水性指它吸收游离的水分子，因此常用作干燥剂（drying agent），可以干燥氯气、二氧化硫等不与它反应的气体。脱水性则指它从化合物中夺取氢和氧元素（以水的比例），这一性质我们将在下一节详细展开。

Concentrated sulfuric acid is also strongly hygroscopic and dehydrating, two concepts that are frequently confused in exams. Hygroscopicity means it absorbs free water molecules, which is why it is used as a drying agent for gases that do not react with it, such as chlorine and sulfur dioxide. Dehydration means it removes hydrogen and oxygen elements (in the ratio of water) from compounds; we will expand on this property in the next section.

6. The Dehydrating Property: Charring Sugar and Concentrating Nitric Acid | 脱水性：蔗糖炭化与制备浓硝酸

浓硫酸的脱水性最经典的演示实验是蔗糖炭化：把浓硫酸倒入蔗糖（C12H22O11）中，蔗糖迅速变黑并膨胀成疏松的碳块，同时放出大量热和水蒸气。反应的实质是浓硫酸按水的比例夺取蔗糖分子中的氢和氧：C12H22O11 → 12C + 11H2O。黑色的固体就是碳，膨胀则是水蒸气逸出造成的。

The classic demonstration of the dehydrating property of concentrated sulfuric acid is the charring of sugar: when concentrated sulfuric acid is poured onto sucrose (C12H22O11), the sugar rapidly turns black and swells into a porous lump of carbon, releasing large amounts of heat and steam. The essence of the reaction is that the acid removes hydrogen and oxygen from the sucrose molecule in the ratio of water: C12H22O11 → 12C + 11H2O. The black solid is carbon, and the swelling is caused by escaping steam.

脱水性的另一个重要应用是制备浓硝酸。实验室制硝酸时，用浓硫酸与硝酸钠反应：NaNO3 + H2SO4 → NaHSO4 + HNO3。由于浓硫酸的沸点高于硝酸，加热时硝酸蒸气逸出，冷凝后得到硝酸。这里浓硫酸既是酸性反应物，又依靠其高沸点把沸点较低的硝酸”赶”出来，体现了”高沸点酸制低沸点酸”的原理。

Another important application of dehydration is the preparation of concentrated nitric acid. In the laboratory, nitric acid is made by reacting concentrated sulfuric acid with sodium nitrate: NaNO3 + H2SO4 → NaHSO4 + HNO3. Because concentrated sulfuric acid boils at a higher temperature than nitric acid, heating drives off nitric acid vapour, which condenses to give the acid. Here the concentrated sulfuric acid acts both as an acidic reactant and, through its high boiling point, drives out the lower-boiling nitric acid, illustrating the principle of preparing a low-boiling acid from a high-boiling one.

7. Sulfuric Acid as a Strong Diprotic Acid: Two-Step Ionisation | 硫酸作为强二元酸：两步电离

硫酸是典型的强二元酸（diprotic acid），它在水中的电离分两步进行。第一步完全电离：H2SO4 → H+ + HSO4-；第二步部分电离：HSO4- ⇌ H+ + SO4^2-。因此0.1 mol/dm³硫酸溶液的pH并不是1，而是略小于1，因为氢离子浓度略高于0.1 mol/dm³。考试中常考这个细节：硫酸的酸性与硫酸根离子的检验。

Sulfuric acid is a typical strong diprotic acid; its ionisation in water occurs in two steps. The first step is complete: H2SO4 → H+ + HSO4-. The second step is partial: HSO4- ⇌ H+ + SO4^2-. Therefore the pH of a 0.1 mol/dm³ sulfuric acid solution is not exactly 1, but slightly less than 1, because the hydrogen ion concentration is slightly above 0.1 mol/dm³. Exams often test this detail, together with the acid properties and the test for sulfate ions.

硫酸根离子的检验是实验题的经典考点：先加入盐酸酸化（排除碳酸根等干扰离子），再加入氯化钡溶液，如果出现白色沉淀（BaSO4），则证明硫酸根离子存在。硫酸钡是难溶盐，且不溶于稀盐酸，这是检验的化学基础。记住这个检验流程的先后顺序，考试时按步骤书写即可得分。

The test for sulfate ions is a classic experimental question: first acidify with hydrochloric acid (to exclude interfering ions such as carbonate), then add barium chloride solution; a white precipitate (BaSO4) confirms the presence of sulfate ions. Barium sulfate is insoluble and does not dissolve in dilute hydrochloric acid, which is the chemical basis of the test. Remember the order of this procedure and write it out step by step in the exam to gain marks.

8. The Oxidising Property: Reactions with Copper and Carbon | 氧化性：与铜和碳的反应

浓硫酸是强氧化剂，尤其在加热条件下。稀硫酸与金属反应体现的是氢离子的酸性，而浓硫酸与金属反应则体现出硫的氧化性（硫酸中的硫为+6价，可被还原为SO2）。例如，加热时浓硫酸与铜反应：Cu + 2H2SO4(浓) → CuSO4 + SO2↑ + 2H2O。注意这里生成的是二氧化硫而不是氢气，这是区分浓硫酸氧化性与稀硫酸酸性的关键。

Concentrated sulfuric acid is a strong oxidising agent, especially when heated. Reactions of dilute sulfuric acid with metals show the acidity of hydrogen ions, whereas reactions of concentrated sulfuric acid with metals show the oxidising ability of sulfur (sulfur in sulfuric acid is in the +6 oxidation state and can be reduced to SO2). For example, when heated, concentrated sulfuric acid reacts with copper: Cu + 2H2SO4(conc) → CuSO4 + SO2↑ + 2H2O. Note that sulfur dioxide is produced rather than hydrogen, which is the key distinction between the oxidising property of concentrated sulfuric acid and the acidity of dilute sulfuric acid.

浓硫酸同样能氧化非金属单质。例如加热时碳被氧化为二氧化碳：C + 2H2SO4(浓) → CO2↑ + 2SO2↑ + 2H2O。这个反应中碳从0价升到+4价被氧化，硫从+6价降到+4价被还原。识别氧化还原中的电子转移、标明氧化剂和还原剂，是CIE化学考试的固定题型。

Concentrated sulfuric acid can also oxidise non-metal elements. For example, when heated, carbon is oxidised to carbon dioxide: C + 2H2SO4(conc) → CO2↑ + 2SO2↑ + 2H2O. In this reaction carbon is oxidised from 0 to +4, while sulfur is reduced from +6 to +4. Identifying electron transfer in redox reactions and naming the oxidising and reducing agents is a standard question type in CIE chemistry exams.

9. Sulphonation: Making Detergents and Dyes | 磺化反应：制造洗涤剂与染料

磺化反应是浓硫酸的另一个重要化学性质：把磺酸基（-SO3H）引入有机分子。最经典的例子是苯的磺化：苯与浓硫酸在加热条件下反应生成苯磺酸（C6H5SO3H）。反应条件通常是约80°C，或使用发烟硫酸。这个反应在CIE大纲中属于苯及其衍生物的必考内容。

Sulphonation is another important chemical property of concentrated sulfuric acid: introducing the sulfonic acid group (-SO3H) into an organic molecule. The classic example is the sulphonation of benzene: benzene reacts with concentrated sulfuric acid on heating to form benzenesulfonic acid (C6H5SO3H). The typical conditions are about 80°C, or the use of fuming sulfuric acid. This reaction is a required topic in the CIE syllabus under benzene and its derivatives.

磺化反应有重要的工业意义：长链烷基苯磺酸盐是合成洗涤剂（洗衣粉、洗洁精）的主要活性成分，它们的分子一端亲水（磺酸根）、一端亲油（长碳链），因此能同时润湿油污和水。磺化也用于合成某些染料和药物中间体。理解”亲水亲油”结构是解释去污原理的关键。

Sulphonation has important industrial significance: long-chain alkylbenzene sulfonates are the main active ingredients of synthetic detergents (washing powders and dishwashing liquids). Their molecules have a hydrophilic end (the sulfonate group) and a hydrophobic end (the long carbon chain), so they can wet both grease and water simultaneously. Sulphonation is also used to synthesise certain dyes and pharmaceutical intermediates. Understanding the “hydrophilic-hydrophobic” structure is the key to explaining the cleaning mechanism.

10. Major Uses: From Fertilisers to Car Batteries | 主要用途：从化肥到汽车电池

硫酸的用途极为广泛，CIE考试常以”列举硫酸的主要用途”为简答题。第一大用途是制造化肥：硫酸与磷矿石反应生产过磷酸钙等磷肥，与氨反应生成硫酸铵（(NH4)2SO4）氮肥。全球约一半的硫酸产量用于化肥工业，可以说硫酸支撑着现代农业。

The uses of sulfuric acid are extremely wide-ranging, and CIE exams often include short-answer questions asking you to list the major uses. The largest use is the manufacture of fertilisers: sulfuric acid reacts with phosphate rock to produce superphosphate fertilisers, and with ammonia to produce ammonium sulfate ((NH4)2SO4) nitrogen fertiliser. About half of the world’s sulfuric acid production goes to the fertiliser industry; one could say sulfuric acid sustains modern agriculture.

第二大用途是铅酸蓄电池（lead-acid battery）：汽车电池的电解液就是约30%的硫酸溶液。放电时硫酸被消耗，充电时硫酸重新生成，电池的充放电循环依赖于硫酸浓度的变化。此外，硫酸还用于石油精炼（作为催化剂和洗涤剂）、金属冶炼前的酸洗（去除金属表面的氧化物）、颜料制造（如钛白粉TiO2）、炸药和纺织工业。

The second major use is the lead-acid battery: the electrolyte of a car battery is about 30% sulfuric acid solution. During discharge sulfuric acid is consumed, and during charging it is regenerated; the charge-discharge cycle depends on the change in sulfuric acid concentration. In addition, sulfuric acid is used in petroleum refining (as a catalyst and wash), pickling of metals before processing (removing surface oxides), pigment manufacture (such as titanium dioxide TiO2), explosives and the textile industry.

11. Acid Rain and Safety: Environmental Impact and Lab Handling | 酸雨与安全：环境影响与实验室操作

硫酸的环境影响主要通过酸雨体现。工业燃烧含硫燃料排放二氧化硫，SO2在大气中被氧化并溶解于水形成亚硫酸和硫酸，使雨水pH降低至4-5甚至更低。酸雨会腐蚀建筑物（尤其是大理石和石灰石）、损害森林和湖泊生态、加速金属腐蚀。这是化学与环境交叉的必考论述题素材。

The environmental impact of sulfuric acid is mainly through acid rain. Burning sulfur-containing fuels in industry releases sulfur dioxide; SO2 is oxidised in the atmosphere and dissolves in water to form sulfurous and sulfuric acids, lowering the pH of rainwater to 4-5 or even lower. Acid rain corrodes buildings (especially marble and limestone), damages forests and lake ecosystems, and accelerates metal corrosion. This is essential material for discussion questions at the interface of chemistry and the environment.

实验室安全方面，浓硫酸具有强腐蚀性，会严重灼伤皮肤和眼睛，操作时必须佩戴护目镜和手套。万一皮肤接触，应立即用大量水冲洗至少15分钟并就医。稀释浓硫酸时务必”酸入水”：将酸沿玻璃棒缓慢倒入水中并搅拌，使热量及时散失；绝不能把水倒入浓硫酸中，否则水在酸表面剧烈沸腾飞溅，极易造成灼伤。

In terms of laboratory safety, concentrated sulfuric acid is highly corrosive and severely burns skin and eyes; goggles and gloves must be worn when handling it. If skin contact occurs, rinse immediately with plenty of water for at least 15 minutes and seek medical attention. When diluting concentrated sulfuric acid, always “add acid to water”: pour the acid slowly down a glass rod into water with stirring so the heat can dissipate. Never pour water into concentrated acid, because the water boils violently and splashes on the acid surface, easily causing burns.

12. Exam Question Patterns: How to Score Full Marks | 常见考试题型：如何拿满分

关于硫酸的题目在CIE考试中主要有四类。第一类是接触法条件分析题，常问”为什么选择450°C””为什么不用更高压力”，答题要点是同时从速率、产率和成本三个角度分析，并引用勒夏特列原理。第二类是性质辨析题，要求区分吸水性和脱水性，给出具体例子（干燥气体 vs 蔗糖炭化）。

Questions about sulfuric acid in CIE exams mainly fall into four categories. The first is analysis of Contact Process conditions, often asking “why 450°C” and “why not a higher pressure”; the answer should consider rate, yield and cost simultaneously, citing Le Chatelier’s principle. The second is property discrimination, requiring you to distinguish hygroscopicity from dehydration with concrete examples (drying a gas versus charring sugar).

第三类是氧化还原方程式书写题，例如与铜、碳的反应，要求配平并标明电子转移、氧化剂和还原剂。第四类是用途与实验题，例如列举硫酸用途、设计硫酸根离子检验流程。答题时注意：方程式必须配平并标注状态符号，氧化还原题要写出氧化数的变化，实验流程题要按”取样→酸化→加试剂→描述现象→得出结论”的逻辑顺序书写。

The third category is writing and balancing redox equations, such as reactions with copper and carbon, including electron transfer, oxidising agent and reducing agent. The fourth is uses and experiments, such as listing the uses of sulfuric acid and designing the sulfate ion test procedure. When answering, remember: equations must be balanced with state symbols, redox questions need oxidation number changes written out, and experimental procedure questions should follow the logical order of “sample → acidify → add reagent → describe observation → draw conclusion”.

Summary | 总结

本文系统梳理了A-Level化学（CIE）中硫酸的核心知识点：工业上通过接触法生产硫酸，经历了制取SO2、催化氧化为SO3、在浓硫酸中吸收生成发烟硫酸并稀释三个阶段，核心条件为450°C、常压和V2O5催化剂；硫酸具有高沸点、吸水性、脱水性、强酸性和氧化性等性质，能发生磺化反应；其主要用途包括制造化肥、铅酸电池电解液、石油精炼和颜料生产等。

This article has systematically reviewed the core knowledge of sulfuric acid in A-Level Chemistry (CIE): industrially, sulfuric acid is produced by the Contact Process through three stages, namely making SO2, catalytic oxidation to SO3, absorption in concentrated sulfuric acid to form oleum and controlled dilution, with key conditions of 450°C, atmospheric pressure and the V2O5 catalyst; sulfuric acid has a high boiling point and shows hygroscopic, dehydrating, strongly acidic and oxidising properties, and undergoes sulphonation; its major uses include manufacturing fertilisers, lead-acid battery electrolyte, petroleum refining and pigment production.

掌握这些内容时，建议把性质与用途联系起来记忆：脱水性和氧化性决定了它在有机反应和金属处理中的角色，吸水性使它成为干燥剂，强酸性则支撑了化肥和电池两大工业用途。配合接触法条件分析题和硫酸根离子检验题反复练习，考试中遇到相关题目就能从容应对。

When mastering this content, it is advisable to connect properties with uses: the dehydrating and oxidising properties determine its role in organic reactions and metal processing, hygroscopicity makes it a drying agent, and strong acidity supports the two major industrial uses of fertilisers and batteries. With repeated practice on Contact Process condition analysis and sulfate ion tests, you will handle related exam questions with confidence.

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