📚 Sulfuric Acid: Production, Properties and Reactions | 硫酸：生产、性质与反应
Sulfuric acid is one of the most important industrial chemicals, manufactured on a vast scale through the Contact process. Its unique combination of strong acidity, powerful dehydrating ability and oxidising nature in the concentrated form makes it essential in fertilisers, chemical synthesis, petroleum refining and metal processing. This article examines its production, the optimisation of the key equilibrium, and the characteristic reactions that define its behaviour.
硫酸是最重要的工业化学品之一，通过接触法大规模生产。它集强酸性、强脱水性和浓酸特有的氧化性于一体，使得它在化肥、化学合成、石油精炼和金属加工中不可或缺。本文将介绍其生产方法、关键平衡的优化，以及定义其行为的特征反应。
1. Introduction to Sulfuric Acid | 硫酸简介
Sulfuric acid, H₂SO₄, is a colourless, viscous liquid that is miscible with water in all proportions, with the dissolution being highly exothermic. It is a diprotic acid, meaning it can donate two protons per molecule. Its industrial significance is reflected in the annual global production exceeding 230 million tonnes, used as a raw material for phosphate fertilisers, detergents, dyes and explosives.
硫酸（H₂SO₄）是一种无色粘稠液体，能与水以任意比例混溶，溶解放出大量热。它属于二元酸，每分子可电离两个质子。其工业重要性体现在全球年产量超过2.3亿吨，用作磷肥、洗涤剂、染料和炸药的原料。
2. The Contact Process: Raw Materials | 接触法：原料
Alternatively, roasting of sulfide ores such as pyrite (FeS₂) or zinc blende (ZnS) can provide SO₂, though direct sulfur combustion is cleaner and dominates current production.
现代接触法以单质硫或硫化物矿石为起点。硫在干燥空气中燃烧生成二氧化硫：S(s) + O₂(g) → SO₂(g)。也可通过焙烧黄铁矿（FeS₂）或闪锌矿（ZnS）获得SO₂，但直接燃烧硫磺杂质更少，在目前生产中占主导地位。
3. Catalytic Oxidation of SO₂ to SO₃ | 二氧化硫的催化氧化
used to accelerate the reaction, which is carried out at around 450°C and a pressure of 1–2 atm. The equilibrium is: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = – ⁻¹. The catalyst works via a redox cycle where V₂O₅ oxidises SO₂ and is regenerated by O₂.
该过程的核心是二氧化硫可逆的放热氧化反应，生成三氧化硫。采用五氧化二钒（V₂O₅）催化剂加速反应，操作温度约450°C，压力1–2个大气压。平衡方程式为：2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = –196 kJ mol⁻¹。催化剂通过氧化还原循环起作用：V₂O₅氧化SO₂，自身被O₂再生。
4. Optimising Conditions for Maximum Yield | 优化条件以获得最高产率
According to Le Chatelier's principle, a lower temperature favours the forward exothermic reaction, increasing equilibrium yield of SO₃. However, too low a temperature makes the rate unacceptably slow. The compromise temperature of about 450°C ensures a high reaction rate with an equilibrium conversion of roughly 97–98%. Regarding pressure, the forward reaction reduces the number of gas molecules (from 3 to 2), so high pressure would shift equilibrium to the right. Yet at 1–2 atm, conversion is already very high, and the extra cost of high-pressure equipment is not justified.
根据勒夏特列原理，低温有利于放热正反应，提高SO₃的平衡产率。但温度过低会使速率过慢。折中温度约450°C可保证高反应速率，平衡转化率约97–98%。在压力方面，正反应气体分子数减少（3变2），高压会使平衡右移。但在1–2 atm下转化率已很高，不值得承担高压设备的额外成本。
5. Absorption of SO₃ and Formation of Oleum | 三氧化硫的吸收与发烟硫酸的形成
uld escape into the atmosphere. Instead, SO₃ is absorbed in concentrated sulfuric acid (98% ic acid: H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l). This two-step method gives safe production of acid of any desired concentration.
三氧化硫并不直接溶于水，因反应过于剧烈，会形成细小硫酸雾滴逸散到大气中。因此，SO₃先被浓硫酸（98%）吸收，生成焦硫酸（发烟硫酸）H₂S₂O₇：SO₃(g) + H₂SO₄(l) → H₂S₂O₇(l)。再用水小心稀释发烟硫酸，制得浓硫酸：H₂S₂O₇(l) + H₂O(l) → 2H₂SO₄(l)。这种两步法可安全地生产任意浓度的硫酸。
6. Acidic Properties | 酸性
SO₄ → H⁺ + HSO₄⁻. The hydrogensulfate ion is a much weaker acid with an equilibrium: HSO₄⁻ ⇌ sodium hydroxide: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. With magnesium: Mg + H₂SO₄ → MgSO₄ + H₂.
作为一种强二元酸，硫酸第一步电离完全：H₂SO₄ → H⁺ + HSO₄⁻。硫酸氢根离子是弱得多的酸，存在平衡：HSO₄⁻ ⇌ H⁺ + SO₄²⁻。稀硫酸与碱、碳酸盐和活泼金属反应，表现出典型酸的性质。例如与氢氧化钠：H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O；与镁：Mg + H₂SO₄ → MgSO₄ + H₂。
7. Dehydrating Action | 脱水作用
m organic compounds such as carbohydrates. For example, when added to sucrose (C₁₂H₂₂O₁₁), a spectacular charring occurs, producing a column of black carbon: C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(l). The acid is also used as a dehydrating agent in esterification and nitration reactions to shift equilibrium by removing water.
浓硫酸对水有极强的亲和力，能从许多物质中夺取水分子。它能脱去有机物（如碳水化合物）中的水元素。例如，将浓硫酸加入蔗糖（C₁₂H₂₂O₁₁），会发生剧烈的碳化现象，产生黑色的碳柱：C₁₂H₂₂O₁₁(s) → 12C(s) + 11H₂O(l)。在酯化和硝化反应中，硫酸也用作脱水剂，通过移除水来推动平衡。
8. Oxidising Properties of Concentrated Sulfuric Acid | 浓硫酸的氧化性
n the reactivity series, such as copper: Cu(s) + 2H₂SO₄(l) → CuSO₄(aq) + SO₂(g) + 2H₂O(l). Non‑ sing behaviour because the oxidising species is molecular H₂SO₄, not the dissociated ions.
热浓硫酸是中等强度氧化剂，自身被还原为SO₂。它能氧化活动性在氢以下的金属，如铜：Cu(s) + 2H₂SO₄(l) → CuSO₄(aq) + SO₂(g) + 2H₂O(l)。碳、硫等非金属也能被氧化：C(s) + 2H₂SO₄(l) → CO₂(g) + 2SO₂(g) + 2H₂O(l)。稀硫酸没有这种氧化性，因为氧化性物种是H₂SO₄分子，而不是解离的离子。
9. Reactions with Halide Salts | 与卤化物盐的反应
reacts with solid halides, producing hydrogen halides. With sodium chloride, only an acid– and are oxidised by concentrated sulfuric acid. For bromide: NaBr(s) + H₂SO₄(l) → NaHSO₄(s) + HBr(g), but the HBr is then oxidised: 2HBr(g) + H₂SO₄(l) → Br₂(g) + SO₂(g) + 2H₂O(l). With iodide, the oxidation can proceed further, producing I₂, SO₂ and even H₂S, showing the increasing reducing power down Group 17.
浓硫酸与固态卤化物反应生成卤化氢。与氯化钠只发生酸碱反应，放出氯化氢气体：NaCl(s) + H₂SO₄(l) → NaHSO₄(s) + HCl(g)。溴离子和碘离子是更强的还原剂，会被浓硫酸氧化。溴化物：NaBr(s) + H₂SO₄(l) → NaHSO₄(s) + HBr(g)，随后HBr被氧化：2HBr(g) + H₂SO₄(l) → Br₂(g) + SO₂(g) + 2H₂O(l)。碘化物则可能进一步氧化，生成I₂、SO₂甚至H₂S，体现了第17族元素还原性从上到下增强的趋势。
10. Key Reactions Summary | 关键反应总结
| Reaction type | 反应类型 | Example equation | 示例方程式 |
|---|---|
| Acid + base | 酸碱中和 | H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O |
| Acid + metal (dilute) | 稀酸与活泼金属 | Mg + H₂SO₄ → MgSO₄ + H₂ |
| Dehydration of sucrose | 蔗糖脱水 | C₁₂H₂₂O₁₁ → 12C + 11H₂O |
| Oxidation of copper (conc.) | 浓酸氧化铜 | Cu + 2H₂SO₄ → CuSO₄ + SO₂ + 2H₂O |
| Halide: chloride | 氯化物反应 | NaCl + H₂SO₄ → NaHSO₄ + HCl |
| Halide: bromide | 溴化物反应 | NaBr + H₂SO₄ → NaHSO₄ + HBr; 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O |
| Halide: iodide | 碘化物反应 | NaI + H₂SO₄ → NaHSO₄ + HI; 8HI + H₂SO₄ → 4I₂ + H₂S + 4H₂O |
This table summarises the essential equations for A-level examination preparation.
本表总结了A-level考试必备的核心方程式。
11. Industrial and Laboratory Uses | 工业与实验室用途
Approximately 60% of sulfuric acid output goes into phosphate fertiliser production via the wet process, converting phosphate rock into phosphoric acid. It is also vital in petroleum refining, metal pickling to remove rust, production of synthetic detergents and rayon, and as the electrolyte in lead-acid batteries. In the laboratory, it serves as a standard strong acid, a dehydrating agent for organic nitrations, and a drying agent for gases that do not react with it.
大约60%的硫酸产量用于磷肥生产，通过湿法将磷矿转化为磷酸。在石油精炼、金属酸洗除锈、合成洗涤剂和人造丝制造中也至关重要，并用作铅酸电池的电解液。实验室中，它用作标准强酸、有机硝化反应的脱水剂，以及不与它反应的气体的干燥剂。
12. Safety and Environmental Notes | 安全与环保要点
Concentrated sulfuric acid is highly corrosive and causes severe burns. When diluting, always add acid to water while stirring to disperse the heat. The Contact process is designed to minimise SO₂ emissions by recycling unreacted gases, and modern plants achieve over 99.7% conversion, limiting acid rain precursors. Spent acid from industrial processes is often regenerated or neutralised with lime to prevent environmental damage.
浓硫酸具有强腐蚀性，会造成严重灼伤。稀释时必须将酸加入水中，同时搅拌以分散热量。接触法通过循环未反应气体最大限度地减少SO₂排放，现代工厂实现99.7%以上的转化率，减少了酸雨前体。工业废酸常被再生或用石灰中和，防止对环境的破坏。
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