📚 Case Study Practice: Applying Chemistry to Real-World Problems | 案例分析实战演练：化学在现实问题中的应用
Case studies are a powerful way to connect the dots between isolated chemical concepts and the messy, real-life challenges that scientists and engineers face every day. This article walks you through a detailed environmental chemistry case, breaking down each step from identifying the problem to proposing chemical solutions and performing calculations. By working through this example, you will sharpen your analytical skills and see how topics like combustion, acid–base reactions, stoichiometry and green chemistry come together.
案例分析是把零散的化学概念和现实中复杂的挑战串联起来的有效方法。本文带你拆解一个详细的环境化学案例，从找出问题到提出化学解决方案、完成计算，每一个步骤都会深入分析。通过这个例子，你将锻炼分析能力，看到燃烧反应、酸碱反应、化学计量和绿色化学等知识点如何在实际中协同作用。
1. The Scenario: Coal-fired Power Plant | 情景：燃煤电厂
A coal-fired power station near a farming region has been linked to declining crop yields and corrosion of nearby metal structures. Local environmental officers suspect that sulfur dioxide (SO₂) emissions from the combustion of sulfur-containing coal are the primary cause. The coal used has an average sulfur content of 2 % by mass, and the plant burns 100 tonnes of coal per day.
一座靠近农业区的燃煤发电站被怀疑与农作物减产以及附近金属结构腐蚀有关。当地环境官员推测，主要原因是含硫煤燃烧排放的二氧化硫（SO₂）。该电厂所用燃煤的平均硫含量为 2 %（质量分数），每天燃烧 100 吨煤。
2. Combustion of Sulfur in Coal | 煤中硫的燃烧
When coal is burned, sulfur present in the coal reacts with oxygen from the air to produce sulfur dioxide. This is a simple combustion reaction that can be represented by the word equation: sulfur + oxygen → sulfur dioxide. The balanced symbol equation is S(s) + O₂(g) → SO₂(g). The reaction is highly exothermic and releases large amounts of heat energy alongside the pollutant gas.
当煤燃烧时，煤中的硫与空气中的氧气反应生成二氧化硫。这是一个简单的燃烧反应，文字表达式为：硫 + 氧气 → 二氧化硫。配平后的符号方程式为 S(s) + O₂(g) → SO₂(g)。该反应剧烈放热，在释放大量热能的同时也产生污染气体。
3. Formation of Acid Rain | 酸雨的形成
Once released into the atmosphere, sulfur dioxide undergoes further oxidation and reacts with water vapour. First, SO₂ is slowly oxidised to sulfur trioxide (SO₃) in the presence of dust particles or nitrogen oxides that act as catalysts. The SO₃ then dissolves in rainwater to form sulfuric acid (H₂SO₄). Even without full oxidation, SO₂ itself can react with water to produce sulfurous acid (H₂SO₃), contributing to acid rain.
二氧化硫排放到大气中后，会进一步氧化并与水蒸气反应。首先，SO₂ 在粉尘颗粒或氮氧化物等催化作用下缓慢氧化为三氧化硫（SO₃）。随后 SO₃ 溶于雨水生成硫酸（H₂SO₄）。即使没有完全氧化，SO₂ 本身也能与水反应生成亚硫酸（H₂SO₃），从而加剧酸雨。
4. Environmental Impact of Acid Rain | 酸雨的环境影响
Acid rain has a pH well below the normal 5.6 of unpolluted rain, typically ranging between 4.2 and 4.6 in affected areas. The increased acidity lowers the pH of soil and water bodies, leaching away essential nutrients like calcium and magnesium and mobilising toxic aluminium ions. Crop roots are damaged, and freshwater ecosystems suffer as fish eggs and aquatic insects cannot survive. In addition, acid rain accelerates the corrosion of limestone buildings and metal structures, especially iron and steel.
酸雨的 pH 值远低于未受污染雨水正常的 5.6，受影响的地区通常在 4.2 至 4.6 之间。增强的酸性降低了土壤和水体的 pH 值，洗去钙、镁等重要养分，并使有毒的铝离子活化。作物根部受损，鱼卵和水生昆虫无法存活，淡水生态系统受到破坏。此外，酸雨会加速石灰石建筑和金属结构（尤其是钢铁）的腐蚀。
5. Measuring Air Pollution | 测量空气污染
Scientists can quantify the SO₂ concentration in air using methods such as gas syringe sampling followed by titration, or by using passive diffusion tubes. A common laboratory simulation involves bubbling a known volume of air through acidified potassium dichromate(VI) solution, where SO₂ reduces orange dichromate(VI) ions to green chromium(III) ions. The colour change can be compared with standards to estimate SO₂ levels.
科学家可以通过气体注射器采样后滴定，或将空气通过酸性重铬酸钾（VI）溶液的方法来定量空气中的 SO₂ 浓度，二氧化硫会将橙色的重铬酸根（VI）离子还原为绿色的铬（III）离子。通过比色与标准溶液对比，可以估算 SO₂ 含量。
6. Chemical Method: Flue Gas Desulfurisation | 化学方法：烟气脱硫
The most widely adopted industrial solution is flue gas desulfurisation (FGD), often using wet scrubbing with limestone (CaCO₃). In this process, the SO₂-containing flue gas is passed through a slurry of limestone and water. The acidic SO₂ reacts with the alkaline calcium carbonate, forming solid calcium sulfite (CaSO₃), which can be further oxidised to calcium sulfate (CaSO₄, gypsum). The cleaned gas is then released into the atmosphere, and the gypsum can be used in construction.
目前工业上最广泛采用的解决方案是烟气脱硫（FGD），通常使用石灰石（CaCO₃）湿法洗涤。在此过程中，含有 SO₂ 的烟气通过石灰石与水的浆液，酸性的 SO₂ 与碱性的碳酸钙反应，生成固态亚硫酸钙（CaSO₃），亚硫酸钙可进一步氧化为硫酸钙（CaSO₄，石膏）。净化后的气体排入大气，石膏则可用于建筑行业。
7. Reactions in the Scrubber | 洗涤塔中的反应
The overall chemistry in the scrubber can be simplified into two steps. First, sulfur dioxide reacts with water and calcium carbonate to form calcium sulfite and carbon dioxide. A balanced equation for the initial scrubbing is: CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g). In a second oxidation step, air is blown through the slurry to convert calcium sulfite to calcium sulfate: 2CaSO₃(s) + O₂(g) → 2CaSO₄(s). This prevents the decomposition of sulfite back to SO₂ at high temperatures.
洗涤塔中的总化学过程可以简化为两步。首先，二氧化硫与水、碳酸钙反应生成亚硫酸钙和二氧化碳。初始洗涤的配平方程式为：CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g)。第二步是氧化，向浆液中鼓入空气将亚硫酸钙转化为硫酸钙：2CaSO₃(s) + O₂(g) → 2CaSO₄(s)。这样可以避免亚硫酸钙在高温下分解回 SO₂。
8. Stoichiometry: Calculating Limestone Required | 化学计量：计算所需石灰石
Using the balanced equation CaCO₃ + SO₂ → CaSO₃ + CO₂, we can calculate the minimum mass of limestone needed per day. The plant burns 100 tonnes of coal containing 2 % sulfur, so the mass of sulfur burned daily is 100 × 0.02 = 2.0 tonnes. The molar mass of S is 32 g mol⁻¹, and the molar mass of CaCO₃ is 100 g mol⁻¹. The mole ratio is 1 : 1. Therefore, the mass of CaCO₃ required = (2.0 × 10⁶ g S ÷ 32) × 100 = 6.25 × 10⁶ g, or 6.25 tonnes. In practice, an excess is used to maximise removal efficiency.
利用配平的方程式 CaCO₃ + SO₂ → CaSO₃ + CO₂，我们可以计算出每天所需石灰石的最低质量。电厂每日燃烧 100 吨煤，含硫 2 %，因此每日燃烧的硫质量为 100 × 0.02 = 2.0 吨。S 的摩尔质量为 32 g mol⁻¹，CaCO₃ 的摩尔质量为 100 g mol⁻¹，物质的量之比为 1 : 1。所以所需 CaCO₃ 质量 = (2.0 × 10⁶ g S ÷ 32) × 100 = 6.25 × 10⁶ g，即 6.25 吨。实际生产中会使用过量石灰石以达到最大脱除效率。
9. Alternative Solutions: Using Low-sulfur Coal | 替代方案：使用低硫煤
Another strategy is to switch to low-sulfur coal or wash the coal before combustion to remove pyrite (FeS₂). Physical cleaning methods such as froth flotation separate sulfur-rich mineral particles from the coal. While this reduces SO₂ at the source, it is often not enough to meet strict emission limits, so it is combined with FGD. Renewable energy sources, such as solar or wind, eliminate the problem entirely but require infrastructure changes and are studied in the context of green chemistry.
另一种策略是改用低硫煤或在燃烧前通过洗煤除去黄铁矿（FeS₂）。泡沫浮选等物理净化方法能将富硫矿物颗粒从煤中分离出来。虽然这样可以从源头减少 SO₂，但通常不足以满足严格的排放限值，因此需要与烟气脱硫结合使用。太阳能、风能等可再生能源可以完全消除这一问题，但需要基础设施的变革，并需从绿色化学的角度加以审视。
10. Evaluation and Further Questions | 评估与延伸问题
When evaluating solutions, you need to consider cost, efficiency, waste products and energy consumption. For example, FGD produces gypsum, which can be sold, but the process requires water and electricity. Questions to ask yourself: What would happen if the limestone slurry became too acidic? (Hint: reaction rate decreases.) How could you test the purity of the gypsum produced? (Use acid test for carbonates.) What impact does burning biomass with coal have on sulfur emissions? This systematic approach of asking ‘why’ and ‘what if’ is the essence of case study practice.
评估方案时，需要综合考虑成本、效率、废弃物和能源消耗。例如，烟气脱硫产生可销售的石膏，但过程消耗水和电。可以问自己：如果石灰石浆液酸度过高会发生什么？（提示：反应速率降低。）如何检测所产石膏的纯度？（用酸检验碳酸盐。）生物质与煤混烧对硫排放有何影响？这种不断追问“为什么”和“如果……会怎样”的系统方法正是案例分析实战的精髓。
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