Case Study Mastery: Year 12 WJEC Chemistry Practice | WJEC Year 12 化学案例分析实战演练

📚 Case Study Mastery: Year 12 WJEC Chemistry Practice | WJEC Year 12 化学案例分析实战演练

Case study questions are a cornerstone of the WJEC Year 12 Chemistry examination. They challenge you to think like a real chemist, piecing together evidence from different branches of the subject to solve a problem. This article presents an extended case study based on an industrial effluent analysis, guiding you step by step through practical reasoning, quantitative calculations, spectroscopic interpretation and the application of key physical chemistry concepts. Work through the exercises, and you will build the integrated problem-solving skills needed for top marks.

案例分析题是 WJEC Year 12 化学考试的核心内容之一。这些题目让你像真正的化学家一样思考，通过汇集来自不同学科分支的证据来解决问题。本文基于一个工业废水分析的实际案例，引导你一步步完成实践推理、定量计算、光谱解析以及关键物理化学概念的应用。跟随这些练习，你将建立起冲击高分所需的综合解题能力。

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1. Understanding the WJEC Case Study Approach | 理解 WJEC 案例分析题的出题思路

A WJEC case study typically presents a real-world scenario followed by a series of interconnected questions. You are expected to retrieve relevant information, perform calculations, and draw conclusions that link different topics such as organic analysis, energetics, kinetics and equilibria. The key is not just knowing the facts, but being able to use them in an unfamiliar context.

WJEC 案例分析题通常会给出一个真实情境，然后接一串相互关联的问题。你需要提取相关信息、进行计算，并得出联系有机分析、能量学、动力学和平衡等不同模块的结论。关键不仅仅是记住知识点，而是能在陌生的背景下灵活运用它们。

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2. The Scenario: Industrial Effluent Sample | 案例背景：工业废水样品

A sample of acidic wastewater was collected from a chemical plant. Preliminary tests showed it contained an organic diprotic acid (H₂A) and possible sulfate impurities. Your task is to identify the acid, determine its concentration, assess its energy release on neutralisation, investigate the rate of its reaction with magnesium, and estimate its acid dissociation constant.

一份来自化工厂的酸性废水样品被采集。初步测试显示它含有一种有机二元酸（H₂A），并可能含有硫酸盐杂质。你的任务是鉴定这种酸，确定其浓度，评估它中和时的能量释放，研究它与镁反应的速率，并估算它的酸离解常数。

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3. Step 1: Qualitative Observations and Preliminary Tests | 第一步：定性观察与初步试验

The effluent had a sharp odour and a pH of approximately 2.8. A few drops of barium chloride solution were added to a separate sample acidified with dilute hydrochloric acid; a faint white precipitate formed, suggesting the presence of sulfate ions. However, the main component was the organic acid. Flame test showed no unusual metal ions.

废水有刺鼻气味，pH 约为 2.8。在另一份用稀盐酸酸化的样品中加入几滴氯化钡溶液，出现了少量白色沉淀，说明可能有硫酸根离子。然而，主要成分是有机酸。焰色反应未显示特殊的金属离子。

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4. Step 2: Volumetric Analysis – Acid-Base Titration | 第二步：容量分析 – 酸碱滴定

A 25.0 cm³ portion of the effluent was titrated with 0.100 mol dm⁻³ sodium hydroxide solution using phenolphthalein indicator. The following results were obtained:

移取 25.0 cm³ 废水，用 0.100 mol dm⁻³ 氢氧化钠溶液滴定，采用酚酞作指示剂，结果如下：

Trial	Initial burette reading (cm³)	Final burette reading (cm³)	Titre (cm³)
1	0.00	23.75	23.75
2	0.05	23.80	23.75
3	0.10	23.85	23.75

The consistent titre is 23.75 cm³. Since the acid is diprotic, the reaction is H₂A + 2NaOH → Na₂A + 2H₂O. Moles of NaOH = (0.100 mol dm⁻³ × 0.02375 dm³) = 2.375 × 10⁻³ mol. Moles of H₂A in 25.0 cm³ = 2.375 × 10⁻³ / 2 = 1.1875 × 10⁻³ mol. Therefore, the concentration of H₂A in the effluent: c(H₂A) = 1.1875 × 10⁻³ mol / 0.0250 dm³ = 0.0475 mol dm⁻³.

滴定管读数一致性很高，平均滴定体积为 23.75 cm³。由于该酸是二元酸，反应式为 H₂A + 2NaOH → Na₂A + 2H₂O。NaOH 的物质的量 = (0.100 mol dm⁻³ × 0.02375 dm³) = 2.375 × 10⁻³ mol。25.0 cm³ 样品中 H₂A 的物质的量 = 2.375 × 10⁻³ / 2 = 1.1875 × 10⁻³ mol。因此，废水中 H₂A 的浓度：c(H₂A) = 1.1875 × 10⁻³ mol / 0.0250 dm³ = 0.0475 mol dm⁻³。

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5. Step 3: Determining Empirical and Molecular Formula | 第三步：确定实验式和分子式

A 2.50 g sample of the pure organic acid (isolated from the effluent) was completely burnt in oxygen. The products were collected: 3.73 g of CO₂ and 1.14 g of H₂O. Mass of C = 3.73 × (12.0/44.0) = 1.017 g. Mass of H = 1.14 × (2.0/18.0) = 0.1267 g. Mass of O = 2.50 – 1.017 – 0.1267 = 1.356 g. Moles: C = 1.017/12.0 = 0.0848; H = 0.1267/1.0 = 0.1267; O = 1.356/16.0 = 0.0848. Dividing by the smallest (0.0848) gives C:H:O = 1:1.5:1, which simplifies to 2:3:2. Empirical formula = C₂H₃O₂, with a formula mass of 59.0.

从废水中分离出的纯有机酸样品 2.50 g 在氧气中完全燃烧。收集到的产物为：3.73 g CO₂ 和 1.14 g H₂O。碳的质量 = 3.73 × (12.0/44.0) = 1.017 g；氢的质量 = 1.14 × (2.0/18.0) = 0.1267 g；氧的质量 = 2.50 – 1.017 – 0.1267 = 1.356 g。物质的量：C = 1.017/12.0 = 0.0848 mol；H = 0.1267/1.0 = 0.1267 mol；O = 1.356/16.0 = 0.0848 mol。除以最小值 0.0848 得 C:H:O = 1:1.5:1，简化为 2:3:2。实验式为 C₂H₃O₂，式量为 59.0。

Mass spectrometry gave a molecular ion peak at m/z = 118. The molecular mass is therefore 118. n = 118 / 59 = 2. The molecular formula is C₄H₆O₄.

质谱分析给出分子离子峰 m/z = 118，因此相对分子质量为 118。n = 118 / 59 = 2。分子式为 C₄H₆O₄。

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6. Step 4: Infrared Spectroscopy and Functional Group Identification | 第四步：红外光谱与官能团鉴定

The infrared spectrum of the isolated acid showed a very broad absorption band around 2500–3300 cm⁻¹, characteristic of the O–H stretch in carboxylic acids. A strong, sharp peak appeared at 1705 cm⁻¹, indicating the presence of a C=O bond. The absence of peaks in the 1600 cm⁻¹ region ruled out alkene or aromatic C=C. These features confirm the compound is a saturated dicarboxylic acid, consistent with butanedioic acid (succinic acid), HOOC–CH₂–CH₂–COOH.

分离出的酸的红外光谱显示在 2500–3300 cm⁻¹ 附近有一个非常宽的吸收带，这是羧酸中 O–H 伸缩振动的特征。同时在 1705 cm⁻¹ 处出现一个强而尖锐的峰，表明存在 C=O 键。在 1600 cm⁻¹ 区域没有峰，排除了烯烃或芳环的 C=C。这些特征证实该化合物是一种饱和二元羧酸，与丁二酸（琥珀酸）HOOC–CH₂–CH₂–COOH 吻合。

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7. Step 5: Energetics – Enthalpy of Neutralisation | 第五步：能量学 – 中和焓

To determine the enthalpy change of neutralisation, 50.0 cm³ of a 0.500 mol dm⁻³ solution of the pure acid was mixed with 50.0 cm³ of 1.00 mol dm⁻³ NaOH in a polystyrene cup. The temperature rose from 21.5 °C to 28.0 °C. The total volume is 100 cm³, assumed to have the density and specific heat capacity of water (4.18 J g⁻¹ K⁻¹). Heat evolved q = 100 g × 4.18 J g⁻¹ K⁻¹ × (28.0 – 21.5) K = 100 × 4.18 × 6.5 = 2717 J.

为测定中和焓变，在聚苯乙烯杯中将 50.0 cm³、0.500 mol dm⁻³ 的纯酸溶液与 50.0 cm³、1.00 mol dm⁻³ 的 NaOH 溶液混合。温度从 21.5 °C 升至 28.0 °C。总体积为 100 cm³，假设其密度和比热容与水相同（4.18 J g⁻¹ K⁻¹）。放出的热量 q = 100 g × 4.18 J g⁻¹ K⁻¹ × (28.0 – 21.5) K = 100 × 4.18 × 6.5 = 2717 J。

Moles of H₂A used = 0.500 mol dm⁻³ × 0.0500 dm³ = 0.0250 mol. The reaction neutralised both protons, so the molar enthalpy of neutralisation (per mole of acid) is ΔH = –2.717 kJ / 0.0250 mol = –108.7 kJ mol⁻¹. This is roughly double the standard enthalpy of neutralisation for a monoprotic strong acid (–57 kJ mol⁻¹), confirming the diprotic nature.

所用 H₂A 的物质的量 = 0.500 mol dm⁻³ × 0.0500 dm³ = 0.0250 mol。该反应中和了两个质子，因此摩尔中和焓（以每摩尔酸计）为 ΔH = –2.717 kJ / 0.0250 mol = –108.7 kJ mol⁻¹。这约为一元强酸标准中和焓（–57 kJ mol⁻¹）的两倍，证实了该酸的二元特性。

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8. Step 6: Kinetics – Order of Reaction with Magnesium | 第六步：动力学 – 与镁反应的级数

The rate of reaction between the acid and magnesium ribbon was studied by measuring the volume of hydrogen gas produced over time. With a large excess of magnesium, the initial rates were determined for different concentrations of H₂A. The data obtained are shown below:

通过测量氢气体积随时间的变化，研究了该酸与镁条反应的速率。在镁大量过量的条件下，测定了不同 H₂A 浓度下的初始速率。所得数据如下：

[H₂A] (mol dm⁻³)	Initial rate (cm³ H₂ min⁻¹)
0.0475	5.40
0.0238	2.70
0.0119	1.35

Halving the acid concentration halves the initial rate, which indicates a first-order dependence with respect to H₂A. The reaction can be written as Mg(s) + 2HA⁻¹ (aq) + 2H⁺ (aq) → Mg²⁺ (aq) + H₂A (aq) + H₂ (g) ; however, the kinetics shows that the overall process is controlled by the supply of protons from the weak acid, leading to rate = k [H₂A].

将酸浓度减半，初始速率也减半，这表明该反应对 H₂A 是一级反应。反应可写作 Mg(s) + 2HA⁻¹ (aq) + 2H⁺ (aq) → Mg²⁺ (aq) + H₂A (aq) + H₂ (g)；然而动力学显示总过程受弱酸提供质子的速率控制，速率方程可表达为 rate = k [H₂A]。

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9. Step 7: Equilibrium – Determining the Acid Dissociation Constant | 第七步：平衡 – 测定酸离解常数

The pH of the 0.0475 mol dm⁻³ acid solution was measured as 2.85. The first dissociation is: H₂A ⇌ H⁺ + HA⁻. At equilibrium, [H⁺] = 10⁻²·⁸⁵ = 1.41 ×

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