📚 Case Study in AQA Year 13 Chemistry: Practical Exercises | 实战演练 AQA 化学案例分析
This case study leads you through the synthesis, purification, and analysis of aspirin, a classic AQA Year 13 practical scenario. It integrates organic chemistry, kinetics, equilibrium, and analytical techniques, exactly as you will encounter in Paper 3 and the required practicals. By working step‑by‑step through the decision‑making and calculations, you will sharpen the skills needed to handle unfamiliar contexts in the examination.
本案例分析将带你完整走一遍阿司匹林的合成、提纯与分析过程，这是 AQA 13 年级经典的实验情境。它融合了有机化学、动力学、平衡和分析技术，与你在卷三和必修实验中将遇到的完全一致。通过逐步参与决策和计算，你将提升处理考试中陌生情境所需的能力。
1. Background and Context of the Case Study | 案例研究的背景与情境
Aspirin (acetylsalicylic acid) can be prepared by the esterification of salicylic acid with ethanoic anhydride. In a typical school laboratory, the reaction is catalysed by a few drops of concentrated phosphoric acid. The crude product is often contaminated with unreacted salicylic acid and the by‑product ethanoic acid, which must be removed via recrystallisation. Understanding the underlying equilibrium and the choice of reagents is a key AQA assessment objective.
阿司匹林（乙酰水杨酸）可通过水杨酸与乙酸酐的酯化反应制得。在典型的学校实验室里，反应由几滴浓磷酸催化。粗产物常夹杂未反应的水杨酸和副产物乙酸，必须通过重结晶除去。理解该过程背后的平衡以及试剂的选择是AQA考试的重要评估目标。
2. Experimental Procedure and Initial Observations | 实验步骤与初步观察
Weigh 2.0 g of salicylic acid into a 50 cm³ conical flask. Add 5 cm³ of ethanoic anhydride and 3 drops of 85 % phosphoric acid. Warm the mixture on a water bath at 60 °C for 20 minutes, stirring occasionally. After cooling, pour the contents into 75 cm³ of ice‑cold water. White crystals of aspirin form rapidly. Record the mass of the crude dry product after filtration.
称取 2.0 g 水杨酸置于 50 cm³ 锥形瓶中。加入 5 cm³ 乙酸酐和 3 滴 85 % 磷酸。将混合物在水浴上于 60 °C 加热 20 分钟，偶尔搅拌。冷却后将内容物倒入 75 cm³ 冰水中。阿司匹林白色晶体迅速析出。过滤后记录粗品干燥质量。
3. Mechanism and Stoichiometry | 机理与化学计量
The reaction is an acid‑catalysed nucleophilic acyl substitution. The –OH group of salicylic acid attacks the carbonyl carbon of ethanoic anhydride, forming a tetrahedral intermediate that collapses to release ethanoic acid. The overall equation is:
C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH
该反应是酸催化的亲核酰基取代。水杨酸的 –OH 基团进攻乙酸酐的羰基碳，形成一个四面体中间体，后者分解并释放出一分子乙酸。总方程式为：
C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH
Molar masses: salicylic acid 138.12 g mol⁻¹, ethanoic anhydride 102.09 g mol⁻¹, aspirin 180.16 g mol⁻¹. In this experiment salicylic acid is the limiting reagent because ethanoic anhydride is used in excess.
摩尔质量：水杨酸 138.12 g mol⁻¹，乙酸酐 102.09 g mol⁻¹，阿司匹林 180.16 g mol⁻¹。本实验中，水杨酸是限制试剂，因为乙酸酐过量使用。
4. Calculation of Theoretical and Percentage Yield | 理论产率与百分产率的计算
Moles of salicylic acid used = 2.0 g ÷ 138.12 g mol⁻¹ = 0.0145 mol. Theoretical moles of aspirin = 0.0145 mol. Theoretical mass = 0.0145 mol × 180.16 g mol⁻¹ = 2.61 g. Suppose the student obtained 2.12 g of crude product. Percentage yield = (2.12 ÷ 2.61) × 100 % = 81.2 %. A yield below 100 % is expected due to mechanical losses, incomplete reaction, and solubility of aspirin in cold water.
使用的水杨酸物质的量 = 2.0 g ÷ 138.12 g mol⁻¹ = 0.0145 mol。阿司匹林的理论物质的量 = 0.0145 mol。理论质量 = 0.0145 mol × 180.16 g mol⁻¹ = 2.61 g。假设学生得到粗产物 2.12 g。百分产率 = (2.12 ÷ 2.61) × 100 % = 81.2 %。产率低于 100 % 是意料之中的，原因包括机械损失、反应不完全以及阿司匹林在冷水中有一定溶解度。
5. Purification by Recrystallisation | 重结晶提纯
The crude product is dissolved in the minimum volume of hot ethanol. Water is then added dropwise until cloudiness persists, followed by cooling in an ice bath. Pure aspirin crystallises out while polar impurities remain dissolved. The crystals are collected by vacuum filtration and dried. This technique exploits the difference in solubility between the desired compound and contaminants.
将粗产物溶于最少量热乙醇中。随后逐滴加入水，直到出现持续的浑浊，然后置于冰浴中冷却。纯阿司匹林结晶析出，而极性杂质则留在溶液中。晶体经抽滤收集并干燥。这一技术利用了目标化合物与杂质在溶解度上的差异。
6. Purity Assessment via Thin‑Layer Chromatography (TLC) | 通过薄层色谱评估纯度
A TLC plate spotted with the recrystallised aspirin, salicylic acid, and a co‑spot is developed in ethyl ethanoate/hexane (1:1). Under UV light, the salicylic acid spot shows a lower Rf (~0.4) due to its polar phenolic –OH, whereas the aspirin spot appears higher (Rf ~0.6). A single spot for the recrystallised product confirms purity; any salicylic acid contamination would give an additional spot.
将重结晶的阿司匹林、水杨酸以及共同点样的薄层板在乙酸乙酯/己烷（1:1）中展开。在紫外灯下，水杨酸斑点因其极性的酚羟基而呈现较低 Rf（约 0.4），而阿司匹林斑点则较高（Rf 约 0.6）。重结晶产物若只有一个斑点，则证明纯度合格；任何水杨酸的污染都会出现额外的斑点。
7. Melting Point Determination | 熔点测定
The literature melting point of aspirin is 138–140 °C. After recrystallisation, the product melts sharply at 139–140 °C, whereas the crude sample might melt over a broader range (130–135 °C). A narrow melting point range, close to the literature value, indicates high purity. Depression and widening of the melting point range are classic signs of impurities.
阿司匹林的文献熔点为 138–140 °C。重结晶后，产物在 139–140 °C 敏锐熔融，而粗样品可能在一个较宽范围（130–135 °C）内熔化。窄的熔程且接近文献值表明纯度高。熔点降低及熔程变宽是存在杂质的典型标志。
8. Spectroscopic Analysis – Infrared Spectroscopy | 光谱分析——红外光谱
The IR spectrum of pure aspirin shows a broad O–H stretch at 2500–3300 cm⁻¹ (carboxylic acid), a sharp C=O stretch at 1680–1750 cm⁻¹ (two overlapping peaks for ester and acid carbonyls), and C–O stretches near 1200–1300 cm⁻¹. The absence of a broad peak around 3300–3500 cm⁻¹ from the phenolic O–H of salicylic acid confirms successful acetylation. Comparing spectra before and after recrystallisation helps assess purity.
纯阿司匹林的红外光谱显示：2500–3300 cm⁻¹ 处的宽 O–H 伸缩振动峰（羧酸），1680–1750 cm⁻¹ 处的尖锐 C=O 伸缩峰（酯和酸羰基的两个重叠峰），以及 1200–1300 cm⁻¹ 附近的 C–O 伸缩振动峰。若没有水杨酸酚 O–H 在 3300–3500 cm⁻¹ 的宽吸收峰，则证明乙酰化完全。比较重结晶前后的光谱有助于评估纯度。
9. Quantitative Analysis – Back Titration | 定量分析——返滴定
A known excess of standard sodium hydroxide is added to a weighed sample of aspirin. The ester group is hydrolysed, and the excess alkali is titrated with hydrochloric acid. The amount of aspirin is deduced from the difference. This back titration method overcomes the slow reaction of the ester and the poor solubility of aspirin in water.
向已称重的阿司匹林样品中加入已知过量的标准氢氧化钠溶液。酯基被水解，然后用盐酸滴定剩余的碱。阿司匹林的量由差值推算。这种返滴定法克服了酯水解反应较慢以及阿司匹林在水中溶解度差的问题。
moles aspirin = (initial moles NaOH – moles HCl) ÷ 2
阿司匹林物质的量 = (NaOH 初始物质的量 – HCl 物质的量) ÷ 2
This calculation assumes the two carboxyl‑derived acid groups from the hydrolysed aspirin both consume hydroxide in a 1:2 stoichiometry. The titration result can be compared with the theoretical mass to evaluate purity.
该计算基于水解后的阿司匹林含有两个源于羧基的酸基团，二者均按 1:2 化学计量消耗氢氧根离子。滴定结果可与理论质量比较，从而评价纯度。
10. Error Analysis and Improvements | 误差分析与改进
Common errors include overheating during the reaction, which can cause decomposition of the product, loss of product during recrystallisation due to hot filtration, and inaccurate measurement of melting point. The percentage yield can be improved by ensuring the reaction goes to completion (extended heating), cooling thoroughly during crystallisation, and washing crystals with a minimal amount of ice‑cold water.
常见误差包括反应过程中过热导致产物分解，热过滤时在重结晶过程中损失产物，以及熔点测量不准确。通过确保反应进行完全（延长加热时间）、在结晶时充分冷却、用最少量的冰冷水洗涤晶体，可以提高百分产率。
11. Application to AQA Exam‑Style Questions | 应对AQA考试风格的题目
In Paper 3, you might be asked to plan an investigation, interpret unfamiliar data, or evaluate the validity of a method. Use this case to practise writing clear, logical sequences for purification steps, justifying the choice of apparatus, and linking observations to underlying chemistry. Always refer back to core principles: green chemistry (atom economy), equilibrium position in esterifications, and the precision of measurement.
在卷三中，你可能需要设计一项探究、解读陌生数据或者评价方法的有效性。利用本案例来练习写出清晰、有逻辑的提纯步骤顺序，论证实验仪器的选择，并将观察现象与背后的化学原理联系起来。始终回顾核心原则：绿色化学（原子经济性）、酯化反应的平衡位置以及测量的精密度。
12. Conclusion and Take‑Home Messages | 总结与要点
This case study demonstrates how the synthesis and analysis of aspirin pull together multiple AQA Year 13 topics: organic synthesis, recrystallisation, TLC, melting point, IR spectroscopy, and volumetric calculations. Successful answers in the exam come from combining practical know‑how with theoretical depth. Practise similar case studies using other compounds, such as the preparation of paracetamol or the hydrolysis of esters, to build confidence in tackling practical‑based questions.
本案例展示了阿司匹林的合成与分析如何串联起 AQA 13 年级的多个主题：有机合成、重结晶、薄层色谱、熔点、红外光谱和容量分析计算。要在考试中成功作答，必须将实验技能与理论深度结合起来。运用其他化合物的类似案例进行练习，例如对乙酰氨基酚的制备或酯的水解，从而增强应对实验类题目的信心。
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