📚 Experimental Procedures in A-Level Chemistry Unit 5 Insert (Jan20): Synthesis of Tetraamminecopper(II) Sulfate | A-Level 化学 Unit 5 实验操作 (2020年1月插入页): 硫酸四氨合铜(II)的合成
This article unpacks the typical experimental techniques embedded in the AQA A-Level Chemistry Unit 5 Insert for January 2020, using the preparation of tetraamminecopper(II) sulfate as a worked example. You will explore weighing, dissolution, controlled reaction, crystallisation, filtration, drying, yield determination, and purity assessment. Every step is linked to underlying chemical principles, common pitfalls and how to minimise errors in the school laboratory.
本文以制备硫酸四氨合铜(II)为例，详解2020年1月AQA A-Level化学第五单元实验插入页中涉及的典型操作。内容覆盖称量、溶解、控温反应、结晶、过滤、干燥、产率计算与纯度评价，每一步都与化学原理、常见错误以及如何减少实验室误差紧密关联。
1. Safety and Risk Assessment | 安全与风险评估
Before any practical work, a thorough risk assessment is mandatory. Copper(II) sulfate pentahydrate is harmful if swallowed and causes serious eye irritation. Concentrated ammonia solution is corrosive, releases toxic fumes and must be handled in a fume cupboard. Wear eye protection, gloves, and a lab coat at all times. Ethanol used for washing is highly flammable; keep away from naked flames.
任何实验开始前都必须完成详尽的风险评估。五水合硫酸铜(II)吞食有害且对眼睛有严重刺激性。浓氨水具有腐蚀性，会释放有毒蒸气，必须在通风橱中操作。全程佩戴护目镜、手套和实验服。用于洗涤的乙醇高度易燃，须远离明火。
2. Accurate Weighing of Starting Materials | 反应物的精确称量
tahydrate (CuSO₄·5H₂O) and record the exact mass. Calculate the number of moles: n = m / M, where M = 249.7 g mol⁻¹. The precise mass is essential for a reliable percentage yield later.
使用可读至0.01克或0.001克的电子天平。归零一个干净、干燥的称量舟或烧杯。称取约5.0克五水合硫酸铜(II) (CuSO₄·5H₂O)，记录精确质量。按n = m / M计算物质的量，其中M = 249.7 g mol⁻¹。精确质量对后续可靠的产率计算至关重要。
3. Dissolution and Controlled Addition of Ligand | 溶解与配体的受控加入
Transfer the solid to a 150 cm³ beaker and dissolve in about 25 cm³ of deionised water, gently warming if necessary to obtain a clear blue solution. Never boil. In a fume cupboard, measure 15 cm³ of concentrated ammonia solution (approx. 2 mol dm⁻³) and add it dropwise with continuous stirring. The colour changes from pale blue to deep royal blue as the tetraamminecopper(II) ion, [Cu(NH₃)₄]²⁺, forms:
将固体转移至150 cm³烧杯中，用约25 cm³去离子水溶解，必要时可微热以得到清澈蓝色溶液。切勿煮沸。在通风橱中量取15 cm³浓氨水(约2 mol dm⁻³)，在持续搅拌下逐滴加入。溶液由浅蓝色变为深宝蓝色，生成四氨合铜(II)离子 [Cu(NH₃)₄]²⁺：
Cu²⁺(aq) + 4NH₃(aq) ⇌ [Cu(NH₃)₄]²⁺(aq)
Adding ammonia too quickly can cause localised high pH, leading to precipitation of copper(II) hydroxide. The dropwise addition, while stirring, ensures the complex is the dominant product.
氨水加入过快会导致局部pH过高，析出氢氧化铜沉淀。滴加并搅拌可确保配离子为主要产物。
4. Crystallisation by Solvent Evaporation | 溶剂蒸发结晶
Once addition is complete, filter the deep blue solution to remove any insoluble residues through a fluted filter paper. Transfer the filtrate to a clean evaporating basin. Place the basin in a warm spot (not exceeding 60 °C) or under a gentle stream of air. Slow evaporation over several hours promotes the growth of well-formed crystals of tetraamminecopper(II) sulfate monohydrate, [Cu(NH₃)₄]SO₄·H₂O.
滴加完成后，用折叠滤纸过滤深蓝色溶液，除去不溶性残留。将滤液转移至干净蒸发皿中。将蒸发皿置于温暖处(不超过60 °C)或微微气流下。数小时的缓慢蒸发可促进硫酸四氨合铜(II)一水合物 [Cu(NH₃)₄]SO₄·H₂O 良好晶体的生长。
5. Filtration under Reduced Pressure | 减压过滤
Set up a Buchner funnel, filter flask, and vacuum pump. Place a circle of filter paper moistened with deionised water inside the funnel. Turn on the vacuum and pour the crystal slurry onto the paper. The rapid suction removes most of the mother liquor. Rinse the crystals with two small portions of ice-cold ethanol to displace water without dissolving the product, then leave under suction to air-dry for a few minutes.
搭建布氏漏斗、抽滤瓶和真空泵。将用去离子水润湿的圆形滤纸放入漏斗内。开启真空，将含晶体的浆液倾倒在滤纸上。快速抽吸移除大部分母液。用少量冰冷的乙醇洗涤晶体两次，以置换水而不溶解产物，然后继续抽吸几分钟使其初步干燥。
6. Final Drying and Packaging | 最终干燥与封装
Transfer the damp crystals to a pre-weighed watch glass or petri dish. Place in a desiccator containing silica gel or calcium chloride, or dry in an oven set to no more than 40 °C. Overheating can decompose the complex. Weigh at intervals until a constant mass is achieved, indicating complete drying. Record the final mass of dry product.
将潮湿晶体转移至已称重的表面皿或培养皿中。放入含硅胶或氯化钙的干燥器中，或在不超过40 °C的烘箱中干燥。过热会分解配合物。间隔称量直至恒重，表明完全干燥。记录干燥产物的最终质量。
7. Determining Percentage Yield | 计算产率
Write the balanced equation. Assuming a 1:1 molar ratio between CuSO₄·5H₂O and [Cu(NH₃)₄]SO₄·H₂O, the theoretical yield is:
写出配平方程式。假设CuSO₄·5H₂O与 [Cu(NH₃)₄]SO₄·H₂O 为1:1摩尔比，理论产率为：
M([Cu(NH₃)₄]SO₄·H₂O) = 245.8 g mol⁻¹
Theoretical mass = moles of CuSO₄·5H₂O × 245.8 g mol⁻¹
Percentage yield = (actual mass / theoretical mass) × 100%
Yields above 90% are rarely obtained in school labs; values around 70–85% are typical due to mechanical losses during crystallisation and transfer.
学校实验室很少能得到超过90%的产率；因结晶与转移中的机械损失，通常在70–85%之间。
8. Purity Assessment by Melting Point or Spectrophotometry | 通过熔点或分光光度法评估纯度
A simple melting point determination (or decomposition point) can serve as a first check. Load a small sample into a capillary tube and heat slowly. Impurities broaden the melting range and depress the decomposition temperature. A more reliable method uses UV-Vis spectrophotometry: the absorbance at λₘₐₓ around 590 nm is proportional to [Cu(NH₃)₄]²⁺ concentration. Compare the absorbance of a solution of the product with that of a pure standard.
简单的熔点(或分解点)测定可作为初步检测。取少量样品封入毛细管缓慢加热。杂质会拓宽熔程并降低分解温度。更可靠的方法是利用紫外-可见分光光度法：在约590 nm处的最大吸收波长λₘₐₓ处，吸光度与 [Cu(NH₃)₄]²⁺ 浓度成正比。将产品溶液的吸光度与纯标准品对比。
9. Identifying Major Sources of Error | 识别主要误差来源
Incomplete transfer: crystal residues left on filter paper, funnel or evaporating basin reduce yield.
转移不彻底：滤纸、漏斗或蒸发皿上残留的晶体会降低产率。
Over-drying or thermal decomposition: excessive heat can drive off ammonia ligands, giving a lighter blue product of lower mass.
过度干燥或热分解：高温会逐出氨配体，导致产物颜色变浅、质量减小。
Co-precipitation of Cu(OH)₂: if ammonia is added too rapidly or without stirring, copper hydroxide may contaminate the crystals.
氢氧化铜共沉淀：若加氨水过快或未搅拌，氢氧化铜可能掺杂在晶体中。
Weighing errors: using a balance affected by air currents, or a damp container, introduces random and systematic uncertainties.
称量误差：受空气流动影响的称量，或容器潮湿，都会引入随机和系统不确定度。
10. Suggested Improvements for Reliability | 提高可靠性的改进建议
Use a Gooch crucible with sintered glass instead of filter paper to avoid fibre contamination and mechanical loss.
使用砂芯玻璃坩埚代替滤纸，避免纤维污染和机械损失。
Scratch the inner walls of the crystallisation vessel with a glass rod to induce nucleation and obtain more uniform crystals.
用玻璃棒刮擦结晶容器内壁，以诱导成核并获得更均匀的晶体。
Cool the filtrate in an ice-water bath before filtration to minimise solubility losses.
将滤液在冰水浴中冷却后再过滤，以减小溶解度损失。
Perform the spectrophotometric analysis using a freshly prepared calibration curve from a known standard to improve accuracy.
用已知标准品配制的新鲜校准曲线进行分光光度分析，提高准确度。
Record temperature and humidity conditions; both affect crystallisation rates and drying time.
记录温度和湿度条件；两者均影响结晶速率和干燥时间。
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