📚 A-Level Chemistry Unit 5 June 2022 Paper: Experimental Techniques | A-Level 化学 Unit 5 2022年6月试卷：实验操作
The June 2022 Unit 5 paper for A-Level Chemistry challenged students with a range of practical scenarios, from the preparation of transition metal complexes to redox titrations and organic synthesis techniques. Mastery of these experimental operations is not only essential for the exam but also for building a solid foundation in practical chemistry. This article revisits the key experimental skills and operations likely featured in that paper, providing bilingual explanations to help you refine your technique and understanding.
2022年6月的A-Level化学Unit 5试卷通过一系列实际场景考查了学生，内容包括过渡金属配合物的制备、氧化还原滴定和有机合成技术。掌握这些实验操作不仅对考试至关重要，也为扎实的化学实践打下基础。本文回顾了该试卷中可能出现的关键实验技能和操作，并以中英双语提供解释，助你完善技巧与理解。
1. Safety and Handling of Transition Metal Compounds | 过渡金属化合物的安全操作
Transition metal salts, such as copper(II) sulphate, iron(II) ammonium sulphate, and cobalt(II) chloride, can be toxic, irritant, or corrosive. Always wear gloves, a lab coat, and safety goggles. Work in a well-ventilated area because some solutions release harmful vapours.
过渡金属盐，如硫酸铜、硫酸亚铁铵和氯化钴，可能有毒、具刺激性或腐蚀性。务必佩戴手套、实验服和护目镜。在通风良好的区域操作，因为某些溶液会释放有害蒸气。
When weighing these solids, use a balance with a draft shield to prevent inhalation of fine powders. Any spills must be cleaned immediately using appropriate methods—for example, neutralising acid spills with sodium hydrogen carbonate.
称量这些固体时，使用带挡风罩的天平以避免吸入细粉。任何泼溅必须立即用适当方法清理——例如，用碳酸氢钠中和酸性泼溅物。
Copper(II) sulphate pentahydrate: CuSO₄·5H₂O
硫酸铜五水合物：CuSO₄·5H₂O
2. Preparation of a Copper(II) Ammine Complex | 铜(II)氨配合物的制备
One classic experiment is the preparation of the deep blue tetraamminecopper(II) ion. Add dilute ammonia solution dropwise to copper(II) sulphate solution. At first, a pale blue precipitate of copper(II) hydroxide, Cu(OH)₂, forms. Upon further addition of excess ammonia, the precipitate dissolves, forming [Cu(NH₃)₄]²⁺.
一个经典实验是制备深蓝色的四氨合铜(II)离子。向硫酸铜溶液中逐滴加入稀氨水。最初生成淡蓝色的氢氧化铜沉淀，Cu(OH)₂。继续加入过量氨水，沉淀溶解，生成[Cu(NH₃)₄]²⁺。
Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)
Cu(OH)₂(s) + 4NH₃(aq) → [Cu(NH₃)₄]²⁺(aq) + 2OH⁻(aq)
During the procedure, constant stirring is essential to prevent localised high concentrations that could form unwanted side products. The intensity of the blue colour depends on complex concentration, making it a good candidate for colorimetry.
操作过程中，不断搅拌必不可少，以防止局部浓度过高而形成不需要的副产物。蓝色的深浅取决于配合物的浓度，因此很适合用比色法分析。
3. Setting Up a Redox Titration – Iron(II) vs. Dichromate(VI) | 氧化还原滴定的建立 – 铁(II)与重铬酸根(VI)
Redox titrations often appear in Unit 5. In a typical procedure, a standard solution of potassium dichromate(VI) is used to determine the concentration of an iron(II) salt. The reaction is carried out in an acidic medium, usually dilute sulphuric acid.
氧化还原滴定经常出现在Unit 5中。一个典型步骤是用标准重铬酸钾溶液测定铁(II)盐的浓度。反应在酸性介质中进行，通常为稀硫酸。
Cr₂O₇²⁻(aq) + 14H⁺(aq) + 6Fe²⁺(aq) → 2Cr³⁺(aq) + 6Fe³⁺(aq) + 7H₂O(l)
Use a burette to dispense the dichromate solution. The iron(II) solution is pipetted into a conical flask along with excess dilute H₂SO₄ and a few drops of a redox indicator, such as sodium diphenylamine-4-sulphonate, which turns purple at the endpoint.
用滴定管盛装重铬酸盐溶液。用移液管移取铁(II)溶液至锥形瓶中，加入过量稀硫酸和几滴氧化还原指示剂，如二苯胺-4-磺酸钠，在终点时变为紫色。
The solution must be swirled continuously. The endpoint approaches when a transient colour change persists longer before disappearing; then add dichromate drop by drop. Record the titre to the nearest 0.05 cm³.
溶液必须持续摇动。当暂时的颜色变化在消失前能持续更久时，表明接近终点；然后逐滴滴加重铬酸盐。记录滴定管读数，精确到0.05 cm³。
4. Preparation of a Standard Solution | 标准溶液的配制
A standard solution of iron(II) ammonium sulphate (Mohr's salt, (NH₄)₂Fe(SO₄)₂·6H₂O) is often prepared. Weigh the solid accurately on a balance, transfer it quantitatively to a volumetric flask, dissolve in distilled water, and make up to the mark.
经常需要配制硫酸亚铁铵（莫尔盐，(NH₄)₂Fe(SO₄)₂·6H₂O）的标准溶液。用天平准确称量固体，定量转移至容量瓶中，加蒸馏水溶解，然后定容至刻度线。
Rinse the weighing boat with distilled water and transfer the washings to the flask to ensure all solid is transferred. Invert the flask several times to ensure homogeneity. Label the flask with the concentration and name.
用蒸馏水冲洗称量舟，将洗涤液转移至容量瓶中，确保全部固体都已转移。反复倒置容量瓶数次以保证均匀。在瓶上标明浓度和名称。
c = n / V (moles = mass / Mᵣ)
浓度 = 物质的量 / 体积 (物质的量 = 质量 / 摩尔质量)
5. Use of Antimony Electrode for pH Measurement | 锑电极测定pH
Some Unit 5 papers include non-conventional pH measurement, such as the antimony/antimony oxide electrode. The electrode is placed in the test solution, and the potential is measured against a reference. The potential varies linearly with pH according to the Nernst equation.
一些Unit 5试卷涉及非常规pH测量，例如锑/氧化锑电极。将电极放入待测溶液中，相对参比测量电位。根据能斯特方程，电位与pH呈线性关系。
Before use, the antimony electrode must be polished lightly with fine sandpaper to remove the oxide layer, then conditioned in a buffer. During measurement, the solution should be stirred and the temperature kept constant.
使用前，需用细砂纸轻磨锑电极以除去氧化层，然后在缓冲液中活化。测量时，溶液需搅拌并保持温度恒定。
6. Reflux Technique for Organic Nitrogen Chemistry | 含氮有机物合成的回流操作
Synthesis of phenylamine from nitrobenzene or preparation of a diazonium salt requires careful temperature control. A reflux apparatus is used: a round-bottom flask, condenser, and heating mantle. Water flows through the condenser to prevent vapour loss.
从硝基苯合成苯胺或制备重氮盐需要精确控温。使用回流装置：圆底烧瓶、冷凝器和加热套。冷凝器中通水以防止蒸气损失。
For the reduction of nitrobenzene, tin and concentrated hydrochloric acid are heated under reflux. After cooling, the mixture is poured onto ice with excess sodium hydroxide to liberate phenylamine, which is separated by steam distillation.
硝基苯还原时，将锡和浓盐酸在回流下加热。冷却后，将混合物倒入加有冰和过量氢氧化钠的烧杯中，释放苯胺，再通过水蒸气蒸馏分离。
Always add anti-bumping granules to ensure smooth boiling. The condenser should be clamped securely. Never stopper the top of the condenser – it must be open to the atmosphere to prevent pressure build-up.
始终加入沸石以确保沸腾平稳。冷凝器应可靠夹紧。切勿塞住冷凝器顶端——必须与大气相通，防止压力积聚。
7. Purification by Recrystallisation | 重结晶提纯
After synthesis, the crude product is purified by recrystallisation. Dissolve the solid in the minimum volume of hot solvent (often water or ethanol) to form a saturated solution. Filter while hot to remove insoluble impurities.
合成后，粗品通过重结晶提纯。将固体溶解在最少量的热溶剂中（通常是水或乙醇），形成饱和溶液。趁热过滤除去不溶性杂质。
Allow the filtrate to cool slowly. Crystals form as solubility decreases. Cooling in an ice bath improves yield. Collect crystals by vacuum filtration using a Büchner funnel and wash with a small amount of cold solvent.
让滤液缓慢冷却。随着溶解度降低，晶体析出。在冰浴中冷却可提高产率。用布氏漏斗真空抽滤收集晶体，并用少量冷溶剂洗涤。
Check purity by melting point; a narrow melting range close to the literature value indicates purity. Dry the crystals in a desiccator or warm oven.
通过熔点检验纯度；熔点范围窄且接近文献值表示纯度高。在干燥器或温暖烘箱中干燥晶体。
8. Thin-Layer Chromatography for Reaction Monitoring | 薄层色谱监测反应
TLC is used to follow the progress of a reaction, for example, in the preparation of an azo dye. Spot the reaction mixture and starting materials on a silica plate, develop in a suitable solvent, and visualise under UV light.
TLC用于跟踪反应进程，例如在偶氮染料制备中。在硅胶板上点样反应混合物和起始原料，在合适溶剂中展开，在紫外灯下观察。
Calculate the Rf value: distance moved by the spot divided by distance moved by the solvent front. Comparing spots allows you to see whether the starting material is consumed and a new product appears.
计算比移值Rf：斑点移动距离除以溶剂前沿移动距离。比较斑点可看出起始原料是否消耗殆尽以及新产物是否出现。
Ensure the baseline is above the solvent level. Use a capillary tube for spotting, and allow spots to dry before eluting. Cover the developing chamber to maintain a saturated atmosphere.
确保基线高于溶剂液面。用毛细管点样，点样后晾干再进行展开。盖上展开缸盖子以保持饱和气氛。
9. Colorimetry to Determine Complex Concentration | 比色法测定配合物浓度
Colorimetry is a common technique in Unit 5. For the [Cu(NH₃)₄]²⁺ complex, prepare a series of standard solutions, measure absorbance at the λmax (around 600 nm), and construct a calibration curve of absorbance vs. concentration.
比色法是Unit 5的常见技术。对于[Cu(NH₃)₄]²⁺配合物，配制一系列标准溶液，在最大吸收波长λmax（约600 nm）处测量吸光度，绘制吸光度-浓度校正曲线。
Use a blank containing all reagents except the complex to zero the colorimeter. Plot a graph of absorbance (y-axis) against concentration (x-axis); it should be a straight line through the origin if Beer-Lambert law is obeyed.
用含有除配合物外所有试剂的空白溶液给比色计调零。以吸光度（y轴）对浓度（x轴）作图；若遵循比尔-朗伯定律，应为一条过原点的直线。
From the calibration curve, determine the unknown concentration. Always handle cuvettes by the frosted sides and wipe the clear sides with lens tissue.
根据校正曲线确定未知浓度。拿取比色皿时只接触磨砂面，并用擦镜纸擦拭透明面。
10. Handling and Disposal of Chemical Waste | 化学废物的处理与处置
Proper waste disposal is integral to any practical assessment. Heavy metal solutions, such as those containing copper or chromium ions, must be collected in labelled waste containers, never poured down the sink. They can be precipitated as carbonates or hydroxides for safer disposal.
正确处理废物是任何实验评估的组成部分。含铜或铬等重金属的溶液必须收集在贴有标签的废液容器中，绝不可倒入水槽。可将其沉淀为碳酸盐或氢氧化物再安全处置。
Used TLC plates and contaminated filter paper should go into solid waste bins. Organic solvents like toluene or ethyl ethanoate require halogen-free waste containers. Always consult the laboratory's safety protocol.
使用过的TLC板和被污染的滤纸应放入固体废物箱。有机溶剂如甲苯或乙酸乙酯需放入无卤废液容器。务必遵循实验室安全规程。
After the experiment, wash hands thoroughly. Reagent bottles must be closed and returned to the designated area. A tidy workspace is part of good experimental technique.
实验结束后彻底洗手。试剂瓶必须盖好并放回指定区域。整洁的工作区域是良好实验技术的一部分。
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