📚 A-Level Chemistry June 18 Insert 3: Experimental Procedures | A-Level化学：2018年6月 Insert 3 实验操作指南
This article explores the essential practical techniques and experimental procedures referenced in the June 2018 A‑Level Chemistry Insert 3. It focuses on accurate measurement, safe handling of chemicals, and the systematic collection of kinetic data, using the iodine clock reaction as a model required practical.
本文详解 2018 年 6 月 A‑Level 化学试卷插入页 3 所涉及的关键实验操作技术。内容围绕精确测量、化学品安全处置和动力学数据的系统采集展开，并以碘钟反应作为典型的必做实验案例进行说明。
1. Overview of Insert 3 | 插入页 3 概况
The June 2018 A‑Level Chemistry Insert 3 typically supports Paper 3, which assesses practical skills and data handling. This insert provides a step‑by‑step method for a kinetics investigation – often the iodine clock reaction. You are expected to follow the procedure exactly, record times and volumes, and then analyse the data to determine the rate equation.
2018 年 6 月的 A‑Level 化学插入页 3 通常用于支持卷 3，该卷评估实验技能与数据处理。插入页给出了一个动力学研究的分步方法——通常是碘钟反应。你需要严格按照步骤操作，记录时间和体积，然后分析数据以确定速率方程。
2. Safety and Risk Assessment | 安全与风险评估
Always wear eye protection, gloves and a lab coat. Hydrogen peroxide (H₂O₂) is an irritant and a strong oxidising agent – avoid contact with skin. Sulfuric acid (H₂SO₄) is corrosive. Handle all glassware with care, and clean up spills immediately. Before starting, complete a full risk assessment as per CLEAPSS guidelines.
务必佩戴护目镜、手套和实验服。过氧化氢 (H₂O₂) 具有刺激性和强氧化性，避免皮肤接触。硫酸 (H₂SO₄) 具有腐蚀性。小心使用所有玻璃器皿，溢出的液体应立即清理。开始实验前，请按 CLEAPSS 指南完成全面的风险评估。
3. List of Apparatus and Chemicals | 仪器与药品清单
You will need: 250 cm³ conical flasks, 10 cm³ and 25 cm³ measuring cylinders, 1 cm³ and 5 cm³ graduated pipettes, a stopwatch accurate to 0.01 s, a white tile, a thermometer, a wash bottle with distilled water, and a dropping bottle of starch indicator. The reagents include 0.10 mol dm⁻³ potassium iodide (KI), 0.050 mol dm⁻³ sodium thiosulfate (Na₂S₂O₃), 0.10 mol dm⁻³ hydrogen peroxide (H₂O₂), and 0.50 mol dm⁻³ sulfuric acid (H₂SO₄).
你需要以下物品：250 cm³ 锥形瓶，10 cm³ 和 25 cm³ 量筒，1 cm³ 和 5 cm³ 刻度移液管，精度为 0.01 s 的秒表，白瓷砖，温度计，装有蒸馏水的洗瓶，以及淀粉指示剂滴瓶。试剂包括 0.10 mol dm⁻³ 碘化钾 (KI)，0.050 mol dm⁻³ 硫代硫酸钠 (Na₂S₂O₃)，0.10 mol dm⁻³ 过氧化氢 (H₂O₂) 和 0.50 mol dm⁻³ 硫酸 (H₂SO₄)。
4. Preparing Solutions Accurately | 精确配制溶液
Use a volumetric flask to prepare standard solutions if indicated; otherwise, the provided solutions are assumed to be accurate. When measuring volumes with pipettes, read the bottom of the meniscus at eye level and allow full drainage time. Rinse pipettes with the solution to be used before taking aliquots to avoid contamination.
若要求，使用容量瓶配制标准溶液；否则，默认所提供的溶液已准确标定。用移液管量取体积时，视线应与弯月面底部平齐，并等待充分排液。在移取液体前，先用待取溶液润洗移液管，以免交叉污染。
5. The Iodine Clock Reaction – Principle | 碘钟反应原理
The two key steps are:
Slow step: H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O
Fast indicator step: 2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻
As long as thiosulfate is present, iodine is consumed immediately. Once all thiosulfate has reacted, free iodine appears and forms a blue‑black complex with starch, signalling the endpoint.
两个关键步骤是：
慢步骤：H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O
快指示步骤：2S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2I⁻
只要硫代硫酸根存在，碘就会被立即消耗。当硫代硫酸根反应完毕，游离碘出现并与淀粉结合形成蓝黑色络合物，标示终点。
6. Carrying Out the Reaction Runs | 进行反应实验
Using a clean conical flask, add the prescribed volumes of KI, Na₂S₂O₃, starch and H₂SO₄, then make up to a fixed total volume with distilled water. Swirl to mix. Quickly pour in the H₂O₂ solution and simultaneously start the stopwatch. Swirl continuously and watch for the first appearance of a blue‑black colour; stop the timer immediately. Record the time to the nearest 0.01 s. Repeat each run at least twice to obtain concordant results (within 0.5 s).
取洁净锥形瓶，按指定体积加入 KI、Na₂S₂O₃、淀粉和 H₂SO₄，然后用蒸馏水补至固定的总体积，旋摇混匀。迅速倒入 H₂O₂ 溶液并同时启动秒表。持续旋摇并观察首次出现蓝黑色的瞬间；立即停止计时。记录时间至 0.01 秒。每个浓度至少重复两次，得到一致的结果（相差不超过 0.5 秒）。
7. Recording Results Systematically | 系统性地记录结果
Prepare a data table before you start. The table should include volumes of each reagent, the total volume, the calculated initial concentrations of I⁻ and H₂O₂, and the measured time and its reciprocal (1/t). Use a clear layout as shown in the insert, and record observations – colour change, temperature, any anomalies.
开始实验前先设计好数据表。表内应列出各试剂的体积、总体积、计算所得的 I⁻ 和 H₂O₂ 的初始浓度，以及测得的反应时间和时间的倒数 (1/t)。按照插入页所示的清晰格式填写，并记录观察现象，如颜色变化、温度以及任何异常情况。
| Run | V(KI) /cm³ | V(Na₂S₂O₃) /cm³ | V(H₂O₂) /cm³ | V(H₂SO₄) /cm³ | V(Water) /cm³ | Time /s | 1/time /s⁻¹ |
|---|---|---|---|---|---|---|---|
| 1 | 10.0 | 5.0 | 10.0 | 10.0 | 15.0 | 45.2 | 0.0221 |
| 2 | 10.0 | 5.0 | 10.0 | 10.0 | 15.0 | 44.8 | 0.0223 |
8. Varying Concentrations Methodically | 有条理地改变浓度
Keep the total volume constant by adjusting the water volume when changing the volume of one reactant. This ensures the concentrations change linearly and the solution depth remains similar for colour comparison. For example, to study the order with respect to I⁻, vary the KI volume while keeping H₂O₂ and H₂SO₄ volumes unchanged, and top up with water. Do this for at least five different concentrations per reactant.
在改变某一种反应物体积时，通过调节水的体积保持总体积恒定。这能确保浓度线性变化，且液面深度相似，便于颜色比对。例如，要研究对 I⁻ 的反应级数，可在 H₂O₂ 和 H₂SO₄ 体积不变的条件下改变 KI 的体积，并用水补至定容。每个反应物至少完成五组不同浓度的实验。
9. Graphical Analysis of Data | 数据的图形分析
The initial rate is proportional to 1/time. Plot a graph of 1/time (y‑axis) against the concentration of the varied reactant (x‑axis). A horizontal line indicates zero order; a straight line through the origin indicates first order; a curve that becomes linear when 1/time² is plotted suggests second order. You can also deduce the order from a rate‑concentration graph or by using log‑log plots.
初始速率与 1/时间 成正比。以 1/时间（y 轴）对改变的反应物浓度（x 轴）作图。水平线指示零级反应；过原点的直线指示一级反应；若画 1/时间² 对浓度得到直线，则表明二级反应。你也可以通过速率‑浓度图或 log‑log 图推导级数。
10. Calculating the Rate Equation and Rate Constant | 计算速率方程和速率常数
[I⁻]ᵐ[H₂O₂]ⁿ. Determine k by substituting a complete set of data (rate in mol dm⁻³ s⁻¹, concentrations in mol dm⁻³) into the equation. Remember to include units: for an overall order of p, the units are mol⁻⁽ᵖ⁻¹⁾ dm³⁽ᵖ⁻¹⁾ s⁻¹. State the final rate law and the value of k with its units.
确定级数后，速率方程的形式为：rate = k [I⁻]ᵐ[H₂O₂]ⁿ。把一组完整数据（速率取 mol dm⁻³ s⁻¹，浓度取 mol dm⁻³）代入方程，求出 k。注意带上单位：若总级数为 p，单位是 mol⁻⁽ᵖ⁻¹⁾ dm³⁽ᵖ⁻¹⁾ s⁻¹。写出最终的速率定律及 k 值（含单位）。
11. Evaluating Errors and Improvements | 误差评估与改进
Common errors include: delay in starting the stopwatch, inconsistent swirling, temperature changes during the reaction, and parallax when reading volumes. To minimise these, use a second person to help with timing, place the flask on a white tile under consistent lighting, and maintain a constant lab temperature. Using a colorimeter with data logging can remove subjective endpoint judgement.
常见误差包括：秒表启动延迟、旋摇力度不一致、反应过程中温度变化，以及读取体积时的视差。为减小这些影响，可请另一人协助计时，将锥形瓶放在白瓷砖上置于恒定光线下，并保持实验室温度稳定。采用配备数据记录器的比色计可以消除主观的终点判断。
12. Conclusion and Exam Readiness | 小结与备考建议
Mastering the procedures outlined in the June 2018 Insert 3 not only secures marks in the practical assessment but also deepens your understanding of kinetics. Practice writing logical plans, constructing clear data tables, and critiquing a given method. Always link back to core concepts: collision theory, activation energy, and the meaning of the rate constant.
熟练掌握 2018 年 6 月插入页 3 所列的操作，不仅能确保在实验考核中得分，还能加深你对动力学的理解。多练习撰写合理方案、设计清晰的数据表并评析给定方法。始终联系核心概念：碰撞理论、活化能以及速率常数的意义。
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