📚 Investigation 651: Effect of Concentration on Reaction Rate | 实验651：浓度对反应速率的影响
In IGCSE Science, understanding how fast a chemical reaction occurs is a fundamental skill. Investigation 651 focuses on a classic experiment: measuring the effect of reactant concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid. This reaction produces a precipitate of sulfur, which turns the solution cloudy, allowing us to measure the time taken for a mark to disappear. This article will guide you through the key concepts, procedure, data handling and evaluation required to master this practical – a popular Edexcel IGCSE Science assessment topic.
在IGCSE科学中，理解化学反应发生得有多快是一项基本技能。实验651聚焦于一个经典实验：测量反应物浓度对硫代硫酸钠与盐酸反应速率的影响。该反应会生成硫沉淀，使溶液变浑浊，这样我们就可以测量一个标记消失所需的时间。本文将引导你掌握这个实验的关键概念、步骤、数据处理和评估——这是爱德思IGCSE科学考试中常见的实操考查主题。
1. Introduction: What is Reaction Rate? | 引言：什么是反应速率？
The rate of a chemical reaction tells us how quickly reactants are turned into products. It can be measured as the change in amount of a reactant or product per unit time. For the reaction between sodium thiosulfate (Na₂S₂O₃) and hydrochloric acid (HCl), solid sulfur forms, making the solution turbid, so a convenient way to monitor rate is by timing how long it takes for the mixture to become opaque.
化学反应速率告诉我们反应物转化为产物有多快。它可以被测量为单位时间内反应物或产物的变化量。对于硫代硫酸钠 (Na₂S₂O₃) 与盐酸 (HCl) 之间的反应，会生成固态硫，使溶液变浑浊，因此监测速率的一种便捷方法是计时混合物变得完全不透明所需的时间。
2. The Collision Theory | 碰撞理论
Reactions occur when particles collide with sufficient energy (activation energy) and with the correct orientation. Increasing the concentration of a reactant means there are more particles per unit volume. This leads to more frequent successful collisions, so the reaction rate increases. Investigation 651 explores this relationship directly by varying the concentration of sodium thiosulfate while keeping other conditions constant.
当粒子以足够的能量（活化能）和正确的取向发生碰撞时，反应就会发生。增加反应物的浓度意味着单位体积内有更多的粒子。这会带来更频繁的成功碰撞，因此反应速率增大。实验651通过改变硫代硫酸钠的浓度、同时保持其他条件不变，直接探索了这一关系。
3. Factors Affecting Reaction Rate | 影响反应速率的因素
Besides concentration, temperature, surface area of solids and the use of catalysts also influence reaction rate. However, in this specific investigation, only concentration is deliberately changed. Temperature is kept constant by doing all trials at room temperature and using the same stock solutions. This ensures a fair test, so we can be confident that any change in time taken is due to the concentration difference alone.
除浓度外，温度、固体的表面积以及催化剂的使用也会影响反应速率。然而，在这个特定的实验中，只有浓度被有意改变。通过在所有试验中保持室温并使用相同的储备溶液，温度被保持不变。这确保了公平测试，因此我们可以确信任何时间上的变化都仅归因于浓度差异。
4. Aim of Investigation 651 | 实验651的目标
The aim is to investigate how the rate of reaction between sodium thiosulfate solution and dilute hydrochloric acid changes as the concentration of sodium thiosulfate is altered. We will measure the time taken for a cross drawn under the reaction flask to disappear, and use reciprocal time (1/time) as a measure of the initial rate.
目标是研究随着硫代硫酸钠浓度的改变，硫代硫酸钠溶液与稀盐酸之间反应速率如何变化。我们将测量反应烧瓶下方画出的十字标记消失所需的时间，并用时间的倒数 (1/时间) 作为初始速率的量度。
5. Variables: Independent, Dependent and Control | 变量：自变量、因变量与控制变量
The independent variable is the concentration of sodium thiosulfate solution. This is varied by diluting the stock solution with distilled water to get different proportions. The dependent variable is the time taken for the cross to disappear, and derived rate (1/t). Controlled variables include: temperature (room temperature), total volume of the reaction mixture (kept constant by adjusting water), concentration and volume of hydrochloric acid, and the size of the cross drawn on paper.
自变量是硫代硫酸钠溶液的浓度。通过用蒸馏水稀释储备溶液来得到不同的比例，从而改变浓度。因变量是十字标记消失所需的时间，以及导出的速率 (1/t)。控制变量包括：温度（室温）、反应混合物的总体积（通过调整水量保持恒定）、盐酸的浓度和体积、以及画在纸上的十字标记的大小。
6. Apparatus and Materials | 仪器与材料
You will need: 0.1 mol/dm³ sodium thiosulfate solution, 1.0 mol/dm³ hydrochloric acid, distilled water, a 10 cm³ measuring cylinder, a 50 cm³ measuring cylinder, a 100 cm³ conical flask, a stopwatch, a white tile or paper with a bold 'X' drawn in black marker, safety goggles, and a lab coat. The conical flask must be clean and dry before each set of trials.
你需要：0.1 mol/dm³ 硫代硫酸钠溶液、1.0 mol/dm³ 盐酸、蒸馏水、一个10 cm³量筒、一个50 cm³量筒、一个100 cm³锥形瓶、一个秒表、一块白瓷砖或一张用黑色记号笔画有粗体 'X' 的纸、护目镜和实验服。每组试验前锥形瓶必须清洁干燥。
7. Method: Step-by-Step Procedure | 方法：逐步实验步骤
1. Place the conical flask on the tile with the cross visible from above. Measure 50 cm³ of sodium thiosulfate solution (the highest concentration) and pour it into the flask. 2. Measure 5 cm³ of hydrochloric acid and add it to the flask, starting the stopwatch immediately. 3. Swirl the flask gently and look down through the solution. Stop the timer as soon as the cross is no longer visible. Record the time. 4. Repeat for different concentrations: use 40 cm³ Na₂S₂O₃ + 10 cm³ water; 30 cm³ Na₂S₂O₃ + 20 cm³ water; 20 cm³ Na₂S₂O₃ + 30 cm³ water; 10 cm³ Na₂S₂O₃ + 40 cm³ water. Always add 5 cm³ of acid last, and keep the total volume at 55 cm³. 5. Carry out two or three trials at each concentration to identify any anomalies and calculate a mean time. Remember to rinse the flask with distilled water between runs.
1. 将锥形瓶放在白瓷砖上，使十字标记从上方可见。量取50 cm³的硫代硫酸钠溶液（最高浓度）并倒入锥形瓶。2. 量取5 cm³盐酸加入锥形瓶，立即启动秒表。3. 轻轻旋转锥形瓶，并从上方向下观察溶液。当十字标记不再可见时立即停止计时。记录时间。4. 对不同浓度重复实验：使用40 cm³ Na₂S₂O₃ + 10 cm³水；30 cm³ Na₂S₂O₃ + 20 cm³水；20 cm³ Na₂S₂O₃ + 30 cm³水；10 cm³ Na₂S₂O₃ + 40 cm³水。最后总是加入5 cm³酸，并保持总体积为55 cm³。5. 每个浓度进行两次或三次试验，以识别异常值并计算平均时间。记得在每次实验之间用蒸馏水冲洗锥形瓶。
8. Recording Results – Data Table | 记录结果 – 数据表
Construct a results table with columns for: concentration of Na₂S₂O₃ (as volume of stock solution used, or as % of original), time trial 1, time trial 2, time trial 3, mean time (s), and 1/mean time (s⁻¹). The unit s⁻¹ indicates 'per second' and represents the rate. Ensure you discard any anomalous result (e.g., a time very different from the others) before calculating the mean. An example table is shown below.
构建一个结果表，各列依次为：Na₂S₂O₃ 浓度（以所用储备溶液的体积表示，或以原始浓度的百分比表示）、时间试验1、时间试验2、时间试验3、平均时间 (s) 和 1/平均时间 (s⁻¹)。单位 s⁻¹ 表示“每秒”，代表速率。在计算平均值之前，确保舍弃任何异常结果（例如与其他结果差异很大的时间）。下表为一个示例。
| Volume of Na₂S₂O₃ (cm³) | Trial 1 (s) | Trial 2 (s) | Trial 3 (s) | Mean time (s) | 1/Mean time (s⁻¹) |
|---|---|---|---|---|---|
| 50 | 22 | 20 | 21 | 21.0 | 0.048 |
| 40 | 30 | 32 | 31 | 31.0 | 0.032 |
| 30 | 45 | 44 | 46 | 45.0 | 0.022 |
| 20 | 68 | 70 | 69 | 69.0 | 0.014 |
| 10 | 130 | 135 | 132 | 132.3 | 0.0076 |
9. Graphing the Data | 绘制数据图表
Plot a graph with 'Concentration of Na₂S₂O₃ (volume of stock in cm³)' on the x-axis and 'Rate (1/mean time in s⁻¹)' on the y-axis. Alternatively, plot concentration vs. mean time to see a curve. Using a line of best fit, describe the trend: as concentration increases, the reaction rate increases, usually in a roughly proportional manner. The graph should show a straight line through the origin if the relationship is directly proportional, but in practice the line may curve slightly due to experimental errors or because the reaction order is not precisely first order.
绘制一张图表，以 'Na₂S₂O₃ 浓度（储备液体积 cm³）' 为x轴，'速率 (1/平均时间 s⁻¹) ' 为y轴。或者，绘制浓度与平均时间的关系图以观察曲线。使用最佳拟合线来描述趋势：随着浓度增加，反应速率增加，通常呈大致成正比的趋势。如果关系是正比的，则图形应为一条通过原点的直线，但在实际操作中，由于实验误差或因为反应级数并非严格一级，线可能会略微弯曲。
10. Analysis: How Concentration Affects Rate | 分析：浓度如何影响速率
₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l). The precipitated sulfur scatters light, causing turbidity. The rate can be approximated as Rate ∝ 1/time.
数据清楚地显示，较高的硫代硫酸钠浓度会导致更快的反应（更短的消失时间）。这是因为在相同体积内存在更多的硫代硫酸根离子，因此与氢离子的碰撞更频繁。理论反应方程式为：Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)。生成的硫沉淀散射光线，导致浑浊。速率可近似为 速率 ∝ 1/时间。
11. Evaluation and Sources of Error | 评估与误差来源
Common sources of error include: human reaction time when starting and stopping the stopwatch; difficulty in judging exactly when the cross disappears (subjectivity – different observers may stop at slightly different turbidity); temperature changes if the flask is handled too much or if the room is not thermally stable; and incorrect measurement of solutions. To improve reliability, use a light sensor or a data logger that measures light transmission, giving a more precise endpoint. Also, ensure that the flask is meticulously rinsed to avoid contamination, and use a thermostated water bath if possible to control temperature more strictly.
常见的误差来源包括：启动和停止秒表时的人为反应时间；准确判断十字标记何时消失存在困难（主观性——不同观察者可能会在稍微不同的浑浊度时停止）；如果过多握住锥形瓶或室温不稳定，温度会发生变化；以及溶液量取不准确。为了提高可靠性，可以使用光传感器或数据记录器来测量透光率，从而给出更精确的终点。此外，确保认真冲洗锥形瓶以避免污染，如果可能的话使用恒温水浴来更严格地控制温度。
12. Conclusion | 结论
Investigation 651 confirms the predictions of collision theory: increasing the concentration of a reactant increases the frequency of successful collisions, and therefore increases the reaction rate. The method using the disappearing cross is a simple yet effective way of measuring rate for reactions that produce a precipitate. By carefully controlling variables and using repeats, we can obtain reliable data. This experiment provides a strong foundation for further studies in kinetics, and it is essential for any student aiming for top grades in Edexcel IGCSE Science practical assessments.
实验651证实了碰撞理论的预测：增加反应物的浓度会增加成功碰撞的频率，从而加快反应速率。使用消失的十字标记的方法是测量生成沉淀的反应速率的一种简单而有效的方式。通过仔细控制变量并使用重复试验，我们可以获得可靠的数据。这个实验为进一步学习动力学打下了坚实基础，对于任何希望在爱德思IGCSE科学实操评估中取得高分的学生来说都是至关重要的。
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