📚 Measuring Reaction Rates: Experimental Methods and Data Analysis | 化学反应速率的测定与实验方法
Chemical kinetics is the study of reaction rates — how quickly reactants turn into products. The rate of a reaction is not just a theoretical concept; it is measured experimentally using a range of techniques. This article reviews the core methods used to determine reaction rates, explains how to process the raw data, and highlights which method suits which type of reaction, as required for A-level chemistry.
化学动力学研究反应速率——即反应物转化为产物的快慢。反应速率不仅是理论概念，还需要通过一系列实验技术来测定。本文回顾确定反应速率的核心方法，解释如何处理原始数据，并指出哪些方法适用于哪些类型的反应，以满足 A-level 化学的要求。
1. What is Reaction Rate? Average vs Instantaneous | 什么是反应速率？平均速率与瞬时速率
The rate of a reaction is defined as the change in concentration of a reactant or product per unit time. For a reaction A → B, the average rate over an interval Δt is:
反应速率定义为反应物或产物浓度随时间的变化率。对于反应 A → B，一段时间 Δt 内的平均速率为：
rate = −Δ[A]/Δt = Δ[B]/Δt
The negative sign shows that the concentration of reactant A is decreasing. The instantaneous rate at any moment is the gradient of the concentration–time graph at that point. Common units are mol dm⁻³ s⁻¹.
负号表示反应物 A 的浓度在减小。任何时刻的瞬时速率是浓度–时间图像上该点的切线斜率。常用单位是 mol dm⁻³ s⁻¹。
2. Choosing a Method: Continuous vs Sampling | 选择方法：连续监测与取样法
Experimental methods fall into two categories: continuous monitoring, where a physical property is recorded throughout the reaction, and sampling, where samples are removed at intervals and chemically analysed. The choice depends on the state of the reactants and products, the speed of the reaction, and the availability of equipment.
实验方法分为两类：连续监测——在整个反应过程中记录某个物理性质；取样法——按一定时间间隔取出样品进行化学分析。选择取决于反应物和产物的状态、反应速度以及可用设备。
For fast reactions, continuous methods with probes are preferred because they capture many data points before the reaction finishes. For slower reactions, sampling with titration or colorimetry can be used, provided the reaction can be effectively stopped at each sample time.
对于快速反应，首选带探头的连续方法，因为它们能在反应完成前捕获大量数据点。对于较慢的反应，可使用滴定或比色法的取样方式，前提是每次取样时能有效停止反应。
3. Measuring Gas Volume | 测量气体体积
When a reaction produces a gas, the volume of gas formed can be measured using a gas syringe or an inverted burette filled with water. For example, the reaction of magnesium ribbon with dilute hydrochloric acid produces hydrogen gas. The gas syringe records the volume directly at set intervals.
当反应产生气体时，可用气筒或充满水的倒置滴定管测量生成气体的体积。例如，镁条与稀盐酸反应生成氢气。气筒可以直接记录设定时间间隔内的气体体积。
- Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) | 镁 ± 盐酸 → 氯化镁 + 氢气
- CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l) | 碳酸钙 + 盐酸 → 氯化钙 + 二氧化碳 + 水
- 2H₂O₂(aq) → 2H₂O(l) + O₂(g) | 过氧化氢分解 → 水 + 氧气
A plot of gas volume against time gives a curve whose gradient is the reaction rate. This method is simple and continuous, but it is limited to reactions that produce an insoluble or sparingly soluble gas so that the gas does not dissolve significantly.
以气体体积对时间作图得到曲线，曲线斜率即为反应速率。该方法简单、连续，但仅限于产生不溶性或微溶性气体的反应，以避免气体显著溶解。
4. Measuring Mass Loss | 测量质量损失
If the gas produced escapes from the reaction flask, the total mass of the flask and contents decreases. A balance connected to a data logger can record mass every few seconds. For example, in the reaction of marble chips with hydrochloric acid, the mass loss equals the mass of carbon dioxide released.
如果产生的气体从反应瓶中逸出，瓶和内容物的总质量会减少。连接数据记录器的电子天平可以每隔几秒记录质量。例如，大理石碎片与盐酸反应时，质量损失等于释放的二氧化碳的质量。
This method is convenient and can be automated, but it is less accurate for fast reactions because gas escapes quickly and the balance may be affected by draughts. A cotton-wool plug is often used to prevent splashing while still letting gas escape.
该方法方便且可自动化，但用于快速反应时精度较低，因为气体逸出快且天平可能受气流影响。通常用棉花塞防止液体溅出，同时仍允许气体逸出。
5. Measuring Pressure Changes | 测量压力变化
For reactions in a closed container where the number of moles of gas changes, the pressure at constant volume is proportional to the total moles of gas. A pressure sensor can measure the rate continuously. For example, the decomposition of hydrogen peroxide in a sealed flask:
对于密闭容器中气体总物质的量发生变化的反应，在恒容条件下压力与气体总物质的量成正比。压力传感器可以连续测量反应速率。例如，过氧化氢在密闭瓶中分解：
2H₂O₂(aq) → 2H₂O(l) + O₂(g)
The pressure increases as oxygen is formed. If the reaction is performed in a sealed vessel with no gas escape, the pressure change can be converted to a concentration change using the ideal gas law, PV = nRT.
随着氧气生成，压力增大。如果反应在无气体逸出的密闭容器中进行，则可利用理想气体定律 PV = nRT 将压力变化转化为浓度变化。
6. Colorimetry and Spectrophotometry | 比色法与分光光度法
When a reactant or product is coloured, the absorbance of light is proportional to its concentration (Beer–Lambert law). A colorimeter or spectrophotometer measures absorbance at a suitable wavelength. For example, in the reaction between acidified potassium manganate(VII) and oxalic acid, the purple MnO₄⁻ is reduced to colourless Mn²⁺, so the absorbance decreases over time.
当反应物或产物有颜色时，吸光度与浓度成正比（Beer–Lambert 定律）。比色计或分光光度计在合适的波长下测量吸光度。例如，酸化高锰酸钾与草酸反应中，紫色的 MnO₄⁻ 被还原为无色的 Mn²⁺，因此吸光度随时间下降。
2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂
The absorbance–time graph can be converted to a concentration–time graph if a calibration curve is prepared. This method is continuous and works well for coloured species but is not suitable for colourless reaction mixtures.
如果制备了校准曲线，吸光度–时间图像可转化为浓度–时间图像。该方法连续且适用于有色物质，但不适用于无色反应混合物。
7. Conductivity Measurements | 电导率测量
Many reactions involve ions, and the electrical conductivity of the solution changes as the concentrations of ions change. For example, in the hydrolysis of an ester with sodium hydroxide, highly conductive OH⁻ ions are consumed, so the conductivity decreases.
许多反应涉及离子，溶液的电导率随离子浓度变化而变化。例如，酯与氢氧化钠的水解反应中，高导电性的 OH⁻ 被消耗，因此电导率下降。
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH
A conductivity probe can record data continuously. This method is particularly useful for ionic reactions in solution, but it requires careful temperature control because conductivity is strongly temperature-dependent.
电导率探头可以连续记录数据。该方法特别适用于溶液中的离子反应，但需要严格控制温度，因为电导率对温度非常敏感。
8. pH Monitoring and Titration Sampling | pH 监测与滴定取样
For reactions that consume or produce H⁺ or OH⁻, a pH electrode can continuously record pH. For example, the reaction of magnesium with acid consumes H⁺, so the pH increases as the acid is used up. In alkaline ester hydrolysis, OH⁻ is consumed, so the pH falls.
对于消耗或产生 H⁺ 或 OH⁻ 的反应，pH 电极可以连续记录 pH。例如，镁与酸反应消耗 H⁺，随着酸被消耗，pH 升高。在碱性酯水解中，OH⁻ 被消耗，pH 下降。
When no useful physical property is available, sampling with titration is used. Samples are withdrawn at known times, the reaction is quenched by dilution, cooling, or adding a reagent, and then a titration is performed to determine the concentration of a reactant or product.
当没有有用的物理性质可用时，采用取样滴定法。在已知时间取出样品，通过稀释、冷却或加入试剂来淬灭反应，然后进行滴定以测定反应物或产物的浓度。
For example, the acid-catalysed hydrolysis of methyl ethanoate can be followed by titrating the ethanoic acid produced with sodium hydroxide. This method is time-consuming but works well for slow reactions with no colour change.
例如，乙酸甲酯的酸催化水解可通过用氢氧化钠滴定生成的乙酸来跟踪。该方法耗时，但对于没有颜色变化的慢反应效果很好。
9. Clock Reactions and Initial Rates | 时钟反应与初始速率
In a clock reaction, a visible change occurs suddenly after a known time, marking the consumption of a fixed amount of reactant. The iodine clock reaction is classic: hydrogen peroxide oxidises iodide to iodine, and the iodine is immediately removed by thiosulfate until the thiosulfate runs out; then starch gives a blue-black colour.
在时钟反应中，经过已知时间后会出现明显变化，标志着固定量反应物被消耗。经典的碘钟反应：过氧化氢将碘离子氧化为碘，碘立即被硫代硫酸根还原除去；当硫代硫酸根耗尽时，淀粉指示剂使溶液变蓝黑色。
The time t for the colour to appear is measured. The initial rate is proportional to 1/t. By varying initial concentrations and measuring t, the order with respect to each reactant can be deduced. This method is excellent for measuring initial rates but does not give a full concentration–time curve.
测量变色所用时间 t，初始速率与 1/t 成正比。通过改变初始浓度并测量 t，可以推断各反应物的反应级数。该方法非常适合测定初始速率，但不能给出完整的浓度–时间曲线。
10. Data Analysis: Gradients and Integrated Rate Laws | 数据分析：斜率与积分速率定律
Once data are collected, the rate is obtained from the gradient of a concentration–time graph. For the initial rate, draw a tangent at t = 0 and measure its slope. For reactions that follow integrated rate laws, plotting appropriate functions gives straight lines whose slopes are related to the rate constant.
收集数据后，通过浓度–时间图像的斜率得到反应速率。对于初始速率，在 t = 0 处作切线并测量其斜率。对于符合积分速率定律的反应，绘制适当的函数得到直线，其斜率与速率常数有关。
For a first-order reaction: ln[A] = −kt + ln[A]₀. A plot of ln[A] against t is linear with gradient = −k.
对于一级反应：ln[A] = −kt + ln[A]₀。以 ln[A] 对 t 作图得到直线，斜率为 −k。
ln[A] = −kt + ln[A]₀
For a second-order reaction: 1/[A] = kt + 1/[A]₀. A plot of 1/[A] against t is linear with gradient = k.
对于二级反应：1/[A] = kt + 1/[A]₀。以 1/[A] 对 t 作图得到直线，斜率为 k。
1/[A] = kt + 1/[A]₀
For first-order reactions, the half-life is constant: t₁/₂ = ln2/k. This provides another graphical method to identify first-order kinetics.
对于一级反应，半衰期恒定：t₁/₂ = ln2/k。这提供了另一种识别一级动力学的作图方法。
11. Summary: Choosing the Right Method | 总结：选择合适的方法
The choice of method depends on the physical state and the optical, electrical or acidic properties of the species involved. Continuous methods such as gas volume, mass loss, pressure, colorimetry, conductivity and pH monitoring are quick and provide many data points, while sampling-titration is suited to slow reactions without a useful physical property.
方法的选择取决于反应物和产物的物理状态以及其光学、电学或酸碱性质。连续方法如气体体积、质量损失、压力、比色、电导和 pH 监测快速且提供大量数据点，而取样滴定适用于没有良好物理性质的慢反应。
- Gas-producing reactions: gas syringe, mass loss, or pressure sensor | 产气反应：气筒、质量损失或压力传感器
- Coloured species: colorimeter or spectrophotometer | 有色物质：比色计或分光光度计
- Ionic reactions: conductivity probe | 离子反应：电导率探头
- H⁺/OH⁻ changes: pH electrode or titration sampling | H⁺/OH⁻ 变化：pH 电极或滴定取样
- Very fast initial-rate studies: clock reactions | 非常快的初始速率研究：时钟反应
Always record temperature carefully to keep the rate constant unchanged. Use reliable equipment, repeat trials, and process data using tangents or integrated rate laws to obtain meaningful kinetic information.
始终仔细记录温度以保持速率常数不变。使用可靠设备，重复实验，并用切线或积分速率定律处理数据，以获得有意义的动力学信息。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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