Edexcel A-Level Chemistry Topic 9 Kinetics: Rates, Orders and Activation Energy | 爱德思A-Level化学专题9动力学：速率、反应级数与活化能

📚 Edexcel A-Level Chemistry Topic 9 Kinetics: Rates, Orders and Activation Energy | 爱德思A-Level化学专题9动力学：速率、反应级数与活化能

Welcome to this revision guide for Edexcel A-Level Chemistry Topic 9 Kinetics I. Kinetics is the study of reaction speed. Exam questions regularly test collision theory, Maxwell-Boltzmann curves, rate equations, orders of reaction, activation energy and catalysts. This guide explains the core ideas step by step with worked-style examples and exam tips.

欢迎阅读本篇爱德思A-Level化学专题9“动力学 I”复习指南。动力学研究的是化学反应速率。考试题目经常考查碰撞理论、麦克斯韦-玻尔兹曼分布曲线、速率方程、反应级数、活化能和催化剂。本指南将逐步讲解核心概念，并配有练习式例题和考试技巧。

1. What is Reaction Rate? | 什么是反应速率？

The rate of a chemical reaction is the change in concentration of a reactant or product per unit time. Common units are mol dm⁻³ s⁻¹. For a reaction A → B, the average rate over a time interval Δt can be written as the change in concentration divided by that time interval. A negative sign is used for reactants because their concentration decreases.

化学反应速率是指反应物或生成物浓度在单位时间内的变化量。常用单位是 mol dm⁻³ s⁻¹。对于反应 A → B，在时间间隔 Δt 内的平均速率可以写成浓度变化量除以该时间间隔。由于反应物浓度在下降，因此反应物的速率表达式需要加上负号。

rate = −Δ[A]/Δt = Δ[B]/Δt

The instantaneous rate at a particular time is found from the gradient of a concentration–time graph. A steeper tangent means a faster reaction at that moment. As reactants are used up, the rate usually decreases unless a catalyst or other factor intervenes.

某一时刻的瞬时速率可以从浓度–时间曲线的切线斜率求得。切线越陡，说明该时刻反应越快。随着反应物被消耗，速率通常会下降，除非有催化剂或其他因素影响。

2. Collision Theory and Activation Energy | 碰撞理论与活化能

For a reaction to occur, particles must collide with the correct orientation and with kinetic energy greater than or equal to the activation energy Eₐ. Only successful collisions lead to product formation. Activation energy is defined as the minimum energy required for a collision to result in a chemical reaction.

反应要发生，粒子必须以正确的取向碰撞，并且动能必须大于或等于活化能 Eₐ。只有有效碰撞才会生成产物。活化能定义为碰撞能够引发化学反应所需的最低能量。

When the activation energy is high, the reaction is slow because only a small proportion of particles have enough energy at a given temperature. When Eₐ is low, the reaction is fast because more particles can overcome the energy barrier.

当活化能较高时，反应较慢，因为在给定温度下只有很少一部分粒子具有足够能量。当 Eₐ 较低时，反应较快，因为更多粒子能够越过能垒。

3. Maxwell-Boltzmann Distribution | 麦克斯韦-玻尔兹曼分布

A Maxwell-Boltzmann curve shows the distribution of molecular kinetic energies in a gas at a given temperature. The curve starts at the origin, rises to a maximum, and then tails off towards higher energies. The area under the entire curve is proportional to the total number of particles.

麦克斯韦-玻尔兹曼曲线表示在某一温度下气体分子动能的分布。曲线从原点开始，上升到一个最大值，然后向高能方向逐渐下降。整条曲线下的面积与粒子总数成正比。

The area under the curve to the right of Eₐ represents the fraction of particles that have enough energy to react. Increasing temperature shifts the distribution to the right and flattens the peak. This greatly increases the area beyond Eₐ, so the reaction rate increases significantly.

曲线在 Eₐ 右侧的面积代表具有足够能量发生反应的粒子比例。升高温度会使分布曲线向右移动，峰值变低。这会显著增加 Eₐ 右侧的面积，因此反应速率会大幅提高。

4. Effect of Concentration and Pressure | 浓度和压强的影响

Increasing the concentration of a solution increases the number of reactant particles per unit volume. This leads to more frequent collisions per second, so the rate of reaction increases. For reactions involving gases, increasing pressure has the same effect as increasing concentration because it forces more gas molecules into the same volume.

增大溶液的浓度会增加单位体积内反应物粒子的数量。这导致每秒碰撞次数增加，因此反应速率增大。对于涉及气体的反应，增大压强与增大浓度效果相同，因为它会把更多气体分子压入同一体积中。

This effect is mostly explained by collision frequency rather than by the energy of individual particles. The particles are no hotter, but they collide more often, so successful collisions become more frequent.

这种影响主要通过碰撞频率来解释，而不是单个粒子的能量。粒子并没有变得更热，但碰撞更加频繁，因此有效碰撞也变得更频繁。

5. Rate Equations and Order of Reaction | 速率方程与反应级数

For a reaction with the general form aA + bB → products, the rate equation may be expressed as:

对于一般形式的反应 aA + bB → 生成物，速率方程可以写成：

rate = k[A]ᵐ[B]ⁿ

In this equation, [A] and [B] are the concentrations of the reactants, m is the order with respect to A, n is the order with respect to B, and k is the rate constant. The overall order of reaction is the sum m + n.

在这个方程中，[A] 和 [B] 是反应物的浓度，m 是相对于 A 的反应级数，n 是相对于 B 的反应级数，k 是速率常数。反应的总级数是 m + n 的和。

Orders can be zero, first, second, or sometimes fractional. They must be determined experimentally. They are not obtained from the stoichiometric coefficients of the balanced equation.

反应级数可以是零级、一级、二级，有时也可以是分数级。级数必须通过实验测定，不能直接从配平方程式中的化学计量数获得。

6. Determining Orders from Initial Rates | 通过初速率确定反应级数

The method of initial rates involves measuring the rate at the very start of a reaction while concentrations are known. To find the order with respect to one reactant, change only that reactant’s concentration and keep all other concentrations constant.

初速率法是指在反应刚开始、浓度已知时测量反应速率。要确定某一反应物的反应级数，只改变该反应物的浓度，并保持其他反应物浓度不变。

Use the following rules: if doubling [A] doubles the rate, the order with respect to A is 1. If doubling [A] quadruples the rate, the order is 2. If doubling [A] has no effect on the rate, the order is 0.

可以使用以下规则：如果 [A] 加倍而速率加倍，则相对于 A 的级数是 1。如果 [A] 加倍而速率变为原来的四倍，则级数是 2。如果 [A] 加倍而速率不变，则级数是 0。

Experiment	[A] / mol dm⁻³	[B] / mol dm⁻³	Initial rate / mol dm⁻³ s⁻¹
1	0.10	0.10	2.0 × 10⁻⁴
2	0.20	0.10	4.0 × 10⁻⁴
3	0.20	0.20	8.0 × 10⁻⁴

From experiments 1 and 2, doubling [A] while [B] stays constant doubles the rate, so order with respect to A is 1. From experiments 2 and 3, doubling [B] while [A] stays constant doubles the rate, so order with respect to B is 1. The rate equation is rate = k[A][B].

从实验1和2可以看出，在 [B] 不变时，[A] 加倍使速率加倍，所以相对于 A 的级数是 1。从实验2和3可以看出，在 [A] 不变时，[B] 加倍使速率加倍，所以相对于 B 的级数是 1。速率方程为 rate = k[A][B]。

7. Measuring Reaction Rates | 测量反应速率

Common methods for measuring reaction rates include monitoring gas volume, measuring mass loss, recording colour change, measuring pH change, or withdrawing samples and titrating. The chosen method must give clear, continuous or regular readings that can be plotted against time.

测量反应速率的常用方法包括监测气体体积、测量质量损失、记录颜色变化、测量 pH 变化，或取出样品进行滴定。选择的方法必须能够给出清晰、连续或定时的读数，并能绘制成随时间变化的曲线。

For example, when a carbonate reacts with dilute acid, carbon dioxide gas is produced. You can measure the volume of gas collected in a gas syringe every 10 seconds. The gradient of the volume–time graph gives the reaction rate at any instant.

例如，碳酸盐与稀酸反应会产生二氧化碳气体。你可以每10秒测量一次气体注射器中收集到的气体体积。体积–时间曲线的斜率可以给出任一时刻的反应速率。

8. Temperature and the Arrhenius Equation | 温度与阿伦尼乌斯方程

Increasing temperature has a much greater effect on reaction rate than increasing concentration or pressure. At a higher temperature, the Maxwell-Boltzmann distribution shifts to the right, and a much larger proportion of particles have energy greater than or equal to Eₐ. This sharply increases the rate of successful collisions.

升高温度对反应速率的影响远大于增大浓度或压强。在较高温度下，麦克斯韦-玻尔兹曼分布曲线向右移动，能量大于或等于 Eₐ 的粒子比例大幅增加。这会显著提高有效碰撞的速率。

The temperature dependence of the rate constant is described by the Arrhenius equation:

速率常数与温度的关系由阿伦尼乌斯方程描述：

k = Ae^(−Eₐ/RT)

Here A is the pre-exponential factor, Eₐ is the activation energy in J mol⁻¹, R is the gas constant 8.31 J K⁻¹ mol⁻¹, and T is the absolute temperature in kelvin. A small temperature rise can cause a large increase in k because of the exponential term.

式中 A 是指前因子，Eₐ 是以 J mol⁻¹ 为单位的活化能，R 是气体常数 8.31 J K⁻¹ mol⁻¹，T 是开尔文温度。由于指数项的存在，温度稍有升高就能使 k 显著增大。

9. Catalysts and Activation Energy | 催化剂与活化能

A catalyst provides an alternative reaction pathway with a lower activation energy. It does not change the position of equilibrium, but it increases the rate of both forward and reverse reactions equally. The catalyst is not used up in the overall reaction.

催化剂提供了一条活化能更低的替代反应路径。它不会改变平衡位置，但会同等程度地加快正反应和逆反应的速率。催化剂在总反应中不会被消耗。

On a Maxwell-Boltzmann curve, lowering Eₐ moves the vertical line to the left. The area under the curve to the right of Eₐ becomes much larger, so a much greater proportion of particles can react without increasing the temperature.

在麦克斯韦-玻尔兹曼曲线上，降低 Eₐ 会使竖直参考线向左移动。Eₐ 右侧曲线下面积变得更大，因此在不升高温度的情况下，能够发生反应的粒子比例也会大幅增加。

10. Common Pitfalls and Exam Tips | 常见错误与考试技巧

Exam questions often ask you to deduce orders from data, calculate the rate constant, and explain rate changes using collision theory and the Boltzmann distribution. The most common errors involve confusing stoichiometric coefficients with reaction orders, or giving vague explanations about particles moving faster without connecting this to the energy threshold Eₐ.

考试题经常要求你根据数据推断反应级数、计算速率常数，并用碰撞理论和玻尔兹曼分布解释速率变化。最常见的错误是把化学计量数误当作反应级数，或者给出粒子运动更快的模糊解释，而没有把解释与能垒 Eₐ 联系起来。

Always derive the rate equation from experimental data, not from the balanced equation.
始终根据实验数据推导速率方程，而不是根据配平方程式。
State the units of k; they depend on the overall order. For example, zero order: mol dm⁻³ s⁻¹, first order: s⁻¹, second order: mol⁻¹ dm³ s⁻¹.
写出 k 的单位；单位取决于总级数。例如零级：mol dm⁻³ s⁻¹，一级：s⁻¹，二级：mol⁻¹ dm³ s⁻¹。
Use the words “more particles have energy greater than Eₐ” rather than just “particles move faster” when explaining temperature effects.
解释温度影响时，要用“更多粒子的能量大于 Eₐ”，而不只是“粒子运动更快”。
Remember that a catalyst lowers Eₐ but does not alter ΔH or the equilibrium position.
记住催化剂会降低 Eₐ，但不会改变 ΔH 或平衡位置。

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Edexcel A-Level Chemistry Topic 9 Kinetics: Rates, Orders and Activation Energy | 爱德思A-Level化学专题9动力学：速率、反应级数与活化能