Rates of Reaction | 反应速率

📚 Rates of Reaction | 反应速率

The rate of a chemical reaction tells us how quickly reactants are turned into products. In IGCSE Science, understanding reaction rates is essential for explaining everyday phenomena, designing experiments, and optimising industrial processes. This article covers the core ideas, measurement techniques, and factors that affect rate.

化学反应速率告诉我们反应物转化为产物的快慢。在 IGCSE 科学中，理解反应速率对解释日常现象、设计实验以及优化工业过程至关重要。本文将涵盖核心概念、测量方法以及影响速率的因素。

1. What is Rate of Reaction? | 什么是反应速率？

Rate of reaction is defined as the change in the amount or concentration of a reactant or product per unit time.

反应速率定义为反应物或产物的物质的量或浓度在单位时间内的变化量。

The rate can be expressed in different units, such as mol dm⁻³ s⁻¹, g s⁻¹, or cm³ s⁻¹, depending on what quantity is being measured.

速率可以用不同单位表示，例如 mol dm⁻³ s⁻¹、g s⁻¹ 或 cm³ s⁻¹，具体取决于测量的物理量。

Rate = Δ(amount) / Δ(time)

The symbol Δ (delta) means ‘change in’. For example, if 0.20 mol of a reactant is used up in 40 s, the average rate is 0.20 ÷ 40 = 0.005 mol s⁻¹.

符号 Δ（delta）表示“变化量”。例如，若 0.20 mol 反应物在 40 s 内被消耗，平均速率为 0.20 ÷ 40 = 0.005 mol s⁻¹。

2. Collision Theory | 碰撞理论

For a reaction to take place, particles must collide with each other.

反应发生的前提是粒子必须相互碰撞。

Not every collision leads to a reaction. A collision is only effective when the particles have at least the activation energy and the correct orientation.

并非每次碰撞都能引发反应。只有当粒子具有至少活化能并且碰撞方向正确时，碰撞才是有效碰撞。

Activation energy (Eₐ) is the minimum energy that colliding particles need in order to react.

活化能（Eₐ）是碰撞粒子发生反应所需的最低能量。

Increasing the frequency of successful collisions increases the rate of reaction.

增加有效碰撞的频率会提高反应速率。

3. Measuring Reaction Rates | 测量反应速率

Several experimental methods can be used to follow the progress of a reaction. The choice depends on which property changes measurably.

有多种实验方法可以追踪反应的进行程度。选择哪种方法取决于哪种性质会发生可测量的变化。

Gas collection: Measure the volume of gas produced at regular intervals using a gas syringe or an upside-down burette.

气体收集：使用气筒或倒置的滴定管每隔一定时间测量产生的气体体积。
Loss in mass: If a gas escapes, record the decreasing mass of the reaction flask.

质量损失：如果气体逸出，记录反应锥形瓶质量的减少。
Precipitation / turbidity: Time how long it takes for a cross drawn under a flask to disappear when a precipitate forms.

沉淀/浑浊度：记录烧瓶下方十字标记因沉淀生成而完全消失所需的时间。
Colour change: Use a colorimeter to measure the intensity of a colour as a reactant is used up or a product forms.

颜色变化：使用比色计测量随着反应物消耗或产物生成而变化的颜色强度。

4. Average Rate Calculations | 平均速率计算

The average rate over a period of time is calculated by dividing the total change in amount by the total time taken.

一段时间内的平均速率等于总量变化除以总时间。

Average rate = (change in amount of reactant or product) / (time taken)

For example, if 5.0 cm³ of carbon dioxide is produced in 50 s, the average rate is 5.0 ÷ 50 = 0.10 cm³ s⁻¹.

例如，若 50 s 内产生了 5.0 cm³ 二氧化碳，平均速率为 5.0 ÷ 50 = 0.10 cm³ s⁻¹。

When calculating with concentration, the units are usually mol dm⁻³ s⁻¹.

如果使用浓度计算，单位通常是 mol dm⁻³ s⁻¹。

5. Factors Affecting Reaction Rates | 影响反应速率的因素

The main factors that affect the rate of a chemical reaction are concentration, pressure, temperature, surface area, and the presence of a catalyst.

影响化学反应速率的主要因素有浓度、压力、温度、表面积和催化剂的存在。

Concentration: Higher concentration means more particles in the same volume, so collisions happen more frequently.

浓度：浓度越高，单位体积内的粒子数越多，碰撞频率越高。
Pressure (gases): Increasing pressure squeezes gas particles closer together, increasing the frequency of collisions.

压力（气体）：增大压力使气体粒子被压缩得更近，增加碰撞频率。
Temperature: Particles gain kinetic energy and move faster. More collisions occur, and a greater proportion have energy above the activation energy.

温度：粒子获得动能并运动得更快。碰撞次数增多，且更多碰撞的能量超过活化能。
Surface area: A larger surface area exposes more reactant particles to collisions.

表面积：更大的表面积使更多反应物粒子暴露在碰撞中。
Catalyst: A catalyst speeds up a reaction by providing an alternative reaction pathway with a lower activation energy.

催化剂：催化剂通过提供活化能较低的反应路径来加快反应速度。

6. Concentration and Pressure | 浓度与压力

For a solution, increasing the concentration of a reactant increases the number of particles per unit volume.

对于溶液，增加反应物浓度会增加单位体积内的粒子数。

This increases the collision frequency, so the rate of reaction increases until the reactant is used up.

这会增加碰撞频率，从而使反应速率增大，直到反应物被消耗完。

For gases, increasing pressure has the same effect as increasing concentration because the gas particles are compressed into a smaller volume.

对于气体，增大压力与增大浓度效果相同，因为气体粒子被压缩在更小体积内。

The graph of amount against time becomes steeper at higher concentration or pressure because the rate is initially greater.

在较高浓度或压力下，物质的量随时间变化的曲线初始阶段更陡，因为初始速率更大。

7. Temperature | 温度

Temperature is usually the most important factor in controlling how fast a reaction proceeds.

温度通常是控制反应进行速度的最重要因素。

When the temperature increases by about 10 °C, the rate of many reactions roughly doubles.

当温度升高约 10 °C 时，许多反应的速率大约翻倍。

This is not only because particles move faster, but also because more particles have the activation energy when they collide.

这不仅是因为粒子运动加快，还因为碰撞时具有活化能的粒子比例增大。

Rate ∝ frequency of successful collisions

A small rise in temperature therefore leads to a much larger increase in rate.

因此，温度小幅升高就会导致速率大幅增加。

8. Surface Area | 表面积

A solid reactant can only collide at its exposed surface. If the solid is broken into smaller pieces, its total surface area increases.

固体反应物只能在其暴露的表面发生碰撞。如果固体被粉碎成更小的颗粒，其总表面积会增加。

For example, powdered calcium carbonate reacts with hydrochloric acid much faster than marble chips of the same mass.

例如，粉末状碳酸钙与盐酸反应的速度远快于相同质量的大理石颗粒。

This is used in industry to increase reaction rates by grinding solid reactants into powders.

工业上常利用这一点，通过将固体反应物研磨成粉末来提高反应速率。

9. Catalysts | 催化剂

A catalyst is a substance that increases the rate of a chemical reaction without being used up in the process.

催化剂是一种在化学反应中能提高反应速率但自身不被消耗的物质。

It works by lowering the activation energy, which means a larger fraction of collisions can successfully react.

它的作用机制是降低活化能，从而使更大比例的碰撞能够成功反应。

Catalysts are specific: different reactions need different catalysts. They are also important in biology as enzymes.

催化剂具有专一性：不同的反应需要不同的催化剂。它们在生物学中以酶的形式也非常重要。

Because catalysts are not consumed, a small amount can speed up a large amount of reactant.

由于催化剂不被消耗，少量催化剂就能加速大量反应物。

10. Rate Graphs and Tangents | 速率图与切线

In a typical reaction, the concentration of a reactant decreases quickly at first and then more slowly as the reactant is used up.

在典型反应中，反应物浓度先快速下降，然后随着反应物消耗而逐渐减慢。

The slope of the curve at any point gives the instantaneous rate at that moment.

曲线上任意一点的斜率表示该时刻的瞬时速率。

To find the instantaneous rate, draw a tangent to the curve at that point and calculate its gradient:

要计算瞬时速率，可以在该点处画一条切线，并计算其梯度：

Rate = (change in y-axis) / (change in x-axis)

The steeper the slope, the faster the reaction. A horizontal line means the reaction has stopped.

斜率越陡，反应越快。水平线表示反应已经停止。

11. Industrial Application: The Haber Process | 工业应用：哈伯法

The Haber process for making ammonia combines nitrogen and hydrogen at high temperature and pressure using an iron catalyst.

哈伯法在高温高压和铁催化剂条件下，将氮气和氢气合成为氨。

N₂ + 3H₂ ⇌ 2NH₃

High temperature increases the rate but decreases the yield of ammonia because the forward reaction is exothermic. A compromise temperature of about 450 °C is used.

高温会加快反应速率，但由于正反应放热，高温会降低氨的产率。因此通常采用约 450 °C 的折中温度。

High pressure (200 atm) both increases the rate and shifts the equilibrium towards more ammonia. The iron catalyst helps the reaction reach equilibrium faster without affecting the yield.

高压（200 atm）既能提高反应速率，又能使平衡向生成更多氨的方向移动。铁催化剂可以帮助反应更快达到平衡，但不影响最终产率。

12. Common Exam Questions and Tips | 常见考题与技巧

In Edexcel IGCSE exams, you may be asked to describe experiments, interpret graphs, or explain the effect of a factor using collision theory.

在 Edexcel IGCSE 考试中，你可能会被要求描述实验、解读图像，或运用碰撞理论解释某一因素的影响。

Use the word ‘successful collisions’ instead of just ‘collisions’ when explaining rate changes.

使用“有效碰撞”而不是只说“碰撞”来解释速率变化。
Quote units in every calculation and every measured rate.

写明单位，在每次计算和测量速率时都要标注单位。
Draw tangents carefully when finding instantaneous rates from curves.

仔细画切线，从曲线求瞬时速率。
Remember catalysts are not used up, they lower activation energy.

牢记催化剂不会被消耗，它们降低活化能。
Link factors to collision frequency or energy: concentration/pressure/surface area affect frequency; temperature affects both frequency and energy.

将因素与碰撞频率或能量联系起来：浓度/压力/表面积影响碰撞频率；温度同时影响频率和能量。

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Rates of Reaction | 反应速率