Rates of Reaction | 化学反应速率

📚 Rates of Reaction | 化学反应速率

In IGCSE Science, the rate of a chemical reaction tells us how quickly reactants are converted into products. Understanding the factors that control reaction rates is essential for both the Edexcel exam and practical laboratory work.

在 IGCSE 科学中，化学反应速率告诉我们反应物转化为产物的快慢。理解控制反应速率的因素，对 Edexcel 考试和实际实验操作都至关重要。

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

The rate of reaction is the change in concentration of a reactant or product per unit time. It can be expressed as the speed at which a reactant is used up or a product is formed.

反应速率是指单位时间内反应物或产物浓度的变化。它可以表示为反应物被消耗的速度或产物生成的速度。

rate = amount of reactant used or product formed ÷ time

速率 = 反应物消耗量或产物生成量 ÷ 时间

Common units include g/s, cm³/s, mol/(dm³·s) or mol dm⁻³ s⁻¹. In experiments, we often measure the change in mass, volume of gas, or colour intensity over time.

常用单位包括 g/s、cm³/s、mol/(dm³·s) 或 mol dm⁻³ s⁻¹。在实验中，我们通常测量质量变化、气体体积或颜色强度随时间的变化。

2. Measuring Reaction Rates | 测量反应速率

There are several practical methods to measure how fast a reaction proceeds. The choice depends on the reaction being studied.

有几种实用的方法可以测量反应进行的快慢。选择哪种方法取决于所研究的反应。

Change in mass – when a gas escapes, the total mass decreases.

质量变化——当气体逸出时，总质量减少。
Volume of gas produced – collected in a gas syringe or over water.

生成气体体积——用气体注射器或排水集气法收集。
Turbidity (cloudiness) – used when a precipitate forms; we time how long it takes to hide a cross.

浊度（浑浊度）——用于有沉淀生成的反应；我们计时到看不见十字标记为止。
Color change – using a colorimeter or simple observation.

颜色变化——使用比色计或简单观察。

A continuous measurement records data at regular time intervals, while an initial rate method measures the time at the very start and calculates the rate as 1/time.

连续测量按固定时间间隔记录数据，而初始速率法测量反应开始时的短段时间，并计算速率为 1/时间。

3. Collision Theory | 碰撞理论

For a reaction to occur, particles must collide with enough energy and the correct orientation. This is the basis of collision theory.

反应发生必须满足两个条件：粒子发生碰撞，且碰撞时具有足够能量和正确取向。这就是碰撞理论的基础。

Only collisions that lead to a reaction are called effective collisions. The minimum energy needed to start a reaction is called the activation energy (Eₐ).

只有能导致反应的碰撞才称为有效碰撞。引发反应所需的最低能量称为活化能（Eₐ）。

effective collision = correct orientation + energy ≥ Eₐ

有效碰撞 = 取向正确 + 能量 ≥ Eₐ

Any factor that increases the frequency or energy of collisions will usually speed up the reaction.

任何能增加碰撞频率或碰撞能量的因素通常都会加快反应速率。

4. Surface Area | 表面积

When a solid reactant is broken into smaller pieces, its total exposed surface area increases. This speeds up the reaction.

当固体反应物被粉碎成更小的颗粒时，其总暴露表面积增大，反应速率因此加快。

With a larger surface area, more solid particles are in contact with the liquid or gas reactant, so the frequency of collisions increases.

表面积更大时，更多固体颗粒与液体或气体反应物接触，碰撞频率增加。

Powder reacts faster than lumps.

粉末比块状反应更快。
Marble chips and hydrochloric acid: small chips give a steeper gas-volume curve.

大理石碎片与盐酸：小碎片产生的气体体积曲线更陡。
In the graph, a faster reaction has a steeper gradient and finishes earlier.

在图中，反应越快，曲线斜率越大，结束得越早。

However, the total amount of product formed is the same if the same mass of solid is used — only the rate changes.

但如果固体的质量相同，最终生成物的总量不变——只是速率发生改变。

5. Concentration and Pressure | 浓度与压强

Increasing the concentration of a solution means there are more reactant particles in the same volume. This raises the collision frequency.

增加溶液浓度意味着相同体积内有更多反应物粒子。这提高了碰撞频率。

higher concentration → more collisions per second → faster rate

浓度越高 → 每秒碰撞次数越多 → 速率越快

For gases, increasing pressure has a similar effect: the same number of particles in a smaller volume forces them closer together, so they collide more often.

对于气体，增大压强有类似效果：相同数量的粒子被压缩到更小体积，粒子间距变小，碰撞更频繁。

Doubling the concentration often (but not always) doubles the rate for simple reactions.

在简单反应中，浓度加倍通常会（但不总是）使速率加倍。
Pressure is only relevant for gaseous reactants.

压强只对气态反应物有效。

Adding an inert gas at constant volume does not change the partial pressures of reactants, so the rate stays the same.

在恒定体积下加入惰性气体，不会改变反应物的分压，因此速率不变。

6. Temperature | 温度

Raising the temperature gives particles more kinetic energy. They move faster and collide more often.

升高温度使粒子获得更多动能。它们运动更快，碰撞更频繁。

More importantly, a larger proportion of collisions now exceed the activation energy, so the fraction of effective collisions increases dramatically.

更重要的是，现在有更高比例的碰撞超过活化能，因此有效碰撞的比例大大增加。

Rule of thumb: a 10 °C rise often doubles the reaction rate

经验法则：温度每升高 10 °C，反应速率大约翻倍

This explains why food spoils faster in warm weather: the bacteria-driven chemical reactions happen more quickly.

这解释了为什么食物在温暖天气下变质更快：细菌驱动的化学反应发生得更快。

7. Catalysts | 催化剂

A catalyst is a substance that speeds up a reaction without being used up in the process. It is chemically unchanged at the end.

催化剂是一种能加快反应速率，但自身在反应过程中不被消耗的物质。反应结束时它在化学性质上保持不变。

A catalyst works by providing an alternative pathway with a lower activation energy. This means more collisions are effective at the same temperature.

催化剂通过提供一条活化能较低的替代反应路径来起作用。这意味着在相同温度下，更多碰撞成为有效碰撞。

Catalysts are specific: different reactions need different catalysts.

催化剂具有选择性：不同反应需要不同的催化剂。
Transition metals like platinum and nickel are common catalysts.

铂和镍等过渡金属是常见的催化剂。
Enzymes are biological catalysts.

酶是生物催化剂。

Because catalysts are regenerated, only a small amount is needed to catalyse a large quantity of reactants.

由于催化剂会再生，只需少量催化剂就能催化大量反应物。

8. Graphs and Rate Calculations | 图表与速率计算

Reaction progress is often shown on a graph of product formed (or reactant remaining) against time.

反应进程通常用生成物（或剩余反应物）对时间的曲线图来表示。

Graph feature 图形特征	Meaning 含义
Steeper gradient 斜率更陡	Faster rate 速率更快
Curve becomes horizontal 曲线变水平	Reaction stops (reactant used up) 反应停止（反应物耗尽）
Final plateau height 最终平台高度	Total product formed (same if same amount of reactant, same conditions) 生成物总量（相同反应物和条件下相同）

To calculate the average rate over a period, divide the change in product by the time interval. The rate at a specific moment is found by drawing a tangent to the curve and calculating its gradient.

要计算一段时间内的平均速率，用产物变化量除以时间间隔。要得到某一时刻的瞬时速率，需要画切线并求其斜率。

tangent gradient = Δy/Δx

切线斜率 = Δy/Δx

9. Factors Summary Table | 因素总结表

The table below summarises how each factor affects the rate of reaction.

下表总结了各因素如何影响反应速率。

Factor 因素	Effect on rate 对速率的影响	Reason 原因
Increase surface area 增大表面积	Increases 增大	More contact area, more collisions 接触面积更大，碰撞更多
Increase concentration/pressure 增大浓度/压强	Increases 增大	More particles per unit volume 单位体积内粒子数更多
Increase temperature 升高温度	Increases significantly 显著增大	More energy, more effective collisions 能量更高，有效碰撞更多
Add catalyst 加入催化剂	Increases 增大	Lowers activation energy 降低活化能

10. Common Exam Pitfalls | 常见考试陷阱

Many students lose marks in this topic due to careless language. Avoid these common mistakes.

很多学生在这个专题中因表达不严谨而失分。请避免以下常见错误。

‘Higher temperature makes particles move faster, so more particles collide.’ — This is correct, but if you say ‘more collisions are effective’ without mentioning activation energy, you are only half right.

“温度升高使粒子运动更快，因此更多粒子碰撞。”——这没错，但如果只说“更多有效碰撞”而不提活化能，只答对了一半。
‘A catalyst increases the rate of reaction.’ — Yes, but the full explanation requires ‘by lowering activation energy’.

“催化剂能增加反应速率。”——是的，但完整解释需要“通过降低活化能”。
‘Increasing the surface area increases the activation energy.’ — Wrong: activation energy is fixed for a given reaction.

“增大表面积会增大活化能。”——错误：对于给定反应，活化能是固定的。
Confusing ‘rate of reaction’ with ‘amount of product’. A catalyst does not change the final amount of product, only how quickly it forms.

混淆“反应速率”与“产物总量”。催化剂不会改变最终产物的量，只改变生成快慢。

Always state both the collision frequency and the effectiveness of collisions when explaining factors like temperature.

在解释温度等因素时，一定要同时提到碰撞频率和碰撞有效性。

Final Tips | 最终提示

In the exam, you may be asked to draw or interpret a graph. Remember that the initial slope is steepest, and the graph becomes flat when one reactant runs out.

考试中，你可能需要绘制或解释曲线图。记住：初始斜率最大，当某一种反应物耗尽时曲线变平。

For an experiment, always identify the variables: the independent variable (what you change), the dependent variable (what you measure), and the control variables (what you keep constant).

对于实验题，一定要辨认变量：自变量（你改变的量）、因变量（你测量的量）和控制变量（你保持不变的量）。

Practise tangent calculations and reversible-reaction rate graphs (where forward and backward rates eventually become equal). These are high-value skills.

练习切线计算和可逆反应的速率曲线（正反应和逆反应速率最终相等）。这些是高分技能。

Mastering rates of reaction gives you a strong foundation for equilibria, catalysis in industry, and biological enzyme kinetics later in your science studies.

掌握反应速率为你后续学习平衡、工业催化和生物酶动力学打下坚实基础。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Rates of Reaction | 化学反应速率