📚 Understanding Rates of Reaction | 理解化学反应速率
Rates of reaction is a core topic in Edexcel IGCSE Chemistry. It explains how quickly chemical reactions happen and why changing conditions can speed them up or slow them down.
化学反应速率是 Edexcel IGCSE 化学的核心内容。它解释了化学反应发生的快慢，以及为什么改变条件可以加快或减慢反应。
1. What are Rates of Reaction? | 什么是反应速率？
The rate of a reaction measures the change in the concentration of a reactant or product per unit of time. It tells us how fast a chemical change occurs.
反应速率衡量的是反应物或产物浓度在单位时间内的变化量，告诉我们化学变化发生的快慢。
- Fast reactions: explosions, neutralisation, burning. | 快速反应：爆炸、中和反应、燃烧。
- Slow reactions: rusting, weathering, food spoiling. | 缓慢反应：生锈、风化、食物变质。
Rate can be expressed as the amount of reactant used or product formed divided by time.
速率可以表示为消耗的反应物量或生成的产物量除以时间。
2. Measuring Reaction Rates | 测量反应速率
To measure a rate, we follow a quantity that changes over time, such as gas volume, mass loss, or colour intensity.
要测量速率，我们需要跟踪一个随时间变化的物理量，例如气体体积、质量损失或颜色深度。
Common methods include collecting gas in a syringe, measuring the loss of mass on a balance, and using a colorimeter for colour changes.
常见方法包括用注射器收集气体、用天平测量质量减少，以及用色度计监测颜色变化。
| Method | What is measured |
| Gas volume | Volume of gas produced over time |
| Mass loss | Decrease in mass as gas escapes |
| Turbidity | Time for precipitate to obscure a cross |
质量损失法适用于产生气体的反应，而浑浊度法则用于生成沉淀的反应。
3. The Collision Theory | 碰撞理论
For particles to react, they must collide with enough energy and the correct orientation. This is called the collision theory.
粒子要发生反应，必须发生碰撞，并且碰撞的能量足够、取向正确。这就是碰撞理论。
An effective collision has energy greater than or equal to the activation energy (Eₐ). Increasing the frequency or success of collisions increases the rate.
有效碰撞的能量必须大于或等于活化能 (Eₐ)。增加碰撞频率或提高碰撞成功率都会加快反应速率。
Rate ∝ frequency of successful collisions
速率 ∝ 有效碰撞频率
4. Factors Affecting Rate: Concentration & Pressure | 影响速率的因素：浓度与压强
Increasing the concentration of a dissolved reactant means more particles in the same volume. This leads to a higher collision frequency and therefore a higher rate.
增加溶液中反应物的浓度，意味着相同体积内有更多粒子，碰撞频率增加，反应速率因此升高。
For gases, increasing pressure pushes particles closer together. The same effect occurs: more collisions per second, faster reaction.
对于气体，增大压强会使粒子更密集，同样会增加每秒碰撞次数，从而加快反应。
- Higher concentration → more particles per unit volume → more collisions per second. | 浓度更高 → 单位体积内粒子更多 → 每秒碰撞更多。
- Higher pressure → same as higher concentration for gases. | 压强更高 → 对气体而言等同于浓度更高。
5. Factor: Temperature | 因素：温度
Raising temperature increases the kinetic energy of particles. They move faster, so collisions become more frequent and more energetic.
升高温度会增加粒子的动能。它们运动得更快，因此碰撞更频繁，且能量更高。
A 10 °C rise often doubles the reaction rate. The number of particles with energy ≥ Eₐ increases dramatically, so successful collisions are much more common.
温度每升高 10 °C，反应速率通常翻倍。能量达到或超过 Eₐ 的粒子数量大幅增加，因此有效碰撞更加频繁。
Higher temperature → more kinetic energy → more successful collisions
温度升高 → 动能增大 → 有效碰撞增多
6. Factor: Surface Area | 因素：表面积
Breaking a solid into smaller pieces increases its total surface area. This exposes more reactant particles to collision with other reactants.
将固体粉碎成更小的颗粒，可以增加其总表面积，暴露出更多可与其它反应物碰撞的粒子。
A powder reacts faster than a single large lump because there are more surface sites available for collisions at any instant.
粉末比大块固体反应更快，因为在任一时刻有更多的表面位置可供碰撞发生。
- Smaller particle size → larger surface area → higher collision frequency. | 颗粒越小 → 表面积越大 → 碰撞频率越高。
- Example: flour dust explodes more easily than a whole grain. | 例：面粉粉尘比整粒谷物更容易爆炸。
7. Factor: Catalysts | 因素：催化剂
A catalyst is a substance that increases the rate of a reaction without being used up itself. It provides an alternative pathway with a lower activation energy.
催化剂是一类能加快反应速率但自身不被消耗的物质。它为反应提供了活化能更低的另一条路径。
With a lower Eₐ, a far greater fraction of collisions become successful, even at the same temperature.
由于活化能降低，即使在相同温度下，也有更大比例的碰撞能够成功。
Catalysts are important in industry because they save energy and reduce costs. Examples include iron in the Haber process and enzymes in biology.
催化剂在工业中非常重要，因为它节省能量、降低成本。例如哈伯法中的铁以及生物体内的酶。
Catalyst lowers Eₐ → increases successful collisions without being consumed
催化剂降低 Eₐ → 增加有效碰撞而不被消耗
8. Calculating Average Rate | 计算平均速率
The average rate of a reaction over a period of time can be calculated by dividing the change in a measurable quantity by the time interval.
一段时间内的平均速率可以通过测量量的变化量除以时间间隔来计算。
Average rate = amount of reactant used or product formed ÷ time taken
平均速率 = 消耗的反应物量或生成的产物量 ÷ 所需时间
For example, if 24 cm³ of carbon dioxide is collected in 60 seconds, the average rate is 0.4 cm³/s.
例如，如果在 60 秒内收集到 24 cm³ 的二氧化碳，那么平均速率为 0.4 cm³/s。
- Units can be cm³/s, g/s, mol/s, or mol dm⁻³ s⁻¹. | 单位可以是 cm³/s、g/s、mol/s 或 mol dm⁻³ s⁻¹。
9. Graphs of Reaction Rates | 反应速率图
Reaction progress is often shown on a graph of product volume or reactant mass against time. The steeper the line, the faster the reaction.
反应进程通常用产物体积或反应物质量对时间的图像表示。曲线越陡，反应越快。
As reactants are used up, the reaction slows down and the graph becomes less steep until it levels off when one reactant is used up.
随着反应物消耗，反应减慢，曲线变平缓，直到某一反应物耗尽后曲线趋于水平。
To find the rate at a specific time, draw a tangent to the curve and calculate its gradient: rate = Δy / Δx.
要计算某时刻的即时速率，可作该点曲线的切线，并计算其斜率：速率 = Δy / Δx。
Gradient of tangent = instantaneous rate
切线的斜率 = 即时速率
10. Real-World Applications | 实际应用
Controlling reaction rates is essential in food preservation, medicine, and industry. For example, refrigeration slows down the spoilage of food by reducing temperature.
控制反应速率在食品保存、医药和工业中至关重要。例如，冷藏通过降低温度来减缓食物腐败。
Catalytic converters in cars speed up the breakdown of pollutants. Industrial manufacturers adjust concentration, pressure, temperature, and catalysts to maximise profit.
汽车催化转换器加速有害物质的分解。工业生产中，人们通过调节浓度、压强、温度和催化剂来最大化利润。
- Steel structures are painted or coated to prevent rusting. | 钢结构涂漆或涂层以防止生锈。
- In the body, enzymes catalyse reactions at body temperature. | 在体内，酶在体温下催化反应。
11. Summary & Exam Tips | 总结与考试提示
To answer rate-of-reaction questions well, always link observations to collision theory: more collisions, higher energy, lower activation energy.
要答好反应速率题目，务必把观察结果与碰撞理论联系起来：更多碰撞、更高能量、更低活化能。
Do not forget to state the activation energy is the minimum energy needed for a successful collision. A catalyst lowers this minimum, not the temperature.
不要忘记说明活化能是发生有效碰撞所需的最低能量。催化剂降低的是这个最低能量，而不是温度。
When calculating rates, check your units and use a tangent on curves if asked for a rate at a specific time. Always show your working.
计算速率时，注意单位；若要求某一时刻的速率，需画切线。一定要写出计算过程。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导