Rates of Reaction | 反应速率

📚 Rates of Reaction | 反应速率

Rates of reaction are a central theme in IGCSE Science. They explain how fast chemical reactions happen and why different conditions change the speed of a reaction.

反应速率是 IGCSE 科学的核心主题之一。它解释化学反应进行得快慢，以及为什么不同条件会改变反应的速度。

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

The rate of a reaction measures how quickly reactants are used up or how quickly products are formed. A fast reaction, like burning magnesium, releases energy in seconds. A slow reaction, like rusting of iron, may take months or years.

反应速率衡量反应物被消耗得快慢，或者产物生成得多快。像镁条燃烧这样的快速反应，在几秒钟内就释放能量；而像铁生锈这样的慢反应，可能需要数月甚至数年。

To measure rate, we compare the change in a measurable quantity, such as mass, volume or colour, with the time taken.

要测量速率，我们需要将某个可测量的量的变化，例如质量、体积或颜色变化，与所用时间进行比较。

Rate = change in quantity ÷ change in time

2. The Collision Theory | 碰撞理论

For a reaction to occur, particles must collide with each other. However, not every collision leads to a reaction. Only collisions with enough energy, called effective collisions, produce a change.

反应发生的前提是粒子之间发生碰撞。但是，并非每一次碰撞都能引发反应。只有那些具有足够能量、能够产生变化的碰撞，即有效碰撞，才能引发反应。

The minimum energy needed for a collision to be successful is called the activation energy, Eₐ. If particles collide with less than this energy, they just bounce apart.

一次碰撞能够成功所需的最小能量称为活化能（Eₐ）。如果粒子碰撞时的能量低于活化能，它们只会弹开。

Successful collision = correct orientation + energy ≥ Eₐ

3. Factors Affecting Reaction Rate | 影响反应速率的因素

There are five main factors that can change how fast a reaction proceeds. These are temperature, concentration, pressure, surface area and the presence of a catalyst.

有五个主要因素可以改变反应进行的快慢。它们是温度、浓度、压强、接触面积以及催化剂的存在。

Temperature: increasing temperature speeds up movement and gives particles more energy.
温度：升高温度使粒子运动加快，并赋予粒子更多能量。
Concentration: more particles in the same volume means more frequent collisions.
浓度：相同体积内粒子数越多，碰撞越频繁。
Pressure: for gases, higher pressure pushes particles closer together.
压强：对气体而言，压强越大，粒子被压得越紧密。
Surface area: smaller pieces create more exposed surface area for collisions.
接触面积：颗粒越小，暴露的表面积越大，碰撞的机会越多。
Catalyst: a catalyst lowers activation energy without being used up.
催化剂：催化剂能降低活化能，而自身不会被消耗。

4. Measuring Reaction Rates | 测量反应速率

Reaction rates can be followed by measuring changes in mass, gas volume, pH, colour or turbidity. For example, in a reaction producing carbon dioxide, we can measure the mass loss from the container as gas escapes.

反应速率可以通过测量质量、气体体积、pH、颜色或浑浊度的变化来追踪。例如，在产生二氧化碳的反应中，我们可以随着气体逸出，测量容器总质量的减少。

When a gas is collected, a gas syringe is often used. The volume of gas is recorded at regular time intervals.

当收集气体时，通常使用气体注射器。每隔一定时间记录气体的体积。

For reactions that change colour, a colorimeter can provide a precise measurement. Alternatively, you can time how long it takes for the solution to become opaque.

对于发生颜色变化的反应，可以使用色度计进行精确测量。或者，也可以计时直到溶液变得不透明为止。

5. How to Calculate the Rate | 如何计算反应速率

From experimental data, the average rate can be calculated using the total change divided by the total time. For example, if 50 cm³ of gas is produced in 20 seconds, the average rate is 2.5 cm³/s.

根据实验数据，平均速率可以用总变化量除以总时间来计算。例如，如果在20秒内产生了50 cm³气体，那么平均速率就是2.5 cm³/s。

However, reactions slow down as reactants are used up. Therefore, the initial rate (the rate at time zero) is often used for comparison, especially when studying how concentration affects the rate.

然而，随着反应物的消耗，反应会减慢。因此，在进行比较时，尤其是在研究浓度对速率的影响时，常用初始速率（时间零点的速率）作为对比。

For calculating initial rate, draw a tangent to the curve at t = 0 and find its gradient.

计算初始速率时，可在 t = 0 处画曲线的切线，然后求出该切线的斜率。

Gradient = Δy ⁄ Δx

6. Concentration and Pressure | 浓度和压强的影响

Increasing the concentration of a reactant adds more particles to the same volume. This increases the collision frequency, so the rate of reaction increases.

增加反应物浓度，相当于在相同体积中加入更多粒子。这提高了碰撞频率，因此反应速率也随之上升。

For gases, increasing pressure has the same effect. The gas particles are compressed into a smaller space, so they collide more often.

对于气体，增大压强也有同样的效果。气体粒子被压缩到更小的空间内，因而碰撞更加频繁。

Higher concentration also increases the chance that collisions have enough energy, because with more particles there are more opportunities for high-energy impacts.

较高的浓度也增加了碰撞具备足够能量的机会，因为粒子越多，出现高能碰撞的机会也越多。

7. Temperature and Activation Energy | 温度与活化能

Temperature has a dramatic effect on reaction rate. A small rise, often about 10°C, can double the rate of many reactions.

温度对反应速率有显著影响。通常温度每升高大约10°C，许多反应的速率就会翻倍。

Heating the mixture makes particles move faster, so they collide more often. More importantly, a higher temperature means a larger proportion of particles have energy greater than the activation energy Eₐ.

加热混合物使粒子运动更快，因此碰撞更频繁。更重要的是，温度升高意味着有更大比例的粒子具有超过活化能 Eₐ 的能量。

Therefore, not only do collisions happen more often, but a higher fraction of them are effective collisions.

因此，碰撞不仅更频繁，而且它们之中有效碰撞的比例也更高。

8. Surface Area and Particle Size | 表面积和颗粒大小

For solids, only particles at the surface are available for collisions. If a solid is broken into smaller pieces, its total surface area increases without changing the total mass.

对于固体来说，只有表面的粒子才能参与碰撞。如果将固体破碎成更小的颗粒，其总表面积会在质量不变的情况下增加。

For example, a 2 g lump of calcium carbonate reacts slower than 2 g of fine powder at the same temperature and concentration of acid. The powder exposes more particles to the acid.

例如，在相同温度和相同酸浓度下，2 g块状碳酸钙比2 g细小粉末反应得更慢。粉末使更多粒子暴露在酸中。

Increasing surface area increases the collision frequency between reactants, which increases the rate.

增加表面积提高了反应物之间的碰撞频率，从而使反应速率增大。

9. Catalysts | 催化剂

A catalyst is a substance that speeds up a chemical reaction but stays chemically unchanged at the end. It provides an alternative route with a lower activation energy.

催化剂是一种能加快化学反应速率，但反应结束后自身化学性质不变的物质。它为反应提供了一条具有较低活化能的替代路径。

Because the activation energy is lower, a much larger number of collisions become effective, even at moderate temperatures.

由于活化能降低了，即使在中等温度下，也有多得多的碰撞变成有效碰撞。

Catalysts are specific: different reactions need different catalysts.
催化剂具有专一性：不同反应需要不同的催化剂。
Catalysts are not used up, so a small amount can catalyse a large amount of reaction.
催化剂不会被消耗，因此少量催化剂能催化大量反应。
Catalysts help industry save energy and money.
催化剂帮助工业界节约能源和资金。

10. Graphical Analysis | 图表分析

When product formation is plotted against time, the curve is steep at first and then becomes flatter. The gradient at any point equals the rate at that moment.

当把产物的生成量对时间作图时，曲线起初很陡，然后逐渐变平。曲线上任一点的斜率等于该时刻的速率。

As reactants are used up, the graph becomes horizontal because the reaction stops when one reactant is completely used up.

随着反应物被消耗，曲线逐渐变为水平，这是因为当某一种反应物完全耗尽时，反应就停止了。

Time interval	Gradient	Rate
Early, near t = 0	Steep	High
Later	Less steep	Lower
Finally	Zero	Zero

11. Real-World Applications | 实际应用

Controlling reaction rate is essential in everyday life. Food spoils slowly at low temperatures, so fridges slow down the bacteria-catalysed reactions that cause decay.

控制反应速率在日常生活中的应用非常重要。低温下食物腐败得慢，因此冰箱能减缓由细菌催化的、导致食物腐败的反应。

In industry, catalysts are used to make fertilisers, plastics and medicines efficiently. For example, the Haber process uses an iron catalyst to increase the rate of ammonia production.

在工业中，催化剂被用来高效地制造化肥、塑料和药品。例如，哈伯法使用铁催化剂来提高氨的生产速率。

Surface area is also important. Flour dust in a mill can explode because the tiny powder particles have a huge surface area and react with oxygen very quickly.

表面积也很重要。面粉厂的粉尘可能爆炸，因为微小的粉末颗粒具有巨大的表面积，能与氧气极快地反应。

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

A typical question will give a table of gas volume against time and ask you to calculate the average rate over a chosen interval. You must read the graph values exactly and use the correct units.

典型考题会给出气体体积随时间的表格数据，并要求你计算某一时间段内的平均速率。你必须准确读取图表数值，并使用正确的单位。

Another common task is to compare two experiments with different conditions, such as changing particle size. You should state which curve is steeper and explain why using collision theory.

另一个常见任务是比较两个不同条件的实验结果，例如改变颗粒大小。你应该说明哪条曲线更陡，并用碰撞理论解释原因。

Remember to include keywords in your answer: more frequent collisions, higher proportion of particles with energy greater than Eₐ, and effective collisions.

记住在答案中使用关键词：碰撞更频繁、具有超过Eₐ能量的粒子比例更高、有效碰撞。

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Rates of Reaction | 反应速率