Rates of Reaction | 反应速率

📚 Rates of Reaction | 反应速率

The rate of a chemical reaction tells us how fast reactants are converted into products. Whether it is a metal fizzing in acid or food spoiling in the fridge, the same underlying principles apply. Understanding rates of reaction helps us control processes in industry, medicine and daily life.

化学反应速率告诉我们反应物转化为产物的快慢。无论是金属在酸中嘶嘶冒泡，还是食物在冰箱中变质，背后遵循的都是同样的原理。理解反应速率，有助于我们在工业、医学和日常生活中控制化学过程。

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

Rate of reaction is the change in quantity of a reactant or product per unit time. It can be measured as the speed at which reactants are used up or products are formed.

反应速率是指单位时间内反应物或产物数量的变化。它可以被定义为反应物被消耗的速度，或者产物生成的速度。

For a reaction such as A → B, the general definition is:

对于反应 A → B，一般的定义是：

Rate = Δquantity ÷ Δtime

Common units include g/s, cm³/s, mol/s and 1/s. A fast reaction has a large rate value; a slow reaction has a small one. For example, an explosion happens in a fraction of a second, while the rusting of iron takes months or years.

常用单位有 g/s、cm³/s、mol/s 和 1/s。反应越快，速率值越大；反应越慢，速率值越小。例如，爆炸在不到一秒内发生，而铁生锈则需数月或数年。

2. Measuring Rates of Reaction | 测量反应速率

Scientists use several practical methods to follow a reaction. The most important in the Edexcel IGCSE course are measuring gas volume, measuring mass loss, and following colour changes.

科学家用多种实验方法来追踪一个反应的进程。在 Edexcel IGCSE 课程中，最重要的是测量气体体积、测量质量损失以及观察颜色变化。

Gas syringe: attach a syringe to the reaction flask and record the volume of gas every 10 seconds.

气体注射器：将注射器连接到反应烧瓶上，每 10 秒记录一次气体体积。
Mass balance: if a gas escapes, the mass of the flask decreases; take readings at regular intervals.

电子天平：如果有气体逸出，烧瓶的总质量会减少；每隔固定时间读取一次读数。
Colour change or turbidity: time how long it takes for a solution to become cloudy or for a colour to disappear.

颜色变化或浑浊度：记录溶液变浑浊或颜色消失所需的时间。

Each method suits different reactions. For example, the reaction between marble chips and hydrochloric acid produces carbon dioxide, so gas volume or mass loss works well.

每种方法适用于不同的反应。例如，大理石碎片与盐酸反应会产生二氧化碳，因此很适合用气体体积或质量损失来测量。

3. Collision Theory | 碰撞理论

For particles to react, they must collide with each other. However, not every collision causes a chemical reaction.

粒子要发生反应，必须相互碰撞。然而，并不是每次碰撞都会引发化学反应。

Two conditions must be met for an effective collision:

一次有效碰撞必须满足两个条件：

The particles must collide with enough energy to break existing bonds, at least equal to the activation energy.

粒子碰撞时必须具备足够能量来断裂原有化学键，至少等于活化能。
The particles must be correctly oriented so that the reactive parts meet.

粒子必须以正确的方向碰撞，使反应部位相互接触。

The activation energy is the minimum energy needed for a reaction to start. Any factor that increases the frequency or the energy of collisions will therefore speed up the reaction.

活化能是反应启动所需的最小能量。因此，任何能提高碰撞频率或碰撞能量的因素，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 surface area increases. This means more particles are exposed to the other reactant.

当固体反应物被粉碎成更小的颗粒时，其总表面积增大。这意味着更多粒子暴露在另一种反应物的接触面上。

Larger surface area → more frequent collisions → faster reaction. For example, powdered calcium carbonate reacts with acid much faster than large marble chips.

表面积越大 → 碰撞越频繁 → 反应越快。例如，粉末状碳酸钙与酸的反应速度远快于大块大理石。

In an experiment, the same mass of powder produces the same total volume of gas but in a shorter time. The final amount of product is unchanged; only the speed is different.

在实验中，相同质量的粉末会产生相同总量的气体，但所需时间更短。产物的最终总量不变，改变的只是反应速度。

5. Concentration and Pressure | 浓度与压强

Increasing the concentration of a solution means more particles are present in the same volume. The particles are closer together, so collisions happen more frequently.

增大溶液浓度意味着相同体积内有更多粒子。粒子之间距离更近，因此碰撞更频繁。

For gases, increasing the pressure has the same effect: the gas particles are compressed into a smaller space, so the number of collisions per second increases.

对于气体来说，增大压强有同样的效果：气体粒子被压缩到更小空间，每秒碰撞次数增加。

Higher concentration or pressure → more frequent collisions → faster rate. Note that these factors do not change the energy of individual particles; they only change how often collisions occur. This is why increasing concentration has a much smaller effect than increasing temperature.

更高的浓度或压强 → 碰撞更频繁 → 反应更快。注意，这些因素并不会改变单个粒子的能量，只会改变碰撞发生的频率。这就是为什么增大浓度的影响远小于升高温度。

6. Temperature | 温度

Raising the temperature affects reactions in two important ways. First, particles move faster, so collisions are more frequent. Second, particles have more kinetic energy, so a greater fraction of collisions exceed the activation energy.

升高温度通过两个重要方式影响反应。第一，粒子运动更快，碰撞更频繁。第二，粒子具有更多动能，因此超过活化能的碰撞比例更大。

The second effect is often more significant. A general rule of thumb is that for many reactions, a 10 °C rise in temperature roughly doubles the rate.

第二个效应往往更重要。一般经验法则是，对许多反应来说，温度每升高 10 °C，反应速率大约翻一倍。

This is why food is stored in refrigerators: low temperature slows down the chemical reactions that cause food to spoil.

这就是为什么食物存放在冰箱中：低温会减慢导致食物变质的化学反应。

7. Catalysts | 催化剂

A catalyst is a substance that speeds up a chemical reaction without being chemically changed at the end. It works by providing an alternative pathway with a lower activation energy.

催化剂是一种能加快化学反应速率、而在反应结束后自身不发生化学变化的物质。它通过提供一条活化能更低的新反应路径来起作用。

Because the activation energy barrier is lower, a much larger fraction of collisions are successful at the same temperature.

由于活化能壁垒降低，在同一温度下，有效碰撞的比例大大增加。

Catalysts are very important in industry. For example, iron is used in the Haber process to make ammonia, and enzymes act as biological catalysts inside living organisms.

催化剂在工业中非常重要。例如，铁被用于哈伯法制氨，而酶则作为生物催化剂在生物体内起作用。

IMPORTANT: A catalyst is not used up in the reaction, so it can be reused. It does not change the amount of product formed, only how quickly it forms.

要点：催化剂在反应中不会被消耗，因此可以重复使用。它不会改变产物的总量，只会改变产物生成的速度。

8. Reaction Graphs and Calculations | 反应图像与计算

Rate of reaction is often represented graphically. On a mass-loss or gas-volume graph, the curve starts steep and gradually becomes flat as the reaction slows down and finally stops.

反应速率通常用图像来表示。在质量损失或气体体积图像中，曲线一开始较陡，随后逐渐变平，表示反应变慢并最终停止。

The steeper the slope at any point, the faster the reaction is at that moment. The slope becomes gentler because as reactants are used up, collisions become less frequent.

任意时刻曲线越陡，说明此刻反应越快。斜率变缓是因为反应物逐渐被消耗，碰撞频率下降。

To calculate the average rate from the curve:

要从曲线计算平均速率：

Average rate = total change ÷ total time

Example: if a reaction produces 80 cm³ of gas in 40 s, the average rate is 80 ÷ 40 = 2 cm³/s.

例如：如果某反应在 40 秒内产生 80 cm³ 气体，则平均速率为 80 ÷ 40 = 2 cm³/s。

To find the rate at a particular moment, draw a tangent to the curve at that point and calculate its gradient.

要计算某一时刻的瞬时速率，可在该点画切线，然后求出切线的斜率。

9. Reversible Reactions and Dynamic Equilibrium | 可逆反应与动态平衡

Some reactions can proceed in both directions. In a reversible reaction, the products can react to reform the reactants. This is shown with the symbol ⇌.

有些反应可以同时向两个方向进行。在可逆反应中，产物可以反应重新生成反应物，用符号 ⇌ 表示。

When the forward and backward reactions happen at the same rate, the system is in dynamic equilibrium. At this point, the concentrations of reactants and products stay constant — not because the reaction has stopped, but because both directions are proceeding at equal speeds.

当正反应和逆反应速率相等时，系统处于动态平衡。此时反应物和产物的浓度保持恒定——不是因为反应停止了，而是因为两个方向正以同样速度进行。

Changing temperature, pressure or concentration can shift the position of equilibrium, which is an important application of rate concepts in industrial chemistry.

改变温度、压强或浓度可以移动平衡的位置，这是速率概念在工业化学中的重要应用。

10. Real-World Applications | 实际应用

Knowledge of reaction rates is used everywhere. In the chemical industry, choosing the right temperature, pressure and catalyst saves time and money.

反应速率的知识应用广泛。在化学工业中，选择合适的温度、压强和催化剂可以节省时间和金钱。

In medicine, drug tablets are designed to dissolve at controlled rates; aspirin is finely divided so that it works quickly. In cooking, food is cut into small pieces to cook faster because the surface area increases.

在医学中，药片被设计为以受控速率溶解；阿司匹林被制成细粉以便快速起效。在烹饪中，食物被切碎后熟得更快，因为表面积增大了。

In reverse, slowing reactions is also useful: painting iron prevents oxygen and water from reaching the metal, slowing rusting.

反过来，减慢反应同样有用：给铁涂漆可阻止氧气和水接触金属，从而减缓生锈。

11. Common Mistakes and Exam Tips | 常见错误与考试提示

Students often confuse concentration and surface area effects, or forget that catalysts are not used up. Here are the key points to remember:

学生经常混淆浓度和表面积的影响，或者忘记催化剂不会被消耗。以下是需要记住的要点：

Only successful collisions lead to reaction: they must have enough energy and the correct orientation.

只有有效碰撞才能引发反应：它们必须有足够的能量和正确的方向。
Graph slope = rate. A steeper curve means a faster reaction at that moment; a horizontal line means the reaction has stopped.

图像斜率 = 速率。曲线越陡，表示此刻反应越快；水平线表示反应已经停止。
Catalysts lower activation energy; they do not appear in the overall equation and are not used up.

催化剂降低活化能；它们不出现在总反应方程式中，也不会被消耗。
Always quote units in rate calculations, e.g. cm³/s or g/s.

在速率计算中始终写上单位，例如 cm³/s 或 g/s。
Remember: a faster rate does not change the final yield of product, only the time taken to reach it.

记住：更快的速率不会改变产物的最终产量，只会缩短达到最终产量所需的时间。

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Rates of Reaction | 反应速率