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 is a central idea that connects particle theory, energy changes, and industrial processes. Understanding how and why reactions occur at different speeds is essential for exams and real-world applications.

在IGCSE科学中，化学反应速率是一个核心概念，它将粒子理论、能量变化与工业生产过程联系起来。理解反应如何以及为何以不同速度进行，对考试和实际应用都至关重要。

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

The rate of reaction is the decrease in the amount of a reactant (or increase in the amount of a product) per unit time.

反应速率是单位时间内反应物数量的减少量（或产物数量的增加量）。

It is usually expressed in units such as g/min, cm³/min, mol/s, or mol/(dm³·s).

其单位通常为g/min、cm³/min、mol/s或mol/(dm³·s)。

For example, the reaction between magnesium and hydrochloric acid produces hydrogen gas: Mg + 2HCl → MgCl₂ + H₂↑. The volume of hydrogen collected at regular intervals gives the rate.

例如，镁与稀盐酸反应产生氢气：Mg + 2HCl → MgCl₂ + H₂↑。通过在不同时刻收集氢气的体积，即可得到反应速率。

2. Measuring Rates | 测量反应速率

Rates can be measured by monitoring how quickly a product forms or a reactant is used up.

速率可以通过监测产物生成的快慢或反应物消耗的快慢来测量。

Common methods include recording the volume of gas released, the loss in mass, the change in pH, the colour intensity, or the formation of a precipitate.

常用方法包括记录释放气体的体积、质量损失、pH变化、颜色强度或沉淀的生成。

When measuring mass loss, the reaction flask is placed on a digital balance and the reading is taken every 10–20 seconds.

测量质量损失时，将反应烧瓶置于电子天平上，每10–20秒记录一次读数。

A more accurate method is to use a gas syringe to collect the gas and plot the volume against time.

更精确的方法是使用气筒收集气体，并绘制体积–时间曲线。

3. Collision Theory | 碰撞理论

For a reaction to occur, reacting particles must collide with each other in the correct orientation and with enough energy.

要使反应发生，反应粒子必须以正确的方向相互碰撞，并具有足够的能量。

This minimum energy needed for a successful collision is called the activation energy (Eₐ).

这种发生有效碰撞所需的最小能量称为活化能（Eₐ）。

Only a small fraction of collisions are successful; anything that increases the frequency or energy of collisions will increase the rate.

只有一小部分碰撞是有效的；任何能增加碰撞频率或碰撞能量的因素都会加快反应速率。

Thus, the rate of reaction depends on the number of successful collisions per second.

因此，反应速率取决于每秒内有效碰撞的次数。

4. Effect of Concentration | 浓度的影响

Increasing the concentration of a reactant means more particles in the same volume, so collisions occur more frequently.

增大反应物浓度意味着相同体积内粒子更多，因此碰撞更加频繁。

For example, when hydrochloric acid is more concentrated, the reaction with sodium thiosulfate to form sulfur occurs faster.

例如，盐酸浓度越高，与硫代硫酸钠反应生成硫的速度就越快。

This results in a steeper slope on a graph of product quantity against time.

在产物量–时间图上，这会表现为更陡的斜率。

However, concentration does not affect the activation energy of the reaction.

但浓度并不改变反应的活化能。

5. Effect of Temperature | 温度的影响

Raising the temperature increases the kinetic energy of particles, making them move faster and collide more often.

升高温度会增加粒子的动能，使它们运动更快，碰撞更频繁。

More importantly, a greater proportion of particles now have energy equal to or greater than the activation energy.

更重要的是，现在有更高比例的粒子具有等于或大于活化能的能量。

As a rough rule, every 10°C rise in temperature roughly doubles the rate of a reaction.

粗略估算，温度每升高10°C，反应速率大约翻倍。

This has a much stronger effect than simply increasing the number of collisions.

这种影响远大于单纯增加碰撞次数所带来的效果。

6. Effect of Surface Area | 表面积的影响

Breaking a solid into smaller pieces increases its total surface area to volume ratio.

将固体粉碎成小块会增加其总体表面积与体积之比。

With more exposed particles, collisions between the solid and the liquid or gas are more frequent.

由于暴露的粒子更多，固体与液体或气体之间的碰撞更加频繁。

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

例如，粉末状碳酸钙与稀酸的反应速度比等质量的石灰石片更快。

In examinations, you should be able to explain why a powder reacts more quickly using collision theory.

在考试中，你需要能用碰撞理论解释为什么粉末反应更快。

7. Effect of Catalysts | 催化剂的影响

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

催化剂是一种能加快反应速率而在反应过程中自身不被消耗的物质。

It works by providing an alternative reaction pathway with a lower activation energy.

它通过提供一条活化能更低的替代反应途径来起作用。

This means more particles have enough energy to react, so successful collisions become more frequent.

这意味着更多粒子拥有足够的能量参与反应，因此有效碰撞更加频繁。

Catalysts are specific: different reactions require different catalysts. For example, manganese(IV) oxide speeds up the decomposition of hydrogen peroxide.

催化剂具有选择性：不同的反应需要不同的催化剂。例如，二氧化锰可加快过氧化氢的分解反应。

8. Calculating Average Rate | 计算平均速率

The average rate over a time interval is calculated as the change in quantity divided by the time taken.

在一段时间间隔内的平均速率等于量的变化量除以所用时间。

rate = (change in quantity) ÷ (time taken)

速率 = 量的变化量 ÷ 所用时间

For example, if 40 cm³ of carbon dioxide is released in 80 seconds, the average rate is 40 ÷ 80 = 0.5 cm³/s.

例如，若80秒内放出40 cm³二氧化碳，则平均速率为40 ÷ 80 = 0.5 cm³/s。

At the start of a reaction, the rate is fastest; it gradually slows down as reactants are used up.

反应刚开始时速率最快；随着反应物消耗，速率逐渐减慢。

9. Graphing Reaction Rates | 反应速率曲线图

When the amount of product is plotted against time, the curve is steep at the start and becomes flat as the reaction finishes.

当以产物量为纵坐标、时间为横坐标作图时，曲线开始时较陡，反应结束时趋于平坦。

The gradient (slope) at any point represents the instantaneous rate at that moment.

曲线上任意一点的梯度（斜率）代表该时刻的瞬时速率。

A steeper line means a faster reaction. If the line is horizontal, the reaction has stopped.

直线越陡表示反应越快。如果直线水平，则反应已经停止。

To compare different experiments, you can draw tangents to the curves and compare their slopes.

要比较不同的实验，可以画出曲线的切线并比较其斜率。

10. Reversible Reactions and Equilibrium | 可逆反应与平衡

Some reactions are reversible, meaning the products can react to form the original reactants again.

有些反应是可逆的，即产物可以再次反应生成原有反应物。

In a closed system, when the forward and backward rates become equal, the reaction has reached dynamic equilibrium.

在封闭体系中，当正反应速率与逆反应速率相等时，反应就达到了动态平衡。

At equilibrium, concentrations of reactants and products remain constant, but both reactions still occur.

平衡时，反应物和产物的浓度保持不变，但正逆反应仍在进行。

Changing concentration, pressure (for gases), or temperature can shift the position of equilibrium, according to Le Chatelier’s principle.

根据勒夏特列原理，改变浓度、压力（对气体而言）或温度，会使平衡位置发生移动。

11. Industrial Applications | 工业应用

The Haber process for making ammonia uses an iron catalyst and a temperature of about 450°C.

哈伯法制氨使用铁催化剂，温度为约450°C。

A compromise between rate and yield is needed: higher temperature increases rate but lowers the equilibrium yield.

需要在速率和产率之间折中：温度更高可加快反应速率，但会降低平衡产率。

Similarly, in the Contact process for manufacturing sulfur trioxide, vanadium(V) oxide is used as a catalyst.

同样，在接触法制备三氧化硫的过程中，使用五氧化二钒作为催化剂。

Catalysts allow reactions to proceed faster at lower temperatures, saving energy and reducing costs.

催化剂使反应在较低温度下也能更快进行，从而节省能源、降低成本。

12. Common Mistakes and Exam Tips | 常见错误与考试技巧

Do not confuse rate of reaction with equilibrium position; rate is about how quickly, equilibrium is about how much.

不要将反应速率与平衡位置混淆；速率涉及的是“多快”，而平衡涉及的是“多少”。

When answering ‘explain’ questions, always mention both the frequency of collisions and the energy of collisions.

回答“解释”类问题时，务必同时提到碰撞频率和碰撞能量。

Use the correct units in your calculations, and always show your working clearly.

在计算中使用正确的单位，并清晰展示计算过程。

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Rates of Reaction | 化学反应速率