Understanding Rates of Reaction | 理解化学反应速率

📚 Understanding Rates of Reaction | 理解化学反应速率

Rates of reaction is a central topic in IGCSE Science (Edexcel). It explains how quickly chemical reactions proceed and the factors that influence them. This article provides a clear, exam-focused revision guide with real-world examples.

化学反应速率是IGCSE科学（爱德思）的核心主题。它解释了化学反应进行的快慢以及影响反应的因素。本文提供一份清晰、紧扣考点的复习指南，并配有真实生活实例。

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

The rate of a chemical reaction measures how fast reactants are used up or products are formed per unit time. It is usually expressed as change in concentration, mass, or volume divided by time.

化学反应速率衡量的是单位时间内反应物被消耗或产物生成的快慢。通常用浓度、质量或体积的变化量除以时间来表示。

For example, when magnesium reacts with hydrochloric acid, the mass of the magnesium decreases, and hydrogen gas is produced. The rate can be found by measuring the loss in mass or the volume of gas released.

例如，镁与盐酸反应时，镁的质量减少，同时生成氢气。我们可以通过测量质量减少量或释放气体的体积来求出反应速率。

Rate = Amount of reactant used or product formed ÷ Time taken

反应速率 = 反应物消耗量或产物生成量 ÷ 所用时间

2. Measuring Rates | 测量反应速率

You can measure rate in several ways. The most common methods measure the volume of gas produced, the mass loss, or the change in colour and turbidity.

测量反应速率的方法有多种。最常见的是测量生成气体的体积、质量减少量，或者颜色和浑浊度的变化。

If the reaction produces a gas, a gas syringe can collect the gas every 10 seconds, allowing a graph of gas volume against time to be plotted.

如果反应产生气体，可以用气密注射器每隔10秒收集气体，然后绘制气体体积随时间变化的曲线。

If the reaction is carried out open to the air, the mass reading will decrease as the gas escapes. This method is simple but only works if the gas is dense enough to give a measurable loss.

如果反应在敞口容器中进行，随着气体逸出，读数会减小。这种方法简单，但仅当气体密度较大、能产生可测量的质量损失时才适用。

3. Collision Theory and Activation Energy | 碰撞理论与活化能

For a reaction to happen, reactant particles must collide with each other. However, not every collision results in a reaction.

反应发生的前提是反应物粒子相互碰撞。然而，并非每一次碰撞都能引发反应。

The particles must collide with enough energy to break the existing bonds. The minimum energy needed for a successful collision is called the activation energy, Eₐ.

粒子必须具有足够的能量才能破坏原有化学键。使碰撞成功所需的最低能量称为活化能，记作 Eₐ。

Increasing the number of successful collisions increases the rate of reaction. Any change that increases the frequency or energy of collisions will make the reaction faster.

有效碰撞次数增加，反应速率就会加快。任何提高碰撞频率或碰撞能量的改变，都会使反应更快。

4. Surface Area | 表面积

When a solid reactant is broken into smaller pieces, its total surface area increases. This exposes more particles to collide with particles of 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。这会暴露出更多粒子，使其能与另一种反应物的粒子碰撞。

For example, powdered calcium carbonate reacts with hydrochloric acid much faster than large marble chips. Powder has a larger surface area, so the frequency of collisions per unit time is higher.

例如，粉末状碳酸钙与盐酸反应的速度远大于块状大理石。粉末的表面积更大，因此单位时间内的碰撞次数更多。

In the lab, you can observe this by timing how long it takes for the same mass of powder and chips to fully react, measuring the gas volume at regular intervals.

在实验室中，你可以称取相同质量的粉末和块状固体，分别测量完全反应所需的时间，并定期记录气体体积来观察这一差异。

5. Temperature | 温度

Raising the temperature increases the average kinetic energy of particles. Particles move faster, so they collide more frequently.

升高温度会增加粒子的平均动能。粒子运动加快，因此碰撞更加频繁。

More importantly, a higher temperature means a larger fraction of particles have energy greater than or equal to the activation energy. This dramatically increases the number of successful collisions.

更重要的是，温度升高意味着有更大比例的粒子具有超过或等于活化能的能量。这极大地增加了有效碰撞的次数。

As a general rule, increasing the temperature by 10 °C roughly doubles the rate of reaction, although this is not exact for every reaction.

一般来说，温度每升高10 °C，反应速率大约翻倍，尽管这一规律并非对所有反应都完全精确。

T ↑ → kinetic energy ↑ → successful collisions ↑ → rate ↑

温度↑ → 动能↑ → 有效碰撞↑ → 速率↑

6. Concentration and Pressure | 浓度与压强

In solutions, increasing the concentration of a dissolved reactant means there are more particles in the same volume. This increases the frequency of collisions.

在溶液中，增大反应物浓度意味着相同体积内有更多粒子。这提高了碰撞频率。

For gases, increasing the pressure pushes the particles closer together, reducing the volume and increasing the number of particles per unit volume. This also increases collision frequency.

对于气体，增大压强会迫使粒子靠得更近，体积缩小，单位体积内的粒子数增加。这同样会提高碰撞频率。

Neither concentration nor pressure changes the activation energy. They speed up the reaction purely by making collisions happen more often.

浓度和压强都不会改变活化能。它们只是通过让碰撞发生得更频繁来加速反应。

A common exam question asks you to explain why a reaction is faster when the concentration is doubled. The answer should mention more particles, more frequent collisions, and therefore more successful reactions.

常见的考题要求解释为什么浓度加倍后反应更快。答案应提到粒子更多、碰撞更频繁，因此有效反应更多。

7. Catalysts | 催化剂

A catalyst is a substance that speeds up a chemical reaction without being used up. It provides an alternative reaction pathway with a lower activation energy.

催化剂是一种能加快化学反应速率而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途径。

Lower activation energy means that at the same temperature, more particles have enough energy to react. Therefore more successful collisions occur every second.

活化能越低，意味着在相同温度下，有更多粒子拥有足够的能量去反应。因此每秒产生的有效碰撞更多。

Catalysts are important in industry because they allow reactions to run at lower temperatures, saving energy and reducing costs. Examples include iron in the Haber process and platinum in the contact process.

催化剂在工业上非常重要，因为它能使反应在较低温度下进行，从而节省能源、降低成本。例如哈伯工艺中的铁和接触法中的铂。

In an exam, always state that a catalyst is chemically unchanged at the end of the reaction, not that it lowers the temperature of the reaction.

考试时，务必说明催化剂在反应结束时化学性质不变，而不是说它降低了反应温度。

8. Reaction Graphs | 反应曲线图

Reaction rate can be shown using graphs. A typical graph plots the volume of gas produced or mass lost against time.

反应速率可以用曲线图表示。常见的是绘制生成气体体积或质量损失随时间变化的曲线。

At the start, the curve is steep because the reaction is fastest. As reactants are used up, the curve becomes less steep. Finally, it flattens when one reactant is completely used up.

开始时曲线最陡，因为反应最快。随着反应物消耗，曲线逐渐变缓。最后，当某一种反应物完全耗尽时，曲线趋于水平。

The average rate over a period of time is calculated from the gradient of the straight line drawn between two points on the curve.

一段时间内的平均速率可以通过连接曲线上两点所画直线的斜率来计算。

For the instantaneous rate at a specific time, draw a tangent to the curve at that point and calculate the gradient of the tangent.

若要计算某一时刻的瞬时速率，需要在该点作曲线的切线，并求出切线的斜率。

9. Calculating Rate from Graphs | 根据图像计算速率

Consider a reaction that produces carbon dioxide. The volume of gas is recorded every 15 seconds.

假设一个反应生成二氧化碳。每15秒记录一次气体体积。

If 40 cm³ of gas is produced after 20 seconds, the average rate during this period is:

如果20秒后生成了40 cm³气体，则这一时期的平均速率为：

Rate = 40 cm³ ÷ 20 s = 2.0 cm³/s

反应速率 = 40 cm³ ÷ 20 s = 2.0 cm³/s

To find the rate at 50 seconds, draw a tangent to the curve at t = 50 s. If the tangent drops from 75 cm³ at 0 s to 15 cm³ at 80 s, the gradient is (75 – 15) ÷ (0 – 80) = 60 ÷ 80 = 0.75 cm³/s. The negative sign simply shows that the calculation direction is reversed.

要求50秒时的速率，在t = 50 s处作切线。如果切线上两个点分别为0秒时75 cm³和80秒时15 cm³，那么斜率为(75 – 15) ÷ (0 – 80) = 60 ÷ 80 = 0.75 cm³/s。负号仅表示计算方向相反。

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

Some reactions are reversible. The products can react together to re-form the reactants. When the forward and backward reactions occur at the same rate, the system is in dynamic equilibrium.

有些反应是可逆的。产物可以重新反应生成反应物。当正反应和逆反应速率相等时，体系处于动态平衡状态。

At equilibrium, the concentrations of reactants and products remain constant, but molecules are still reacting. This is an important concept in the Haber process and the contact process.

平衡时，反应物和产物的浓度保持恒定，但分子仍在不断反应。这是哈伯工艺和接触法中的重要概念。

Changing conditions such as temperature, pressure, or concentration can shift the position of equilibrium, according to Le Chatelier’s principle. A catalyst does not change the equilibrium position; it only helps the system reach equilibrium faster.

改变温度、压强或浓度会使平衡位置移动，这符合勒夏特列原理。催化剂不会改变平衡位置，只是帮助体系更快达到平衡。

In IGCSE exams, you may be asked to predict which conditions favour the forward reaction. Always link this to energy changes and the number of gas molecules on each side.

在IGCSE考试中，你可能会被要求判断哪些条件有利于正反应。一定要将判断依据与能量变化以及两侧气体分子数目联系起来。

11. Industrial Application: The Haber Process | 工业应用：哈伯工艺

The Haber process produces ammonia from nitrogen and hydrogen. The reaction is:

哈伯工艺利用氮气和氢气合成氨。反应方程式为：

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ/mol

This reaction is exothermic, so a lower temperature would increase the yield of ammonia at equilibrium. However, lower temperatures make the reaction too slow.

该反应放热，因此降低温度会提高平衡时氨的产率。但温度过低会导致反应速率过慢。

A temperature of about 450 °C and an iron catalyst are used as a compromise. A high pressure of 200 atm increases the yield because there are more gas molecules on the left side.

工业上采用约450 °C的温度和铁催化剂，作为平衡与速率的折中。200 atm的高压会增加产率，因为左侧气体分子总数更多。

In answers, mention that the catalyst allows a faster rate at a lower temperature, reducing energy costs without lowering equilibrium yield.

回答时，要提到催化剂使反应在较低温度下仍能保持较快速率，从而节约能源，同时不降低平衡产率。

12. Common Exam Mistakes and Revision Tips | 常见考试错误与复习建议

One common mistake is saying that increasing concentration increases the energy of particles. Concentration only affects collision frequency, not particle energy.

常见错误之一是说增大浓度会增加粒子能量。浓度只影响碰撞频率，不影响粒子能量。

Another mistake is confusing the terms ‘rate’ and ‘extent’. Rate is how fast a reaction proceeds; extent is how much product is formed at equilibrium. A catalyst affects rate only, not extent.

另一个错误是混淆“速率”和“限度”。“速率”指反应进行得快慢；“限度”指平衡时生成产物的多少。催化剂只影响速率，不影响限度。

When drawing graphs, label both axes with units. Mark the point where the reaction stops when one reactant runs out. Compare steepness to judge which reaction is faster.

绘制曲线图时，请标出坐标轴和单位。在某种反应物耗尽、反应停止的位置做出标记。通过比较曲线陡峭程度来判断哪个反应更快。

Finally, always use the words ‘successful collisions’, ‘activation energy’ and ‘frequency of collisions’ in explanations. These keywords earn marks in the Edexcel mark scheme.

最后，在解释中务必使用“有效碰撞”、“活化能”和“碰撞频率”等关键词。这些词语能在爱德思考评标准中获得分数。

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Understanding Rates of Reaction | 理解化学反应速率