📚 Rates of Reaction | 反应速率
In IGCSE Edexcel Science, rates of reaction is one of the most frequently tested topics in both Papers 1 and 2. Understanding how quickly reactants turn into products, and what changes that speed, is essential for obtaining full marks in data-analysis questions and core practical questions.
在 IGCSE Edexcel 科学考试中，反应速率是 Paper 1 和 Paper 2 中最常考的知识点之一。理解反应物转化为产物的快慢，以及哪些因素会改变这一速度，对于在数据分析和核心实验题中拿到满分至关重要。
1. What is Rate of Reaction? | 什么是反应速率？
The rate of a reaction tells us how fast a chemical change happens. You can measure it by tracking how quickly a reactant is used up or how quickly a product appears.
反应速率告诉我们化学反应发生的快慢。你可以通过追踪反应物消耗的快慢或产物生成快慢来测量它。
Rate is defined as the change in the amount of a reactant or product divided by the time taken for that change.
速率的定义是：反应物或产物的数量变化量除以发生该变化所花的时间。
rate of reaction = amount of reactant used or product formed ÷ time taken
Common units used in Edexcel practical questions include:
Edexcel 实验题中常用的单位包括：
- g/s — mass lost or gained per second | g/s — 每秒损失或增加的质量
- cm³/s — volume of gas produced per second | cm³/s — 每秒生成的气体体积
- mol/dm³/s — concentration change per second | mol/dm³/s — 每秒的浓度变化
2. Measuring Reaction Rates | 测量反应速率
There are three main methods used in IGCSE Edexcel core practicals to follow a reaction as it happens.
在 IGCSE Edexcel 核心实验中，主要有三种追踪反应进程的方法。
Collecting a gas. Use a gas syringe, or a measuring cylinder filled with water over a beehive shelf, to collect the gas produced and record the volume at regular time intervals.
收集气体法。使用气体注射器，或倒扣在水中装满水的量筒，来收集生成的气体，并每隔一定时间记录体积。
Loss of mass. Place the flask on a balance; if the reaction produces a gas that escapes, the mass reading falls, and you record it every few seconds.
质量损失法。将反应容器放在天平上；如果反应生成的气体逸出，天平读数会下降，每隔几秒记录一次读数。
Disappearing cross. When a solid precipitate forms, the solution becomes cloudy. Place a flask on top of a paper marked with a cross and time how long the cross takes to vanish.
十字消失法。当生成固体沉淀时，溶液变浑浊。在纸上画一个十字，把容器放在纸上，记录十字完全消失所需的时间。
3. Collision Theory | 碰撞理论
For a reaction to happen, reactant particles must collide with each other. However, not every collision leads to a reaction.
反应要发生，反应物粒子之间必须发生碰撞。然而，并非每一次碰撞都会引发反应。
A successful collision needs two conditions: the particles must have energy equal to or greater than the activation energy, and they must hit each other with the correct orientation.
一次有效碰撞需要两个条件：粒子的能量必须大于或等于活化能，并且粒子必须以正确的方向相互撞击。
Activation energy (Eₐ) is the minimum energy that particles need in order to break bonds and start a reaction.
活化能（Eₐ）是粒子为了断开化学键并启动反应所需的最低能量。
Any factor that increases the frequency of successful collisions will increase the rate of reaction. This is the key idea behind all the factors below.
任何能增加有效碰撞频率的因素都会加快反应速率。这是下面所有因素背后的核心原理。
4. Concentration | 浓度
When the concentration of a dissolved reactant increases, there are more reactant particles in the same volume of solution.
当溶液中反应物的浓度增大时，相同体积内就有更多的反应物粒子。
The particles are packed more closely, so they collide more frequently. More frequent collisions mean more successful collisions per second, and therefore a higher rate.
粒子排列得更紧密，因此碰撞更频繁。每秒有效碰撞的次数增多，反应速率随之加快。
On a concentration–rate graph, a simple reaction often shows increasing rate up to a limit, because at very high concentrations the particles are already so crowded that adding more has little extra effect.
在浓度–速率图像中，简单的反应速率通常随浓度增大而升高并趋于极限，因为在极高浓度下粒子已经非常拥挤，再增加浓度的效果很小。
5. Pressure | 压强
Pressure only affects reactions that involve gases. Increasing the pressure compresses the gas particles into a smaller volume, so the same number of particles now occupies less space.
压强只影响涉及气体的反应。增大压强会把气体粒子压缩到更小的体积中，因此相同数量的粒子占据的空间更小。
This makes collisions between particles more frequent, so the rate of reaction increases. In industry, many gaseous reactions, such as the Haber process, are run at high pressure for exactly this reason.
这使粒子之间的碰撞更加频繁，反应速率因此提高。在工业上，许多气体反应（如哈伯法制氨）正是在高压下进行的，原因就在于此。
6. Temperature | 温度
Raising the temperature gives particles more kinetic energy, so they move faster. This increases the frequency of collisions between reactant particles.
升高温度使粒子获得更多动能，运动得更快。这增加了反应物粒子之间碰撞的频率。
More importantly, a greater proportion of the particles now have energy greater than the activation energy. This dramatically increases the number of successful collisions.
更重要的是，现在有更大比例的粒子具有高于活化能的能量。这大大增加了有效碰撞的次数。
As a rough guide, for many reactions every 10 °C rise in temperature roughly doubles the rate of reaction.
粗略估计，对很多反应来说，温度每升高 10 °C，反应速率大约翻一倍。
7. Surface Area | 表面积
When a solid reactant is broken into smaller pieces, its total surface area increases. For example, cutting a cube into eight smaller cubes exposes far more surface than the original cube.
当固体反应物被粉碎成更小的颗粒时，其总表面积增大。例如，把一个立方体切成八个小立方体，暴露的表面积远大于原来。
A reaction can only happen at the surface of a solid, where its particles meet the other reactant. A larger surface area means more solid particles are exposed, so collisions are more frequent and the rate increases.
反应只发生在固体表面，即固体粒子与另一种反应物接触的地方。表面积越大，暴露的固体粒子越多，碰撞越频繁，速率越快。
On a graph, using powdered marble instead of large lumps gives a steeper initial curve, but the final amount of product is the same because the mass of reactant is unchanged.
在图像上，用大理石粉末代替大块大理石，初始曲线会更陡，但最终产物的总量相同，因为反应物的质量没有改变。
8. Catalysts | 催化剂
A catalyst is a substance that speeds up a reaction but is chemically unchanged and not used up at the end. It provides an alternative reaction pathway with a lower activation energy.
催化剂是一种能加快反应速率、但本身在反应前后化学性质和质量都不变的物质。它为反应提供了一条活化能更低的替代路径。
With a lower Eₐ, a much larger proportion of particles have enough energy to react successfully at the same temperature, so the rate increases without needing extra heat.
由于活化能更低，在相同温度下就有更大比例的粒子具有足够的能量进行有效碰撞，因此无需额外加热即可提高反应速率。
Catalysts are specific: different reactions need different catalysts. Transition metals are widely used in industry, and biological catalysts are called enzymes. Enzymes have an optimum temperature and pH; beyond these they denature and lose their activity.
催化剂具有选择性：不同的反应需要不同的催化剂。过渡金属在工业中应用广泛，而生物催化剂被称为酶。酶有最适温度和 pH；一旦超过这些范围，酶就会变性失活。
9. Rate Graphs | 速率曲线图
In Edexcel practical questions, you are often asked to plot a graph of gas volume or mass loss against time.
在 Edexcel 实验题中，经常要求你绘制气体体积或质量损失随时间变化的曲线图。
The steeper the line, the faster the reaction. As the reactant is used up, the curve flattens and reaches a plateau where the reaction stops.
曲线越陡，反应越快。随着反应物被消耗，曲线逐渐变平，最终达到平台，表示反应停止。
Average rate is calculated as total change divided by total time, for example the total gas volume divided by the time taken to reach the plateau.
平均速率 = 总变化量 ÷ 总时间。例如，气体总体积除以到达平台所用的时间。
When describing a rate–time graph, say: “The rate decreases with time because the concentration of the reactant falls, so successful collisions happen less often.”
在描述速率–时间图像时，应说：”速率随时间减小，因为反应物浓度下降，有效碰撞发生的频率降低。”
10. Core Practicals | 核心实验
Edexcel expects you to know two key practical setups in detail, including their apparatus, measurements and variables.
Edexcel 要求你详细掌握两个关键实验装置，包括仪器、测量量和变量控制。
Practical A: Marble chips and hydrochloric acid. Measure the loss of mass on a balance, or collect the CO₂ in a gas syringe.
实验 A：大理石与稀盐酸反应。用天平测量质量损失，或使用气体注射器收集 CO₂。
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Keep the mass of marble chips and the volume of acid constant; only change one factor — for example the concentration of the acid.
保持大理石的质量和酸的体积不变；每次只改变一个因素，例如酸的浓度。
Practical B: Sodium thiosulfate and hydrochloric acid. Add acid to sodium thiosulfate; a sulfur precipitate forms and the cross underneath the flask disappears.
实验 B：硫代硫酸钠与盐酸反应。将酸加入硫代硫酸钠溶液中；生成硫沉淀，瓶底下的十字逐渐消失。
Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)
Repeat with different concentrations of sodium thiosulfate and measure the time for the cross to disappear. Higher concentration gives a shorter time, so the rate is higher.
用不同浓度的硫代硫酸钠重复实验，记录十字消失的时间。浓度越高，时间越短，说明速率越快。
11. Exam Tips | 考试提示
Common mistakes lose marks every year. Use these tips to avoid them.
每年都有考生因常见错误而丢分。运用下面这些提示来避免。
- Always write “more frequent successful collisions” rather than just “more collisions” — many mark schemes specifically require the word successful. | 一定要写”有效碰撞更频繁”，而不是只写”碰撞更多”——许多评分标准明确要求”有效（successful）”一词。
- When comparing rates, quote readings from the data, e.g. “20 cm³ of gas in 40 s compared with 5 cm³ in 40 s.” | 比较速率时要引用数据，例如”40 秒内产生 20 cm³ 气体，而原来 40 秒只有 5 cm³”。
- Do not confuse surface area with the amount of reactant; powdering a solid does not change its mass. | 不要把表面积与反应物的量混淆；把固体研成粉末不会改变其质量。
- For temperature, state that “a greater proportion of particles have energy greater than or equal to the activation energy.” | 关于温度，应写明”更大比例的粒子具有大于或等于活化能的能量”。
- Always include units on calculated rates. A number without a unit is not worth full marks. | 计算速率时一定要带单位。没有单位的数值无法得到满分。
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