Rates of Reaction | 化学反应速率

📚 Rates of Reaction | 化学反应速率

The rate of a chemical reaction tells us how quickly reactants are converted into products. It is a core topic in the Edexcel IGCSE Science specification and is essential for understanding everything from industrial processes to biological systems.

化学反应速率告诉我们反应物转化为产物的快慢。它是 Edexcel IGCSE 科学课程的核心内容，也是理解工业流程和生物系统等一切过程的基础。

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

The rate of reaction measures the change in concentration (or amount) of a reactant or product per unit time. It can be expressed as the amount of reactant used up, or the amount of product formed, divided by the time taken.

反应速率衡量的是单位时间内反应物或产物浓度（或物质的量）的变化。它可以用反应物消耗的量或产物生成的量除以所花的时间来表示。

Rate = amount of reactant used or product formed ÷ time

速率 = 反应物消耗量或产物生成量 ÷ 时间

For example, if 2.0 g of magnesium reacts completely with excess hydrochloric acid in 50 seconds, the average rate is 2.0 g ÷ 50 s = 0.040 g/s.

例如，若 2.0 g 镁与过量稀盐酸在 50 秒内完全反应，则平均速率 = 2.0 g ÷ 50 s = 0.040 g/s。

2. Measuring Rates | 测量反应速率

Several methods can be used to follow the progress of a reaction. The choice depends on the substances involved and the physical property being observed.

跟踪反应进程有多种方法，具体选择取决于所涉及的物质以及观察的物理性质。

Measuring the volume of gas produced over time using a gas syringe or an inverted burette.
通过气体注射器或倒置滴定管测量一定时间内产生气体的体积。
Measuring the loss in mass (if a gas escapes) using a balance.
用天平测量质量损失（当气体逸出时）。
Monitoring a colour change or turbidity (e.g. the disappearing cross method for sodium thiosulfate and acid).
监测颜色变化或浑浊度（例如硫代硫酸钠与酸反应时“十字消失法”）。
Measuring changes in pH or electrical conductivity if ions are produced or consumed.
若产生或消耗离子，可测量 pH 或电导率的变化。

Each method gives data that can be plotted as a graph of quantity against time. The steeper the graph, the faster the reaction.

每种方法都能得到数据，并可绘制成“量—时间”图。图像越陡，反应越快。

3. Collision Theory | 碰撞理论

For a chemical reaction to occur, reactant particles must collide with each other. However, not every collision leads to a reaction. Only collisions with energy greater than or equal to the activation energy (Eₐ) and with the correct orientation are successful.

化学反应发生时，反应物粒子必须相互碰撞。但并非每次碰撞都会引发反应。只有能量大于或等于活化能（Eₐ）并且方向正确的碰撞才是有效碰撞。

Successful collision = correct orientation + energy ≥ Eₐ

有效碰撞 = 正确的方向 + 能量 ≥ Eₐ

The rate of reaction depends on two things: how often particles collide (collision frequency) and the fraction of collisions that are successful. Increasing temperature, concentration, pressure or surface area all increase the collision frequency or the proportion of successful collisions.

反应速率取决于两个因素：粒子碰撞的频率，以及碰撞中有效碰撞所占的比例。升高温度、增大浓度、压强或表面积，都会提高碰撞频率或有效碰撞比例。

4. Factor: Temperature | 因素：温度

Raising the temperature gives reactant particles more kinetic energy, so they move faster. This increases both collision frequency and, more importantly, the proportion of particles with energy equal to or greater than Eₐ. A rule of thumb: a 10 °C rise can roughly double the rate.

升高温度使反应物粒子获得更多动能，运动加快。这不仅提高了碰撞频率，更重要的是提高了能量 ≥ Eₐ 的粒子比例。粗略经验：温度每升高 10 °C，反应速率约增加一倍。

At higher temperatures, particles move faster ⇒ more collisions per second.
温度越高，粒子运动越快 ⇒ 每秒碰撞次数增多。
More particles have energy > Eₐ ⇒ a greater fraction of collisions are effective.
更多粒子具有能量 > Eₐ ⇒ 有效碰撞占比增大。
The rate therefore increases rapidly with temperature.
因此反应速率随温度迅速增加。

The Maxwell–Boltzmann distribution diagram shows that at a higher temperature, the curve shifts to the right and flattens, with a larger area under the tail beyond Eₐ.

麦克斯韦–玻尔兹曼分布图表明，温度升高时，曲线右移且变平坦，超过 Eₐ 的尾部区域更大。

5. Factor: Concentration and Pressure | 因素：浓度与压强

For solutions, increasing the concentration of a reactant means more particles are present in the same volume. This increases the frequency of collisions between reactant particles, so the rate increases.

对于溶液，增大反应物浓度意味着相同体积内有更多粒子。这增大了反应物粒子之间的碰撞频率，从而使反应速率提高。

For gases, increasing the pressure is equivalent to increasing the concentration. The particles are squeezed into a smaller volume, so they collide more often.

对于气体，增大压强相当于增大浓度。粒子被挤压到更小的体积内，因此碰撞更频繁。

Higher concentration ⇒ more particles per unit volume ⇒ higher collision frequency.
浓度更高 ⇒ 单位体积内粒子数更多 ⇒ 碰撞频率更高。
Higher pressure ⇒ same number of particles in a smaller volume ⇒ more frequent collisions.
压强更高 ⇒ 相同粒子数占据更小体积 ⇒ 碰撞更频繁。
The activation energy is unchanged; only the number of successful collisions per second increases.
活化能不变；只是每秒的有效碰撞次数增加。

Note: In gaseous reactions, pressure changes affect rate only if at least one reactant is a gas.

注意：在气体反应中，压强变化只影响至少一种反应物为气体的反应速率。

6. Factor: Surface Area | 因素：表面积

When a solid reactant is broken into smaller pieces, its total surface area increases. More particles of the solid are exposed to the other reactant, so collisions occur more frequently at the solid–liquid or solid–gas interface. This increases the rate.

当固体反应物被粉碎成更小的颗粒时，其总表面积增大。更多固体颗粒暴露在另一种反应物中，因此在固—液或固—气界面上碰撞更频繁，从而加快反应速率。

A lump of calcium carbonate reacts slowly with acid; powdered calcium carbonate reacts much faster.
块状碳酸钙与酸反应缓慢；粉末状碳酸钙反应快得多。
Surface area can be increased by grinding, cutting, or using a fine powder.
可通过研磨、切割或使用细粉来增大表面积。
For a given mass, smaller particles have a larger total surface area.
对于相同质量，颗粒越小，总表面积越大。

Graphically, a powder gives a steeper initial slope and reaches completion sooner than a lump of the same mass.

从图像看，与同质量的块状固体相比，粉末的初始斜率更大，并且更快达到反应终点。

7. Factor: Catalysts | 因素：催化剂

A catalyst is a substance that increases the rate of a reaction without being used up itself. It works by providing an alternative reaction pathway with a lower activation energy.

催化剂是一种能加快反应速率但自身不被消耗的物质。它的作用是通过提供一条活化能更低的新反应路径来加速反应。

Catalysts are specific: different reactions usually require different catalysts.
催化剂具有选择性：不同的反应通常需要不同的催化剂。
They do not affect the final amount of product; they only affect how quickly it is formed.
催化剂不影响最终产物的量，只影响生成快慢。
Catalysts can be biological (enzymes) or chemical (e.g. manganese dioxide for hydrogen peroxide decomposition).
催化剂可以是生物催化剂（酶）或化学催化剂（例如二氧化锰催化过氧化氢分解）。

Lower Eₐ ⇒ more particles have enough energy ⇒ faster reaction

活化能降低 ⇒ 更多粒子获得足够能量 ⇒ 反应加快

In industry, catalysts such as iron (Haber process) and vanadium(V) oxide (Contact process) reduce energy costs and increase production rates.

在工业中，如哈伯法中的铁和接触法中的五氧化二钒等催化剂，能降低能源成本并提高生产效率。

8. Graphs and Calculations | 图像与计算

Reaction progress is usually shown on a graph of product volume (or mass, or concentration) against time. The gradient at any instant gives the instantaneous rate. The average rate between two times is found from the total change divided by the time interval.

反应进程通常用产物体积（或质量、浓度）随时间的曲线图表示。任何时刻的曲线斜率即为该时刻的瞬时速率。两个时间点之间的平均速率等于总变化量除以时间间隔。

For example, if 60 cm³ of carbon dioxide is collected after 2 minutes, the average rate is 60 cm³ ÷ 120 s = 0.5 cm³/s.

例如，若 2 分钟内收集到 60 cm³ 二氧化碳，则平均速率为 60 cm³ ÷ 120 s = 0.5 cm³/s。

Quantity	Unit
Volume of gas	cm³ or dm³
Mass lost	g
Concentration	mol/dm³
Time	s or min

When a reactant is used up, the graph becomes flat. A steeper line at the start indicates a faster initial rate compared to a flatter line.

当反应物耗尽时，曲线变平。起点处更陡的线意味着初始速率比更平缓的线更快。

9. Real-World Applications | 实际应用

Understanding reaction rates is vital in both everyday life and industrial chemistry.

理解反应速率对日常生活和化学工业都至关重要。

Food preservation: refrigeration slows down the chemical reactions that cause food to spoil.
食品保鲜：冷藏减缓导致食物变质的化学反应。
Industrial synthesis: the Haber process uses high pressure and a catalyst to achieve a viable rate of ammonia production.
工业合成：哈伯法利用高压和催化剂使氨的生产速率达到经济可行的水平。
Rust prevention: painting or coating iron limits surface contact with oxygen and water, reducing the rate of corrosion.
防锈：给铁刷漆或镀层可限制其与氧气和水的接触，降低腐蚀速率。
Pharmaceutical design: drug tablets are made with different coatings to control the rate at which the medicine is released in the body.
药物设计：药片采用不同包衣来控制药物在体内的释放速率。

In each case, changing a factor such as temperature, surface area, concentration or a catalyst controls whether the reaction is fast enough for our needs.

在每种情况中，通过改变温度、表面积、浓度或催化剂等因素，可以控制反应是否达到我们需要的快慢程度。

Conclusion | 总结

Reaction rate is governed by collision theory. The rate increases when we raise temperature, concentration, pressure or surface area, or when we add a suitable catalyst. These ideas are tested through practical experiments, graphs and calculations, and they link directly to many industrial and biological processes.

反应速率由碰撞理论主导。升高温度、增大浓度、压强或表面积，或加入合适的催化剂，都会使反应速率加快。这些概念通过实验、图像和计算来考察，并与许多工业和生物过程直接相关。

Mastering this topic requires not only memorising the factors, but also explaining them using particle behaviour and energy distributions.

掌握这一主题不仅需要记住影响因素，还要能运用粒子行为和能量分布来进行解释。

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Rates of Reaction | 化学反应速率