📚 Rates of Reaction | 反应速率
Chemical reactions occur at different speeds. Some, like explosions, happen in a flash; others, like rusting, take years. In IGCSE Science, understanding how to measure and control reaction rates is a key skill that links theory to practical applications.
化学反应以不同的速度发生。有些反应，比如爆炸，瞬间完成；另一些反应，比如生锈，则需要数年。在 IGCSE 科学中，理解如何测量和控制反应速率是一项关键技能，它将理论与实际应用联系起来。
1. What Is Reaction Rate? | 什么是反应速率？
Reaction rate is the change in concentration 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.
反应速率是单位时间内反应物或产物浓度的变化量。它可以表示为消耗的反应物的量，或生成的产物的量，除以所用的时间。
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 0.02 mol of a product appears in 10 seconds, the average rate is 0.002 mol/s.
例如，如果 0.02 mol 产物在 10 秒内出现，则平均速率是 0.002 mol/s。
2. Collision Theory | 碰撞理论
For a reaction to occur, reactant particles must collide with each other. However, not every collision leads to a reaction. Only collisions with enough energy and the correct orientation are successful.
为了使反应发生，反应物粒子必须相互碰撞。然而，并非每次碰撞都会导致反应。只有具有足够能量且方向正确的碰撞才是有效碰撞。
This minimum energy required 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）。
A higher frequency of successful collisions means a faster reaction rate.
有效碰撞的频率越高，反应速率越快。
3. Factors Affecting Reaction Rate | 影响反应速率的因素
Several factors change how quickly a reaction proceeds. In the Edexcel IGCSE specification, you must know at least five: concentration, pressure, surface area, temperature, and catalysts.
有几个因素会改变反应进行的快慢。在 Edexcel IGCSE 考纲中，你至少需要掌握五个因素：浓度、压强、表面积、温度和催化剂。
Increasing concentration or pressure raises the number of particles per unit volume.
增加浓度或压强会提高单位体积内的粒子数。
Increasing surface area exposes more particles to collision.
增加表面积会暴露更多粒子以供碰撞。
Increasing temperature gives particles more kinetic energy.
升高温度赋予粒子更多动能。
Adding a catalyst provides an alternative pathway with lower activation energy.
添加催化剂提供了活化能更低的新反应途径。
4. Concentration and Pressure | 浓度与压强
When the concentration of a solution is increased, the number of particles in the same volume increases. As a result, collisions happen more frequently, and the rate of reaction increases.
当溶液的浓度增加时，相同体积内的粒子数会增加。结果，碰撞发生得更频繁，反应速率加快。
For gases, increasing pressure has the same effect as increasing concentration: the particles are squeezed into a smaller volume, so they collide more often.
对于气体，增加压强与增加浓度效果相同：粒子被压缩到更小的体积中，因此它们碰撞更频繁。
In both cases, the activation energy is unchanged, but the frequency of successful collisions is higher.
在这两种情况下，活化能不变，但有效碰撞的频率更高。
5. Temperature | 温度
Raising the temperature increases the average kinetic energy of particles. Particles move faster, so they collide more frequently. More importantly, a much larger proportion of particles now have energy greater than or equal to the activation energy.
升高温度会增加粒子的平均动能。粒子运动得更快，因此碰撞更频繁。更重要的是，现在有更大比例的粒子拥有大于或等于活化能的能量。
A useful rule of thumb: for many reactions, the rate roughly doubles for every 10 °C rise in temperature. This is because the number of energetic particles increases dramatically.
一个有用的经验法则：对于许多反应，温度每升高 10 °C，速率大约翻倍。这是因为高能粒子的数量急剧增加。
Note that a small temperature change has a far greater effect on rate than a small concentration change, because it changes both collision frequency and collision energy.
注意，小的温度变化对速率的影响远大于小的浓度变化，因为它同时改变了碰撞频率和碰撞能量。
6. Surface Area | 表面积
When a solid reactant is broken into smaller pieces, its total surface area increases. More particles are exposed to the other reactant, so the frequency of collisions at the surface increases.
当固体反应物被破碎成更小的颗粒时，其总表面积增大。更多粒子暴露给其他反应物，因此表面上的碰撞频率增加。
For example, a lump of calcium carbonate reacts slower with acid than powdered calcium carbonate of the same mass. Powder has a much larger surface area.
例如，一块碳酸钙与酸反应比相同质量的粉末状碳酸钙更慢。粉末的表面积大得多。
Increasing surface area does not change the energy of the particles; it only increases the chance of collision.
增加表面积不会改变粒子的能量；它只是增加了碰撞的机会。
7. Catalysts | 催化剂
A catalyst is a substance that increases the rate of a reaction without being used up. 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。它通过提供一条活化能更低的替代反应路径来实现这一作用。
Catalyst + reactants → products + Catalyst (unchanged)
催化剂 + 反应物 → 产物 + 催化剂（不变）
Catalysts are specific: different reactions need different catalysts. In biological systems, enzymes are biological catalysts that work at body temperature.
催化剂具有专一性：不同的反应需要不同的催化剂。在生物系统中，酶是在体温下起作用的生物催化剂。
Catalysts do not change the position of equilibrium in reversible reactions, but they help the system reach equilibrium faster.
催化剂不改变可逆反应的平衡位置，但能帮助体系更快达到平衡。
8. Measuring Reaction Rates | 测量反应速率
Several experimental methods can track how fast a reaction proceeds. The choice depends on the reaction being studied.
有几种实验方法可以追踪反应进行的速度。选择哪种方法取决于所研究的反应。
Collecting gas: A gas syringe measures the volume of gas produced over time.
收集气体：用气筒测量随时间产生的气体体积。
Loss of mass: If a gas escapes, the mass of the reaction flask decreases.
质量损失：如果气体逸出，反应瓶的质量会减小。
Monitoring turbidity: A precipitate formed in a solution makes it cloudy; the time for a cross to disappear can be measured.
监测浊度：溶液中生成的沉淀会使溶液变浑浊；可以测量十字标记消失所需的时间。
Color change or pH change: Use a colorimeter or pH probe to record changes.
颜色变化或 pH 变化：使用色度计或 pH 探针记录变化。
| Method 方法 | What is measured 测量内容 | Example reaction 示例反应 |
| Gas collection | Volume of gas (cm³) | Mg + HCl → H₂ |
| Loss of mass | Mass (g) | CaCO₃ + HCl → CO₂ |
| Turbidity | Time for cross to vanish (s) | Na₂S₂O₃ + HCl → S |
9. Interpreting Graphs | 解读曲线图
Reaction rate can be determined from graphs of product or reactant concentration against time. The steeper the curve, the faster the reaction.
反应速率可以从产物或反应物浓度随时间的曲线图中确定。曲线越陡，反应越快。
As a reaction proceeds, reactants are used up, so the rate gradually decreases. The curve becomes less steep and eventually flattens when the reaction stops.
随着反应进行，反应物被消耗，速率逐渐减小。曲线变缓，当反应停止时最终变平。
To find the instantaneous rate at a particular time, draw a tangent to the curve at that point and calculate its gradient:
要找到某一特定时间的瞬时速率，可以在该点画一条切线，并计算其斜率：
Rate = gradient = Δy ÷ Δx
速率 = 斜率 = Δy ÷ Δx
The average rate over a whole reaction is the total change in a measured quantity divided by the total time taken.
整个反应的平均速率是测量量的总变化除以所用的总时间。
10. Real-World Applications | 现实应用
Controlling reaction rate is vital in industry, medicine, and everyday life.
控制反应速率在工业、医学和日常生活中至关重要。
Food preservation: refrigeration slows down the enzymatic reactions that cause food to spoil.
食品保存：冷藏减缓导致食物变质的酶促反应。
Industrial Haber process: an iron catalyst and high pressure speed up ammonia production.
工业哈伯法：铁催化剂和高压加快氨的生产。
Car airbags: sodium azide decomposes extremely quickly to fill the airbag instantly.
汽车安全气囊：叠氮化钠极速分解，瞬间充满气囊。
Medicines: tablets are often powdered or made into finer particles so the drug dissolves and reacts faster in the body.
药物：药片常被粉碎或制成更小颗粒，以便药物在体内更快溶解和反应。
Understanding reaction rates helps chemists design safer and more efficient processes.
理解反应速率帮助化学家设计更安全、更高效的工艺。
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