📚 Rates of Reaction | 化学反应速率
In IGCSE Chemistry, understanding how fast chemical reactions proceed is essential. This article covers the factors that affect reaction rates, how to measure them, and how to interpret graphs using collision theory.
在 IGCSE 化学中，理解化学反应进行的快慢至关重要。本文涵盖影响反应速率的因素、如何测量速率，以及如何运用碰撞理论解释相关图表。
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 tells us how quickly reactants are used up or products are formed. Common units include mol dm⁻³ s⁻¹ or g s⁻¹.
反应速率是指单位时间内反应物或产物浓度的变化量。它告诉我们反应物被消耗或产物生成的快慢。常用单位有 mol dm⁻³ s⁻¹ 或 g s⁻¹。
For a simple reaction A → B, the rate can be expressed as:
rate = −Δ[A]/Δt = Δ[B]/Δt
where Δ[A] means the change in concentration of A over time Δt. The negative sign shows that reactant concentration decreases.
其中 Δ[A] 表示反应物 A 的浓度在时间 Δt 内的变化量。负号表示反应物浓度减小。
2. Collision Theory | 碰撞理论
For a reaction to occur, particles must collide with each other. However, not every collision leads to a reaction. A collision is successful only when two conditions are met: the particles must have enough energy (greater than or equal to the activation energy, Eₐ), and they must be oriented correctly.
要使反应发生，粒子之间必须发生碰撞。然而，并非每次碰撞都能引发反应。只有当两个条件同时满足时，碰撞才是有效的：粒子具有足够的能量（大于或等于活化能 Eₐ），并且碰撞时的取向正确。
Reaction rate depends on the frequency of successful collisions. The higher the frequency of successful collisions, the faster the reaction.
反应速率取决于有效碰撞的频率。有效碰撞频率越高，反应越快。
3. Surface Area | 表面积
Increasing the surface area of a solid reactant exposes more particles to the other reactant. For example, a powdered solid has a much larger surface area than a single lump of the same mass. More surface area means more frequent collisions between reactant particles, so the rate increases.
增大固体反应物的表面积，可以使其更多粒子暴露在另一种反应物中。例如，粉末状固体的表面积远大于同质量的一块固体。表面积越大，反应物粒子之间碰撞越频繁，因此速率增大。
When acid reacts with calcium carbonate, a powder produces carbon dioxide much faster than marble chips of equal mass. The total amount of gas produced eventually is the same, but the reaction time is shorter.
当酸与碳酸钙反应时，粉末产生的二氧化碳比等质量的大理石碎片快得多。最终产生的气体总量相同，但反应时间更短。
4. Temperature | 温度
Raising the temperature gives particles more kinetic energy. They move faster, so they collide more often. More importantly, a greater proportion of particles have energy equal to or greater than the activation energy. Thus, the fraction of successful collisions increases sharply. As a rough rule, a 10 °C rise can double or triple the rate.
升高温度使粒子获得更多动能。它们运动更快，因此碰撞更频繁。更重要的是，具有等于或大于活化能能量的粒子比例更大。因此，有效碰撞的比例急剧增加。粗略估计，温度每升高 10 °C，速率可能加倍或增加两倍。
In terms of the Maxwell–Boltzmann distribution, the curve shifts to the right and flattens. The area under the curve beyond Eₐ increases, showing that more particles can react.
从麦克斯韦-玻尔兹曼分布来看，曲线向右移动并变得平缓。活化能右侧曲线下的面积增大，说明更多粒子能够发生反应。
5. Concentration and Pressure | 浓度与压强
For solutions, increasing the concentration means more solute particles per unit volume. This leads to more frequent collisions, so the rate increases. For gases, increasing the pressure (or decreasing the container volume) pushes the particles closer together, also raising the collision frequency.
对于溶液，增大浓度意味着单位体积内溶质粒子更多。这导致碰撞更频繁，从而速率增大。对于气体，增大压强（或缩小容器体积）使粒子靠得更近，同样增加了碰撞频率。
It is important to note that concentration and pressure changes do not affect the activation energy; they only change the number of collisions per second.
必须注意，浓度和压强的变化不会改变活化能，它们只改变每秒的碰撞次数。
6. Catalysts | 催化剂
A catalyst is a substance that speeds up a reaction without being used up in the process. It provides an alternative reaction pathway with a lower activation energy. This means at the same temperature, many more particles have enough energy to react, so the rate increases.
催化剂是一种能够加快反应速率但在反应过程中自身不被消耗的物质。它提供了一条活化能更低的新反应路径。这意味着在相同温度下，有更多粒子具有足够的能量发生反应，因此速率增大。
Catalysts are used widely in industry, such as iron in the Haber process and vanadium(V) oxide in the contact process. They reduce energy costs and increase efficiency. Catalysts are specific: different reactions need different catalysts.
催化剂在工业中应用广泛，例如哈伯法中使用的铁、接触法中使用的五氧化二钒。它们降低了能源成本并提高了效率。催化剂具有选择性：不同的反应需要不同的催化剂。
7. Measuring Reaction Rates | 测量反应速率的方法
Several techniques allow us to follow the progress of a reaction. The choice depends on the nature of the reactants and products. Common methods include measuring gas volume, recording the loss of mass, monitoring the change in colour or turbidity, and using a pH probe.
有几种技术可以跟踪反应的进程。选择哪种方法取决于反应物和产物的性质。常用方法包括测量气体体积、记录质量减少、监测颜色或浑浊度的变化，以及使用 pH 探头。
- Gas volume: collect the gas produced in a gas syringe and record the volume at regular time intervals.
- Mass loss: place the flask on a balance and record the decrease in mass as gas escapes.
- Turbidity: place the flask above a cross on paper and measure how long it takes for the cross to disappear.
- 气体体积：用注射器收集产生的气体，按规定时间间隔记录体积。
- 质量减少：将烧瓶放在天平上，记录因气体逸出而导致的的质量减少。
- 浊度：在烧瓶下方纸上画一个十字，测量十字消失所需的时间。
8. Interpreting Rate Graphs | 速率图表的解读
When we plot the volume of gas produced (or the concentration of a reactant) against time, we obtain a curve that starts steeply and gradually becomes flatter. The slope (gradient) at any point represents the instantaneous rate at that moment. Initially the rate is fastest because reactant concentrations are highest. As the reaction proceeds, reactants are used up, so the slope decreases. Finally, the curve becomes horizontal, meaning the reaction has stopped.
当我们绘制产生气体的体积（或反应物浓度）随时间变化的曲线时，得到的曲线起始较陡，然后逐渐变平。任意一点的斜率（梯度）代表那时刻的瞬时速率。初始时速率最快，因为反应物浓度最高。随着反应进行，反应物被消耗，斜率减小。最终，曲线变为水平，表示反应停止。
To calculate the average rate over a period, divide the total change in quantity by the total time taken. To find the instantaneous rate at a particular time, draw a tangent to the curve and calculate its gradient.
要计算一段时间内的平均速率，将总量的变化除以总时间。要找出某一时刻的瞬时速率，可画出该点的切线并计算其斜率。
9. Activation Energy and Energy Profile Diagrams | 活化能与能量剖面图
Every reaction has a minimum energy threshold called the activation energy. An energy profile diagram shows the energy of reactants and products, with the activation energy as the 'hill' that must be overcome. For an exothermic reaction, products have lower energy than reactants; for an endothermic reaction, products have higher energy.
每个反应都有一个最小能量阈值，称为活化能。能量剖面图显示反应物和生成物的能量，活化能则表现为必须跨越的“能量山”。对于放热反应，产物能量低于反应物；对于吸热反应，产物能量高于反应物。
Adding a catalyst lowers the height of the 'hill' without changing the energy change of the whole reaction. This is why a catalyst increases the rate without altering the position of equilibrium in reversible reactions.
加入催化剂可以降低“能量山”的高度，但不改变整个反应的能量变化。这就是为什么催化剂能提高速率，但不改变化学平衡的位置。
10. Practical: The Effect of Concentration on Rate | 实验：浓度对速率的影响
A classic experiment to investigate concentration is the reaction between sodium thiosulfate and hydrochloric acid. The solution becomes cloudy as solid sulfur forms. Place a flask containing sodium thiosulfate on a piece of white paper with a black cross drawn on it. Add hydrochloric acid, start the timer, and record how long it takes for the cross to become invisible.
一个经典的探究浓度的实验是硫代硫酸钠与盐酸的反应。随着固体硫生成，溶液变浑浊。将装有硫代硫酸钠的烧瓶放在画有黑色十字的白纸上。加入盐酸，启动计时器，记录十字消失所需的时间。
If you repeat the experiment with more dilute sodium thiosulfate, the cross takes longer to disappear. This shows that a lower concentration gives a slower rate. To make the comparison fair, keep the volume of acid, temperature, and the same observer. You can calculate the rate as 1/time taken (s⁻¹).
如果用更稀的硫代硫酸钠重复实验，十字消失所需时间会更长。这说明浓度越低，速率越慢。为保持公平比较，应保持酸的体积、温度相同，且由同一观察者进行。可以计算速率作为 1/所用时间（s⁻¹）。
11. Rate and Collision Frequency: Summary Table | 速率与碰撞频率：总结表
| Factor | How it increases rate | Reason |
| Surface area ↑ | More collisions per second | More exposed particles |
| Temperature ↑ | More collisions and more successful collisions | Particles move faster; more particles have energy ≥ Eₐ |
| Concentration/pressure ↑ | More frequent collisions | More particles in the same volume |
| Catalyst added | More particles have energy ≥ Eₐ | Activation energy lowered |
In the table above, factors achieving faster reactions all result in a higher frequency of successful collisions, except for catalysts, which increase the fraction of successful collisions at a given temperature.
在上表中，加快反应的因素都导致有效碰撞频率升高，但催化剂例外，它提高了给定温度下有效碰撞所占的比例。
12. Real-World Applications | 实际应用
Reaction rate concepts are used in food preservation: low temperatures slow down the chemical reactions that cause food to spoil. In medicine, drugs are designed to break down at specific rates in the body. In industry, choosing the right temperature, pressure, and catalyst maximizes product output while minimising cost.
反应速率概念用于食品保存：低温能减缓导致食物腐败的化学反应。在医学中，药物被设计为在体内以特定速率分解。在工业中，选择合适的温度、压强和催化剂，可以在降低成本的同时最大化产物产量。
Every time you cook food, you are using temperature to control reaction rates. Frying an egg speeds up protein denaturation, while refrigeration slows down the reactions that make milk go sour. Understanding rates of reaction helps scientists design better chemical processes and safer products.
每次烹饪食物时，你都在利用温度控制反应速率。煎蛋加速了蛋白质变性，而冷藏则减缓了使牛奶变酸的反应。理解反应速率帮助科学家设计更好的化学工艺和更安全的产品。
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