📚 Rates of Reaction | 化学反应速率
In IGCSE Edexcel Science, understanding rates of reaction is essential for predicting how quickly chemical processes occur and for explaining the factors that affect them. This revision guide breaks down the core ideas, using clear language and worked examples.
在爱德思 IGCSE 科学考试中，理解反应速率对于预测化学过程发生的快慢以及解释影响它们的因素至关重要。本复习指南用清晰的语言和典型例题分解核心概念。
1. What Is Rate of Reaction? | 什么是反应速率？
The rate of reaction measures how quickly reactants are used up or products are formed. It is defined as the change in concentration of a reactant or product per unit time.
反应速率衡量反应物被消耗或产物生成的快慢。其定义为反应物或产物浓度在单位时间内的变化量。
- Unit: mol/dm³/s (or g/s, cm³/s).
- 公式单位： mol/dm³/s（或 g/s、cm³/s）。
Rate = change in amount ÷ time taken
反应速率 = 物质的量变化 ÷ 所用时间
For example, if 10 cm³ of gas is produced in 20 seconds, the average rate is 0.5 cm³/s.
例如，如果在 20 秒内产生了 10 cm³ 气体，则平均速率为 0.5 cm³/s。
2. Collision Theory | 碰撞理论
For a reaction to occur, particles must collide with sufficient energy (greater than or equal to the activation energy) and the correct orientation. Only effective collisions lead to a reaction.
反应发生的前提是：粒子必须发生碰撞，且碰撞能量足够（大于或等于活化能），并且方向合适。只有有效碰撞才能引发反应。
- Activation energy (Eₐ): the minimum energy needed for a reaction to start.
- 活化能（Eₐ）： 反应启动所需的最小能量。
Increasing the frequency of effective collisions increases the rate of reaction.
提高有效碰撞的频率会增大反应速率。
3. Effect of Concentration and Pressure | 浓度和压强的影响
Higher concentration of a solution means more particles per unit volume. This leads to more frequent collisions, so the rate increases. For gases, increasing pressure has the same effect.
溶液浓度越高，单位体积内的粒子越多，碰撞越频繁，因此速率加快。对于气体，增大压强具有同样的效果。
| Factor | Effect on rate |
| Higher concentration | Increases |
| Higher pressure (gas) | Increases |
Lowering concentration or pressure reduces the number of collisions per second, slowing the reaction.
降低浓度或压强会使每秒碰撞次数减少，反应变慢。
4. Effect of Temperature | 温度的影响
tantly, a greater proportion of particles now have energy ≥ Eₐ, so the rate rises sharply.
升高温度会增加粒子的平均动能。粒子运动加快，碰撞更频繁。更重要的是，更多的粒子能量达到或超过活化能，因此速率急剧上升。
Rate roughly doubles for every 10 °C rise (for many reactions)
对许多反应，温度每升高 10 °C，速率约翻倍
The increase in rate is not just due to more collisions; the energy factor is more significant.
速率增大不仅因为碰撞更多，能量因素更为关键。
5. Effect of Surface Area | 表面积的影响
When a solid reactant is broken into smaller pieces, its total surface area increases. This exposes more particles to the other reactant, increasing the frequency of collisions.
当固体反应物被粉碎成更小的颗粒时，其总表面积增大。这会暴露出更多与另一种反应物接触的粒子，从而增加碰撞频率。
| Form | Surface area | Rate |
| Large lumps | Small | Slow |
| Powder | Large | Fast |
This is why powdered calcium carbonate reacts with hydrochloric acid far more quickly than marble chips.
这就是为什么碳酸钙粉末与盐酸的反应速度远快于大理石碎块。
6. Catalysts | 催化剂
A catalyst is a substance that speeds up a 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反应但自身不被消耗的物质。它为反应提供了一条活化能更低的替代路径。
- Enzymes are biological catalysts.
- 酶是生物催化剂。
- Transition metals (e.g. iron in the Haber process) are common industrial catalysts.
- 过渡金属（如哈伯法中的铁）是常见的工业催化剂。
Catalysts do not affect the position of equilibrium, only how quickly it is reached.
催化剂不影响平衡位置，只影响到达平衡的快慢。
7. Reading Rate Graphs | 解读速率曲线图
Reaction progress can be shown as a graph of product volume (y-axis) against time (x-axis). The steeper the slope, the faster the reaction.
反应进程可以用产物体积（y 轴）对时间（x 轴）的曲线图表示。斜率越陡，反应越快。
- Initial slope: fastest, because reactant concentrations are highest.
- 初始斜率：最快，因为反应物浓度最高。
- Slope becomes horizontal: reaction has stopped (one reactant used up).
- 斜率变为水平：反应停止（某反应物已耗尽）。
To compare rates, calculate the gradient of the tangent at a given time.
要比较速率，可计算某一时刻切线的斜率。
8. Experiments to Measure Rate | 测定速率的实验方法
Common methods include collecting a gas in a gas syringe, monitoring mass loss on a balance, or measuring the time for a solution to become turbid.
常见方法包括：用气体注射器收集气体、用天平监测质量减少，或测量溶液变浑浊所需的时间。
| Method | What is measured | Example reaction |
| Gas syringe | Volume of gas produced | Mg + HCl → MgCl₂ + H₂ |
| Mass loss | Loss of gas mass | CaCO₃ + HCl → CO₂ |
| Turbidity | Time for a cross to disappear | Na₂S₂O₃ + HCl → S |
Each method requires careful control of variables to get reliable results.
每种方法都需要严格控制变量才能获得可靠结果。
9. Real-World Applications | 实际应用
Understanding rates is used in food preservation (cold storage slows spoilage), industrial production (choosing catalysts to balance speed and cost), and medicine (how quickly tablets dissolve).
理解反应速率用于食品保存（冷藏减缓变质）、工业生产（选择催化剂以平衡速度与成本）以及医学领域（药片溶解的快慢）。
- High surface area in medicines helps them absorb quickly.
- 药物中的高表面积有助于快速吸收。
- Industrial reactions often use high pressure and temperature to increase yield rate, but cost and safety matter.
- 工业反应常采用高压高温来提高产率，但需考虑成本和安全。
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