📚 Rates of Reaction | 化学反应速率
Understanding how fast chemical reactions occur is essential not only in the laboratory but also in industry, where controlling speed can save time, energy and money. In IGCSE Edexcel Science, the rate of reaction is defined as the change in amount of reactant or product per unit time. Factors such as concentration, temperature, surface area and the presence of a catalyst can dramatically alter this speed. Grasping these concepts, together with the collision theory, will enable you to interpret experimental data and graphs confidently.
了解化学反应进行的快慢，不仅在实验室中至关重要，在工业生产上同样关键，因为控制反应速度可以节省时间、能源和成本。在IGCSE Edexcel科学课中，反应速率定义为单位时间内反应物或生成物量的变化。浓度、温度、表面积以及催化剂的存在等因素都会显著改变反应速率。掌握这些概念，并结合碰撞理论，将帮助你自信地解读实验数据和图表。
1. Defining Rate of Reaction | 反应速率的定义
quantity could be mass, volume of gas produced, or concentration change. For example, if a reaction produces 24 cm³ of gas in 2 minutes, its average rate is 12 cm³ per minute. A faster reaction has a steeper gradient on a graph of quantity versus time.
化学反应的速率告诉我们反应物被消耗或生成物产生的快慢。它可以表示为：速率 = 量的变化 / 所用时间。这里的“量”可以是质量、生成气体的体积或浓度的变化。例如，一个反应在2分钟内生成了24 cm³气体，其平均速率就是每分钟12 cm³。在“量-时间”图上，反应越快，曲线的斜率就越陡。
2. Collision Theory | 碰撞理论
For a reaction to occur, particles must collide with sufficient energy and with the correct orientation. This is known as the collision theory. The minimum energy required for a successful collision is called the activation energy (Eₐ). Only collisions with energy equal to or greater than Eₐ will result in a reaction. Increasing the frequency of effective collisions or the proportion of particles with energy above Eₐ will raise the rate of reaction.
要发生反应，粒子必须以足够的能量和正确的方向碰撞。这就是碰撞理论。成功碰撞所需的最低能量称为活化能 (Eₐ)。只有能量等于或大于 Eₐ 的碰撞才会导致反应。提高有效碰撞的频率，或者增加能量超过 Eₐ 的粒子比例，都会加快反应速率。
3. Effect of Concentration | 浓度的影响
Higher concentration means more reactant particles are present in the same volume. This increases the frequency of collisions between particles, making effective collisions more likely per unit time. As a result, the rate of reaction increases. For reactions involving solutions, doubling the concentration often roughly doubles the rate, provided other conditions remain constant. However, once the limiting reactant is used up, the rate drops to zero.
浓度越高，意味着相同体积内含有更多的反应物粒子。这会增加粒子之间的碰撞频率，使单位时间内发生有效碰撞的可能性更大，因此反应速率加快。对于溶液中的反应，在其他条件不变的情况下，浓度加倍通常会使速率大致加倍。不过，当限制性反应物消耗殆尽时，速率会降为零。
4. Effect of Temperature | 温度的影响
Raising the temperature has two main effects: particles gain kinetic energy and move faster, leading to more frequent collisions. More importantly, a greater proportion of particles now have energy equal to or above the activation energy. Even a small temperature rise can greatly increase the rate of reaction because the number of particles exceeding the Eₐ threshold can rise exponentially. In general, a 10 °C increase often doubles the rate for many reactions at room temperature.
升高温度有两个主要效应：粒子获得动能，运动得更快，碰撞更加频繁。更重要的是，此时有更大比例的粒子达到或超过了活化能。即使是很小的温度升高也会使反应速率大大提高，因为超过 Eₐ 阈值的粒子数目可能呈指数增长。通常，室温附近的许多反应，温度每升高10 °C，速率大约翻倍。
5. Effect of Surface Area | 表面积的影响
When a solid reactant is broken into smaller pieces, its total surface area exposed to other reactants increases. This allows more particles to be available for collision at any one time, raising the collision frequency without changing the concentration or temperature. Thus, powdered solids react faster than larger lumps. This is why catalysts in industry are often used in finely divided forms, and why dust explosions can occur with combustible powders.
当固体反应物被粉碎成更小的颗粒时，其暴露于其他反应物的总表面积增大。这使任何时刻都有更多的粒子可供碰撞，提高了碰撞频率，而浓度和温度不变。因此，粉末状固体比块状固体反应更快。这就是工业上催化剂常以细微粉末形态使用的原因，也说明了可燃粉尘为何会引起粉尘爆炸。
6. Catalysts and Rate of Reaction | 催化剂与反应速率
biological catalysts that work under mild conditions. In the laboratory, manganese(IV) oxide catalyses the decomposition of hydrogen peroxide into water and oxygen: 2H₂O₂ → 2H₂O + O₂.
催化剂是一种能提高反应速率，而在反应结束时既没有化学变化也没有被消耗的物质。它的工作原理是提供一条活化能较低的替代反应路径，因此有更多的粒子具备成功反应所需的能量。酶是生物催化剂，在温和条件下起作用。在实验室中，二氧化锰 (MnO₂) 可催化过氧化氢分解为水和氧气：2H₂O₂ → 2H₂O + O₂。
7. Measuring Rate of Reaction | 测量反应速率
In the IGCSE context, common methods to follow reaction rate include: measuring the volume of gas evolved with a gas syringe or over water; measuring the loss in mass using a balance; and observing the time taken for a cross to disappear when a precipitate forms. The rate can be calculated as the slope of the tangent on a curve at a given time, giving the instantaneous rate. Average rate is simply the total change divided by total time.
在IGCSE范畴中，常见的跟踪反应速率的方法有：使用气体注射器或排水法测量放出气体的体积；使用天平测量质量损失；以及观察生成沉淀时十字标记消失所需的时间。瞬时速率可以通过曲线上某点切线的斜率求得，平均速率则是总变化量除以总时间。
8. Interpreting Rate Graphs | 速率图的解读
A graph of quantity of product versus time typically shows a steep initial slope that gradually levels off to a plateau. The steepest part of the curve represents the fastest rate, usually at the start when reactant concentrations are highest. As the reaction proceeds, reactants are used up, collisions become less frequent, and the rate decreases. The reaction is complete when the graph becomes horizontal, indicating that no more product is being formed. Comparing curves from different conditions allows you to deduce the effect of a variable.
生成物量-时间图通常显示：初始曲线很陡，随后逐渐趋于平坦。曲线最陡的部分代表最快的速率，一般在反应起始阶段反应物浓度最高时出现。随着反应进行，反应物被消耗，碰撞频率降低，速率下降。当曲线变为水平时，反应结束，不再有产物生成。通过比较不同条件下的曲线，可以推断出某个变量带来的影响。
9. Practical Investigation Example | 实验探究示例
A classic IGCSE experiment investigates how changing the concentration of hydrochloric acid affects the rate of its reaction with magnesium ribbon. Magnesium ribbon is added to a known volume and concentration of HCl, and the volume of hydrogen gas evolved is recorded every 10 seconds. The experiment is repeated with different acid concentrations while keeping the total volume, temperature and length of magnesium constant. Results typically show that higher acid concentrations produce a steeper initial gradient and a shorter time to completion.
一个经典的IGCSE实验探究是改变盐酸浓度对它与镁带反应速率的影响。将镁带加入已知体积和浓度的盐酸中，每隔10秒记录一次产生氢气的体积。实验在不同的酸浓度下重复进行，同时保持溶液总体积、温度以及镁带长度不变。结果通常显示，酸浓度越高，初始斜率越陡，反应完成所需时间越短。
10. Collision Theory Summary Table | 碰撞理论总结表
| Factor | Effect on collision frequency | Effect on proportion with E ≥ Eₐ | Rate change |
|---|---|---|---|
| Increasing concentration | Increases | No change | Increases |
| Increasing temperature | Increases slightly | Increases significantly | Increases greatly |
| Increasing surface area | Increases (solid only) | No change | Increases |
| Adding a catalyst | No direct change | Increases (lowers Eₐ) | Increases |
This table summarises how each factor influences collision frequency and the proportion of particles meeting the activation energy requirement. Notice that concentration and surface area only alter the number of collisions, while temperature and catalysts change the energy distribution of particles.
这张表格总结了各个因素如何影响碰撞频率以及达到活化能要求的粒子比例。注意，浓度和表面积仅改变碰撞次数，而温度和催化剂则改变粒子的能量分布。
11. Rate Equations and Calculations | 速率方程与计算
The mathematical expression for average rate is:
average rate = Δquantity / Δtime
inutes on a volume-time graph has a slope of 20 cm³/min, that is the instantaneous rate. Remember that rates usually decrease over time, so instantaneous rate is lower later in the reaction.
平均速率的数学表达式为：
平均速率 = Δ量 / Δ时间
若一个反应在5分钟内产生60 cm³气体，平均速率 = 60 cm³ ÷ 5 min = 12 cm³/min。如果在体积-时间图上，2分钟时切线的斜率为20 cm³/min，这就是瞬时速率。记住，反应速率通常会随时间下降，所以反应后期的瞬时速率更低。
12. Real-World Applications and Exam Tips | 实际应用与考试技巧
Chemical industries rely on rate control: the Haber process uses an iron catalyst to make ammonia economically. Food preservation uses low temperatures to slow spoilage reactions. In exams, always describe the effect on collision frequency and activation energy when explaining rate changes. Use precise terminology and refer to particle diagrams or graph shapes for high marks. When drawing curves, make sure that the final amount of product is the same if the same amount of limiting reactant is used, even if the rate differs.
化工生产依赖速率控制：哈伯法使用铁催化剂来经济地生产氨。食品保存利用低温来延缓腐坏反应。考试时，解释速率变化一定要说明对碰撞频率和活化能的影响。使用精确的术语，并结合粒子图或曲线形状来答题，有助于获得高分。绘制曲线时，要注意如果限制性反应物的量相同，即使反应速率不同，最终产物总量也应当一致。
Published by TutorHao | Science Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图