📚 GCSE WJEC Chemistry: Rates of Reaction – Essential Revision Notes | GCSE WJEC 化学：反应速率考点精讲
Understanding the rate of a chemical reaction is key to controlling industrial processes, biological systems, and everyday changes. This WJEC GCSE Chemistry revision guide covers the essential concepts – from measuring rates to collision theory, and the influence of temperature, concentration, surface area, and catalysts. Master the graphical analysis and calculations that regularly appear on exam papers.
理解化学反应的速率是控制工业流程、生物系统和日常变化的关键。本 WJEC GCSE 化学复习指南涵盖核心概念——从测量速率到碰撞理论，以及温度、浓度、表面积和催化剂的影响。掌握试卷中经常出现的图表分析和计算。
1. What is Rate of Reaction? | 什么是反应速率？
The rate of a reaction tells us how quickly reactants are converted into products. It can be defined as the change in amount of a reactant or product per unit time. For reactions producing a gas or losing mass, we commonly use: Rate = quantity of product formed / time, or Rate = mass of reactant used up / time. The units could be g/s, cm³/s, or mol/s depending on what is measured.
反应速率表明反应物转化为产物的快慢。它可以定义为单位时间内反应物或产物量的变化。对于产生气体或质量减少的反应，我们通常使用：速率 = 生成的产物量 / 时间，或速率 = 消耗的反应物质量 / 时间。单位可能是 g/s、cm³/s 或 mol/s，取决于测量对象。
average rate = change in quantity ÷ time taken
2. Measuring Rate: Monitoring Mass Loss | 测量速率：监测质量减少
This method is ideal when a gas is given off and escapes. For example, placing marble chips (calcium carbonate) in a flask with dilute hydrochloric acid on a balance. The reaction produces CO₂ gas which leaves the flask, causing a measurable mass loss. The equation: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g). Record mass at regular intervals; the steepest part of the mass-loss curve indicates the fastest rate.
当有气体释放并逸出时，此方法最理想。例如，将大理石碎片（碳酸钙）和稀盐酸放在天平上的锥形瓶中。反应生成 CO₂ 气体并离开烧瓶，导致质量可测量的减少。方程式：CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)。定期记录质量；质量减少曲线最陡峭的部分表明速率最快。
If the mass decreases by 0.44 g in 80 seconds, the average rate = 0.44 ÷ 80 = 0.0055 g/s. Always note that the reaction slows down over time as reactants are used up.
如果质量在 80 秒内减少 0.44 g，平均速率 = 0.44 ÷ 80 = 0.0055 g/s。务必注意，随着反应物被消耗，反应速率随时间减慢。
3. Measuring Rate: Collecting Gas Over Water or with a Gas Syringe | 测量速率：排水集气法或气体注射器
When a gas is produced, its volume can be measured. A common setup uses a conical flask connected to a delivery tube leading to an inverted measuring cylinder filled with water (downward displacement of water). Alternatively, a gas syringe gives a direct volume reading. Record the gas volume every 10 seconds. The rate = volume of gas / time, often in cm³/s.
当产生气体时，可以测量其体积。常见的装置是用锥形瓶连接导管，通入一个装满水倒置的量筒（向下排水集气法）。或者，使用气体注射器直接读取体积。每 10 秒记录一次气体体积。速率 = 气体体积 / 时间，常以 cm³/s 为单位。
For the same marble-acid reaction, if 36 cm³ of CO₂ is collected in the first 60 seconds, the average rate = 36/60 = 0.60 cm³/s. As before, the rate decreases as the acid concentration drops.
对于同一个大理石与酸的反应，如果在头 60 秒内收集到 36 cm³ CO₂，平均速率 = 36/60 = 0.60 cm³/s。如前所述，随着酸浓度下降，速率减小。
4. Measuring Rate: The Disappearing Cross (Turbidity) | 测量速率：消失的十字（浊度法）
This experiment uses the reaction between sodium thiosulfate (Na₂S₂O₃) and hydrochloric acid. The equation: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l). The sulfur precipitate makes the solution cloudy. A cross drawn on paper is placed under the reaction flask; the time taken for the cross to ‘disappear’ from view is recorded. Rate ∝ 1 / time (a shorter time means a faster rate).
此实验使用硫代硫酸钠 (Na₂S₂O₃) 与盐酸的反应。方程式：Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + SO₂(g) + S(s) + H₂O(l)。硫沉淀使溶液变浑浊。在反应瓶下方放置一张画有十字的纸；记录十字从视线中“消失”所需的时间。速率 ∝ 1 / 时间（时间越短表示速率越快）。
For a reliable result, keep the viewing distance and the size of the cross constant. This method is especially useful to demonstrate the effect of temperature or concentration on rate without needing numerical measurements.
为获得可靠结果，观察距离和十字大小应保持不变。此方法特别适合演示温度或浓度对速率的影响，而无需数值测量。
5. Collision Theory | 碰撞理论
Reactions occur only when reacting particles collide. For a collision to be successful and lead to products, two conditions must be met: the colliding particles must have energy equal to or greater than the activation energy (Eₐ), and they must collide with the correct orientation. The rate of reaction depends on the frequency of successful collisions per second.
只有当反应粒子发生碰撞时，反应才会发生。要成功碰撞并生成产物，必须满足两个条件：碰撞粒子的能量必须等于或大于活化能 (Eₐ)，并且必须以正确的取向碰撞。反应速率取决于每秒成功碰撞的频率。
Activation energy (Eₐ) is the minimum energy required for a reaction to occur. It acts as an energy barrier; only particles with energy ≥ Eₐ can react when they collide.
活化能 (Eₐ) 是反应发生所需的最低能量。它像一个能量屏障；只有能量 ≥ Eₐ 的粒子在碰撞时才能发生反应。
6. Effect of Temperature on Rate | 温度对速率的影响
Increasing the temperature significantly speeds up most reactions. There are two reasons: (i) particles gain kinetic energy and move faster, so they collide more frequently. (ii) More importantly, a much larger fraction of particles now have energy equal to or greater than the activation energy, leading to many more successful collisions per second.
升高温度会显著加速大多数反应。有两个原因：(i) 粒子获得动能，运动更快，因此碰撞更频繁。(ii) 更重要的是，现在具有等于或大于活化能的粒子比例大大增加，导致每秒成功碰撞次数多得多。
As a rough guide, for many reactions at room temperature, a 10°C rise approximately doubles the rate. The Maxwell–Boltzmann distribution explains this: the curve flattens and shifts to the right, so the area under the curve beyond Eₐ increases sharply.
粗略估计，对于室温下的许多反应，温度每升高 10°C，速率大约翻倍。麦克斯韦–玻尔兹曼分布解释了这一点：曲线变平并向右移动，因此超过 Eₐ 的曲线下面积急剧增加。
7. Effect of Concentration and Pressure | 浓度和压力的影响
For reactions in solution, increasing the concentration of a reactant means there are more dissolved particles per unit volume. This leads to more frequent collisions and thus a faster rate. Similarly, for gases, increasing the pressure forces particles closer together, increasing the collision frequency.
对于溶液中的反应，增加反应物浓度意味着单位体积内有更多溶解粒子。这导致碰撞更频繁，从而速率更快。同样，对于气体，增加压力使粒子靠得更近，增加了碰撞频率。
Note: concentration changes only affect the rate if the reactant involved is part of the rate-determining step. Pressure changes only apply to reactions involving gases and effectively increase the concentration of gaseous reactants.
注意：浓度变化只有在所涉及的反应物是速率控制步骤的一部分时才会影响速率。压力变化仅适用于涉及气体的反应，并有效地增加了气态反应物的浓度。
8. Effect of Surface Area (Particle Size) | 表面积（颗粒大小）的影响
For solid reactants, breaking them into smaller pieces or powder increases the total surface area exposed to the other reactants. Only particles on the surface can collide immediately. With a larger surface area, more solid particles are available for collision at any moment, so the frequency of successful collisions rises, making the reaction faster.
对于固体反应物，将其破碎成更小的碎片或粉末可增加暴露给其他反应物的总表面积。只有表面的粒子能够立即碰撞。表面积更大时，任何时刻都有更多固体粒子可供碰撞，从而增加成功碰撞的频率，使反应更快。
A large marble chip reacts relatively slowly with acid; the same mass of powdered marble effervesces vigorously and finishes much sooner. In terms of collision theory, more reactant ‘faces’ are accessible.
大块大理石与酸反应较慢；相同质量的大理石粉末则会剧烈起泡并更快完成。从碰撞理论来说，有更多的反应“面”可以接触。
9. Catalysts and Activation Energy | 催化剂与活化能
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 (Eₐ). This means that a much larger proportion of particles now possess the required energy, so the frequency of successful collisions multiplies.
催化剂是一种能够增加反应速率而自身不被消耗的物质。它通过提供具有较低活化能 (Eₐ) 的替代反应路径来起作用。这意味着现在具备所需能量的粒子比例大大增加，因此成功碰撞的频率成倍增加。
Catalysts are not included in the overall chemical equation because they are regenerated. Biological catalysts are called enzymes. In the lab, manganese(IV) oxide (MnO₂) catalyses the decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂. Only a tiny mass of catalyst is needed.
催化剂不包含在总化学方程式中，因为它们被再生。生物催化剂称为酶。在实验室中，二氧化锰 (MnO₂) 催化过氧化氢的分解：2H₂O₂ → 2H₂O + O₂。只需要极少质量的催化剂。
10. Interpreting Rate Graphs | 解读速率图表
A graph of ‘amount of product formed’ or ‘mass of reactant remaining’ against time offers deep insight. The slope of the tangent at any point gives the instantaneous rate. Initially, the slope is steepest because reactant concentrations are highest. Over time, the slope decreases as reactants are consumed, and eventually the line becomes horizontal when the reaction stops (limiting reactant used up).
“产物生成量”或“剩余反应物质量”对时间的关系图能提供深层见解。任意点处切线的斜率给出瞬时速率。开始时斜率最陡，因为反应物浓度最高。随时间推移，斜率随着反应物的消耗而减小，当反应停止（限制反应物耗尽）时，线条最终变为水平。
To compare two conditions, e.g. higher temperature, the curve will be steeper and reach completion sooner, but the final amount of product is the same (if the same amounts of reactants are used). Average rate can be calculated as total product formed / total time.
比较两种条件时，例如较高温度，曲线会更陡峭并更快达到完成，但最终产物量相同（如果使用相同量的反应物）。平均速率可计算为总产物生成量 / 总时间。
- Steeper initial gradient = faster initial rate
- Flattening = rate decreasing
- Horizontal line = reaction complete
- 初始梯度更陡 = 初始速率更快
- 变平 = 速率下降
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