📚 Reaction Rates | 反应速率 考点精讲
Reaction rates are a central pillar of OCR A-Level Chemistry, bridging the gap between the feasibility of a reaction (thermodynamics) and its practical timeline. A thorough grasp of collision theory, experimental methods and the quantitative rate equation is essential for both examination success and deeper understanding of chemical processes.
反应速率是 OCR A-Level 化学的核心支柱之一，它连接了反应的自发性（热力学）与实际时间尺度。深入掌握碰撞理论、实验测量方法以及定量的速率方程，对应试成功和深刻理解化学过程都至关重要。
1. Defining Reaction Rate | 反应速率的定义
The rate of a chemical reaction is defined as the change in concentration of a reactant or product per unit time. For a reactant A, rate = –Δ[A]/Δt; for a product, rate = Δ[product]/Δt. The conventional units are mol dm⁻³ s⁻¹.
化学反应速率定义为反应物或产物浓度在单位时间内的变化量。对于反应物 A，速率 = –Δ[A]/Δt；对于产物，速率 = Δ[产物]/Δt。常用单位是 mol dm⁻³ s⁻¹。
Instantaneous rate is found from the gradient of a tangent to the concentration–time curve at a specific time. The initial rate is the instantaneous rate at t = 0, where it carries the maximum value and is least complicated by reverse reactions or side products.
瞬时速率由浓度–时间曲线上某点切线的斜率求得。初始速率是 t = 0 时的瞬时速率，此时速率最大且受逆反应或副产物干扰最小。
2. Experimental Measurement of Rates | 实验测量反应速率
Several experimental techniques allow us to follow the progress of a reaction. The choice depends on the physical and chemical properties of reactants and products.
多种实验技术可用于追踪反应进程，选择依据取决于反应物与产物的物理化学性质。
Common methods include:
- Gas collection – measuring the volume of gas evolved using a gas syringe or displacement of water.
- Mass loss – monitoring the decrease in mass of the reaction mixture as a gas escapes.
- Colorimetry – tracking absorbance changes for coloured species such as I₂ or Br₂.
- Sampling and quenching – withdrawing samples at timed intervals, quenching to stop the reaction and titrating for remaining reactant.
- Conductivity – measuring changes in electrical conductivity when the number of ions changes.
常见方法包括：
- 气体收集 – 用气筒或排水法测量生成气体的体积。
- 质量损失 – 随气体逸出监测反应混合物质量的减少。
- 比色法 – 追踪有色物种（如 I₂ 或 Br₂）的吸光度变化。
- 取样与淬灭 – 定时取样，淬灭使反应停止后滴定剩余反应物。
- 电导法 – 当离子数目改变时测量电导率变化。
3. Collision Theory and Activation Energy | 碰撞理论与活化能
For a reaction to occur, reactant particles must collide with energy equal to or greater than the activation energy, Ea, and with an appropriate orientation that allows bonds to break and form. Only a small fraction of collisions meet both criteria.
反应发生要求反应物粒子以等于或高于活化能 Ea 的能量碰撞，并且取向适当使化学键能够断裂和生成。只有一小部分碰撞同时满足这两个条件。
Activation energy is the minimum energy required for a reaction to take place. It represents the energy barrier that must be overcome. The lower the Ea, the greater the proportion of successful collisions at a given temperature.
活化能是反应发生所需的最低能量，它代表必须克服的能垒。Ea 越低，在给定温度下有效碰撞的比例就越大。
4. The Maxwell-Boltzmann Distribution | 麦克斯韦-玻尔兹曼分布
The Maxwell-Boltzmann distribution shows the spread of kinetic energies among particles in a gas or liquid at a constant temperature. The curve starts at the origin, rises to a peak that represents the most probable energy, and then tails off towards high energies.
麦克斯韦-玻尔兹曼分布展示了恒定温度下气体或液体中粒子动能的分布。曲线始于原点，上升至代表最可几能量的峰值，然后向高能端拖尾。
The area under the curve beyond the Ea line corresponds to the fraction of particles that possess enough energy to react. This area is very sensitive to temperature and to the presence of a catalyst that lowers Ea.
曲线下 Ea 线右侧的面积对应于具有足够能量发生反应的粒子比例。该面积对温度以及能降低 Ea 的催化剂非常敏感。
5. Effect of Concentration and Pressure | 浓度和压力的影响
Increasing the concentration of a reactant in solution means more particles are present per unit volume. This raises the frequency of collisions, leading to a higher reaction rate without changing the energy distribution.
增大溶液中反应物的浓度意味着单位体积内有更多粒子，这提高了碰撞频率，从而加快反应速率，但不改变能量分布。
For gaseous reactions, an increase in pressure (or decrease in volume) has an analogous effect: it raises the concentration of gas molecules and therefore the collision frequency, increasing the rate.
对于气体反应，增加压强（或减小体积）具有类似效果：它提高了气体分子的浓度，因而增加碰撞频率，使速率加快。
It is worth noting that not every collision leads to a reaction; only those with energy ≥ Ea and correct orientation are successful. So the rate increase is proportional to the increase in total collisions only if the energetic requirement remains unchanged.
需要注意并非每次碰撞都发生反应；只有能量 ≥ Ea 且取向正确的碰撞才能成功。因此速率的增加与总碰撞次数的增加成正比，前提是能量要求不变。
6. Effect of Temperature | 温度的影响
Raising the temperature has a dual effect. Firstly, particles move faster, so collision frequency increases slightly. More importantly, the distribution of kinetic energies shifts: the curve flattens and moves to the right, greatly increasing the proportion of particles with energy ≥ Ea.
升高温度有双重效应。首先，粒子运动变快，碰撞频率略有增加。更重要的是，动能分布发生变化：曲线变平、右移，极大地增加了能量 ≥ Ea 的粒子比例。
It is this exponential increase in the fraction of energetic particles that makes temperature such a powerful factor. Often a 10 °C rise will double or triple the reaction rate, a rule of thumb that underlines the sensitivity of rate to temperature.
正是高能粒子比例的指数式增加，使温度成为一个十分强有力的因素。通常温度升高 10 °C 会使速率翻倍或增至三倍，这一经验法则突显了速率对温度的敏感性。
Plotting a Maxwell-Boltzmann distribution before and after heating visually demonstrates the greater area beyond Ea, which directly accounts for the observed rate enhancement.
绘制加热前后的麦克斯韦-玻尔兹曼分布可直观展示 Ea 右侧面积增大，这直接解释了所观察到的速率升高。
7. Surface Area and Catalysts | 表面积与催化剂
For heterogeneous reactions involving solids, breaking a solid into smaller pieces increases its surface area. A larger surface allows more frequent collisions between reactant particles and the solid surface, accelerating the reaction rate.
对于有固体参与的非均相反应，将固体粉碎成小块可增大其表面积。更大的表面积使反应物粒子与固体表面的碰撞更频繁，从而加快反应速率。
A catalyst provides an alternative reaction pathway with a lower activation energy. It participates in the reaction mechanism but is regenerated, remaining chemically unchanged at the end. By lowering Ea, a catalyst shifts the Maxwell-Boltzmann distribution so that a much larger fraction of molecules can react at the same temperature.
催化剂提供一条活化能较低的替代反应路径。它参与反应机理，但被再生，结束时化学性质不变。通过降低 Ea，催化剂使麦克斯韦-玻尔兹曼分布中能反应的分子比例在相同温度下大幅增加。
The table below summarises how each factor influences reaction rate in the framework of collision theory.
下表总结了各因素在碰撞理论框架下对反应速率的影响。
| Factor | Effect on rate | Collision theory explanation |
|---|---|---|
| Concentration / Pressure | Increase | More particles per unit volume → higher collision frequency |
| Temperature | Strong increase | Larger fraction of particles with E ≥ Ea; minor increase in collision frequency |
| Surface area (solid) | Increase | Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com
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