Kinetics | 动力学 - aleveler.com

📚 Kinetics | 动力学

Chemical kinetics is the study of reaction rates and the factors that control how fast reactions occur. In AQA A-level Chemistry topic 1.6, the focus is on collision theory, the Maxwell–Boltzmann distribution, activation energy and the role of catalysts. These ideas explain why changing concentration, pressure, temperature or surface area can dramatically alter the speed of a reaction.

化学动力学研究反应速率以及控制反应快慢的因素。在AQA A-level化学1.6中，重点包括碰撞理论、麦克斯韦-玻尔兹曼分布、活化能以及催化剂的作用。这些概念解释了为什么改变浓度、压强、温度或表面积会显著改变反应速度。

1. What Is Reaction Rate? | 什么是反应速率？

Reaction rate is a measure of how quickly reactants are consumed or products are formed. It can be expressed as the change in concentration of a reactant or product divided by the time taken for that change. The standard unit for concentration is mol dm⁻³, so rate is usually given in mol dm⁻³ s⁻¹.

反应速率衡量反应物消耗得多快或产物生成得多快。它可以表示为单位时间内反应物或产物浓度的变化。浓度的常用单位是mol dm⁻³，因此速率通常以mol dm⁻³ s⁻¹表示。

rate = −Δ[reactant] / Δt = +Δ[product] / Δt

Several experimental methods can be used to follow a reaction, depending on the substances involved:

根据所涉及的物质，可以用多种实验方法跟踪反应进程：

Measuring the volume of gas produced over time.

随时间测量生成气体的体积。
Measuring the loss of mass when a gas escapes from the reaction flask.

当气体从反应瓶中逸出时测量质量损失。
Using a colorimeter to follow a colour change.

使用比色计跟踪颜色变化。

2. Collision Theory | 碰撞理论

For a reaction to take place, particles must collide with enough energy and with the correct orientation. Collision theory explains reaction rates in terms of effective collisions. An effective collision is one that successfully converts reactants into products.

反应发生的前提是粒子必须以足够的能量并以正确的取向发生碰撞。碰撞理论用有效碰撞来解释反应速率。所谓有效碰撞，是指成功将反应物转变成产物的碰撞。

The conditions for an effective collision are:

发生有效碰撞需要满足以下条件：

The reactant particles must physically collide with each other.

反应物粒子之间必须发生实际的相互碰撞。
The collision must have energy equal to or greater than the activation energy, Eₐ.

碰撞能量必须大于或等于活化能Eₐ。
The particles must be oriented correctly so that the relevant bonds can break and form.

粒子的取向必须正确，使得相关化学键能够断裂并形成。

3. Activation Energy and the Maxwell–Boltzmann Distribution | 活化能与麦克斯韦-玻尔兹曼分布

The activation energy, Eₐ, is the minimum energy that colliding particles must have for a reaction to occur. Even in a small sample of gas, individual molecules have different kinetic energies. The Maxwell–Boltzmann distribution is a graph showing the spread of molecular energies at a fixed temperature.

活化能Eₐ是碰撞粒子发生反应所需的最低能量。即使在一小份气体样品中，单个分子也具有不同的动能。麦克斯韦-玻尔兹曼分布是一幅描述在一定温度下分子能量范围的曲线图。

Key features of the Maxwell–Boltzmann distribution you must be able to identify:

你必须能够识别麦克斯韦-玻尔兹曼分布的关键特征：

The curve starts at the origin: no particles have zero energy.

曲线从原点开始：没有粒子的能量为零。
The area under the curve represents the total number of particles present.

曲线下的面积代表存在的粒子总数。
The peak of the curve corresponds to the most probable energy.

曲线的峰值对应最概然能量。
The mean energy is slightly greater than the most probable energy because the curve has a long high-energy tail.

由于曲线有一条较长的高能尾巴，平均能量略高于最概然能量。
Only particles with energy greater than or equal to Eₐ, i.e. those in the shaded region to the right of the activation energy line, can react.

只有能量大于或等于Eₐ的粒子，即位于活化能竖线右侧阴影区域内的粒子，才能发生反应。

4. Effect of Concentration and Pressure | 浓度和压强的影响

When the concentration of a solution is increased, more reactant particles are present in the same volume. This raises the collision frequency, so the number of effective collisions per unit time increases and the rate of reaction is faster. Increasing concentration does not alter the activation energy or the average kinetic energy of the particles.

当溶液浓度增大时，相同体积内含有更多的反应物粒子。这提高了碰撞频率，因此单位时间内有效碰撞的次数增加，反应速率加快。增大浓度不会改变活化能或粒子的平均动能。

For gaseous reactions, increasing pressure at constant temperature compresses the gas, bringing particles closer together. As a result, the collision frequency increases, leading to more effective collisions and a higher reaction rate.

对于气体反应，在恒温下增大压强会使气体被压缩，粒子彼此更加靠近。结果是碰撞频率增加，产生更多有效碰撞，从而反应速率提高。

5. Effect of Temperature | 温度的影响

Raising the temperature increases the average kinetic energy of particles, so a greater proportion of particles has energy greater than or equal to Eₐ. On the Maxwell–Boltzmann curve, a higher temperature shifts the peak to the right and makes the curve lower and broader. The total area under the curve remains the same because the number of particles is unchanged.

升高温度会增加粒子的平均动能，因此有更大比例的粒子能量达到或超过Eₐ。在麦克斯韦-玻尔兹曼曲线上，更高的温度会使峰值向右移动，同时曲线变得更低、更宽。曲线下的总面积保持不变，因为粒子总数并未改变。

Even a small increase in temperature leads to a large increase in the number of effective collisions. This explains why reaction rates often double or even triple when the temperature is raised by only about 10 K.

即使温度小幅升高，也会使有效碰撞次数大幅增加。这也解释了为什么温度仅升高约10 K时，反应速率常常会成倍甚至增大数倍。

6. Effect of Surface Area | 表面积的影响

Solid reactions can only occur when particles collide at the surface of the solid. If a solid is cut, ground or powdered, its total surface area increases significantly. This exposes more reactant particles to the other reactant, raising the collision frequency at the interface and therefore increasing the rate of reaction.

固相反应只能发生在固体表面上。如果把固体切割、研磨或制成粉末，其总表面积会显著增加。这会暴露出更多反应物粒子与其他反应物接触，提高了界面上的碰撞频率，因此反应速率增大。

Surface area has no effect on the activation energy or the average kinetic energy of particles. It simply makes effective collisions more frequent.

表面积不影响活化能或粒子的平均动能。它只是使有效碰撞更加频繁。

7. Catalysts | 催化剂

A catalyst is a substance that speeds up a chemical reaction without being used up. It does this by providing an alternative reaction pathway with a lower activation energy. Catalysts are important in industry because they reduce the energy cost of a process and help reactions reach completion faster.

催化剂是能够加快化学反应而又不被消耗的物质。它通过提供一条活化能更低的替代反应途径来实现这一点。催化剂在工业中非常重要，因为它们降低了工艺所需的能量成本，并帮助反应更快达到完全转化。

At a given temperature, lowering Eₐ means that a much larger fraction of particle collisions are effective. Therefore, the rate of reaction rises. Catalysts are also selective: different reactions usually require different catalysts.

在给定温度下，降低Eₐ意味着更大比例的粒子碰撞是有效的，因此反应速率上升。催化剂还具有选择性：不同的反应通常需要不同的催化剂。

Examples of catalysts you should know:

你应该了解的催化剂例子：

Manganese(IV) oxide catalyses the decomposition of hydrogen peroxide into water and oxygen.

二氧化锰催化过氧化氢分解为水和氧气。
Iron is used as a catalyst in the Haber process for ammonia synthesis.

铁用于哈伯法合成氨反应中作催化剂。
Enzymes are biological catalysts that are highly specific to particular reactants.

酶是对特定反应物具有高度特异性的生物催化剂。

8. Catalysts and the Maxwell–Boltzmann Distribution | 催化剂与麦克斯韦-玻尔兹曼分布

On a Maxwell–Boltzmann distribution diagram, the presence of a catalyst does not change the shape, position or total area of the curve, provided the temperature is constant. The molecular energy distribution is still exactly the same.

在麦克斯韦-玻尔兹曼分布图中，如果温度保持恒定，催化剂的存在不会改变曲线的形状、位置或总面积。分子能量分布仍然完全相同。

However, the activation energy line is moved to the left, from Eₐ to a lower value, Eₐ(cat). The shaded area to the right of this new line is larger, showing that many more particles now have sufficient energy to react than in the uncatalysed reaction.

然而，活化能线会向左移动，从Eₐ移动到

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Kinetics | 动力学