Rates of Reaction | 反应速率

📚 Rates of Reaction | 反应速率

Chemical reactions occur at different speeds. Some, like explosions, are extremely fast, while others, such as rusting, are very slow. Understanding the factors that control reaction rates is essential in both chemistry and industry.

化学反应以不同速度发生。有些反应极快，如爆炸；有些反应则非常缓慢，如生锈。理解控制反应速率的因素在化学和工业领域都至关重要。

1. What is Rate of Reaction? | 什么是反应速率？

The rate of a reaction measures how quickly reactants are used up or products are formed. It is defined as the change in the amount of a substance per unit time.

反应速率衡量反应物被消耗或产物生成的快慢。它定义为反应物或产物在单位时间内发生的变化量。

The general equation for rate is:

反应速率的一般公式为：

Rate = ΔAmount / ΔTime

Here, Δ (delta) means ‘change in’. The amount can be measured in grams (g), cubic centimetres (cm³), moles (mol), or as a change in concentration (mol/dm³).

这里的 Δ 表示“变化量”。物质的量可以用克（g）、立方厘米（cm³）、摩尔（mol）或浓度变化（mol/dm³）来度量。

For example, if a reaction produces 10 cm³ of gas in 20 seconds, the average rate is 0.5 cm³/s.

例如，若某一反应在 20 秒内产生 10 cm³ 气体，则平均速率为 0.5 cm³/s。

2. Measuring Rates | 测量反应速率

Several experimental methods can be used to follow the progress of a reaction. The choice depends on the physical state of the reactants and products.

多种实验方法可以用来跟踪反应的进程。选择哪种方法取决于反应物和产物的物理状态。

Mass loss: If a gas is given off, the total mass of the flask decreases. A balance can record the change over time.

质量损失法：若反应释放出气体，烧瓶总质量会减少。可用天平记录随时间的变化。
Gas volume: A gas syringe or measuring cylinder over water can collect the gas produced.

气体体积法：可以使用气筒或排水集气法收集产生的气体。
Turbidity: For reactions forming a precipitate, the cloudiness of the mixture increases. This can be observed visually or with a light sensor.

浊度法：对于生成沉淀的反应，混合物的浑浊度会增加。可以通过肉眼或光传感器观察。
Colour change: In titrations or reactions with coloured substances, the time for a colour change can be measured.

颜色变化法：在滴定或涉及有色物质的反应中，可以测量颜色变化所需的时间。

These methods allow us to calculate the rate at different moments, or an average rate for the whole reaction.

这些方法使我们能够计算不同时刻的速率，或整个反应的平均速率。

3. Collision Theory | 碰撞理论

For a reaction to happen, reactant particles must collide with each other. However, not every collision leads to a reaction.

为了使反应发生，反应物粒子必须相互碰撞。然而，并非每一次碰撞都会导致反应。

Two conditions are necessary for a successful collision:

发生有效碰撞需要满足两个条件：

The particles must have enough energy to break existing bonds, at least equal to the activation energy (Eₐ).

粒子必须具有足够的能量来断裂原有的化学键，至少达到活化能（Eₐ）的要求。
The particles must collide with the correct orientation so that the atoms are positioned appropriately for the new bonds to form.

粒子必须以正确的取向碰撞，使原子位置合适，从而能够形成新的化学键。

Increasing the frequency of successful collisions increases the rate of reaction. Any factor that makes collisions more likely or more energetic will speed up the reaction.

加快有效碰撞的频率能提高反应速率。任何使碰撞更频繁或更具能量的因素都会加速反应。

4. Effect of Concentration | 浓度的影响

Concentration is the amount of solute dissolved in a given volume of solution. In a more concentrated solution, there are more particles in the same volume.

浓度是指一定体积溶液中所含溶质的量。在更浓的溶液中，相同体积内的粒子数更多。

When concentration increases, particles are packed more closely together. This means they collide with one another more frequently. Therefore, the frequency of successful collisions increases, and the rate of reaction increases.

当浓度增加时，粒子排列更紧密。这意味着它们相互碰撞的频率更高。因此，有效碰撞的频率增加，反应速率加快。

More particles per unit volume → more collisions per second → higher rate

This can be observed by adding dilute hydrochloric acid to magnesium ribbon. With a higher concentration of acid, the hydrogen gas is produced more quickly.

可以通过将稀盐酸与镁条反应来观察此现象。使用更高浓度的酸时，氢气产生得更快。

5. Effect of Pressure | 压力的影响

Pressure affects reactions involving gases. Increasing the pressure compresses the gas, reducing its volume while the number of particles remains constant.

压力影响涉及气体的反应。增加压力会压缩气体，使其体积减小，而粒子数目保持不变。

As a result, the concentration of gas particles increases. They are closer together, so collisions become more frequent. The rate of reaction therefore increases.

结果，气体粒子的浓度增大。它们相互靠近，碰撞更加频繁。因此反应速率增大。

Pressure changes have very little effect on solids and liquids because they are already nearly incompressible.

压力变化对固体和液体影响很小，因为它们几乎不可压缩。

Higher pressure → higher gas concentration → faster reaction

The Haber process for making ammonia uses high pressure (about 200 atmospheres) to increase the yield and rate of reaction.

哈伯法制氨气时使用高压（约 200 个大气压）以提高产率和反应速率。

6. Effect of Surface Area | 表面积的影响

For solid reactants, reaction occurs at the surface where particles are exposed. Only the particles on the surface can collide with other reactant particles.

对于固体反应物，反应发生在粒子暴露的表面上。只有表面上的粒子才能与其他反应物粒子碰撞。

If a solid is broken into smaller pieces, its total surface area increases. More particles are available for collisions at the same time, so the frequency of successful collisions rises.

如果将固体碎裂成更小的颗粒，其总表面积增大。同时有更多粒子可用于碰撞，因此有效碰撞频率升高。

For example, a large lump of calcium carbonate reacts slowly with hydrochloric acid, but powdered calcium carbonate reacts vigorously because it has a much larger surface area.

例如，大块碳酸钙与稀盐酸反应缓慢，而粉末状碳酸钙反应剧烈，因为其表面积大得多。

Smaller particle size → larger surface area → faster reaction

In industry, reactants are often ground into powders before mixing, such as in cement and fuel combustion, to speed up reactions.

在工业中，反应物通常先研磨成粉末再混合，例如水泥生产和燃料燃烧，以加快反应。

7. Effect of Temperature | 温度的影响

Temperature strongly influences reaction rates. Heating a mixture gives particles more kinetic energy, so they move faster.

温度对反应速率影响显著。加热混合物使粒子获得更多动能，从而运动更快。

This has two important effects:

这有两个重要影响：

Particles collide more frequently because they move around faster.

粒子运动更快，因此碰撞更加频繁。
A greater proportion of particles have energy equal to or greater than the activation energy. These collisions are more likely to be successful.

更多比例的粒子能量达到或超过活化能。这些碰撞更有可能成为有效碰撞。

Roughly, a 10°C increase in temperature doubles the rate of many reactions. This is a useful rule of thumb in chemistry.

粗略地说，温度每升高 10°C，许多反应的速率会翻倍。这是化学中一个很有用的经验法则。

Higher temperature → more energetic collisions → much faster reaction

Food is kept in fridges because low temperatures slow down the chemical reactions that cause spoilage.

食物放在冰箱里，是因为低温能减缓导致腐败的化学反应。

8. Catalysts | 催化剂

A catalyst is a substance that speeds up a chemical reaction without being used up in the process. It remains chemically unchanged at the end of the reaction.

催化剂是一种能加快化学反应速率而自身不被消耗的物质。反应结束时，它保持化学性质不变。

Catalysts provide an alternative pathway for the reaction that has a lower activation energy. This means a larger proportion of collisions have sufficient energy to react.

催化剂为反应提供了另一种低活化能的途径。这意味着更大比例的碰撞具有足够的能量来发生反应。

Catalyst lowers Eₐ → more successful collisions per second

Catalysts are highly specific. Different reactions require different catalysts. For example, iron is used in the Haber process, and manganese(IV) oxide is used in the decomposition of hydrogen peroxide.

催化剂具有很强的选择性。不同的反应需要不同的催化剂。例如，铁用于哈伯法，二氧化锰用于过氧化氢的分解。

Biological catalysts are called enzymes. They control metabolic reactions inside living organisms.

生物催化剂被称为酶。它们控制生物体内的代谢反应。

9. Activation Energy and Reaction Profiles | 活化能与反应能垒图

Activation energy is the minimum energy that colliding particles need in order to start a chemical reaction. It is often shown on an energy profile diagram.

活化能是碰撞粒子引发化学反应所需的最低能量。它通常显示在能量能垒图中。

In a reaction profile, the x-axis is the progress of the reaction, and the y-axis is the potential energy of the system.

在反应能垒图中，横轴表示反应进程，纵轴表示体系的势能。

The curve rises from the reactants to a peak, then falls to the products. The height of the peak above the reactants is the activation energy. The overall energy change (ΔH) is the difference between products and reactants.

曲线从反应物上升到峰值，然后下降到产物。峰值高于反应物的高度即为活化能。总能量变化（ΔH）是产物与反应物之间的差值。

Reactants → transition state → products

A catalyst changes the pathway, reducing the activation energy as shown by a lower peak. This allows more particles to react without increasing the temperature.

催化剂改变了反应路径，降低了活化能，如图所示峰值较低。这使得更多粒子无需升温就能参与反应。

10. Applications and Examples | 应用与实例

Understanding reaction rates has many practical applications in everyday life and industry.

理解反应速率在日常生活和工业中有许多实际应用。

Food preservation: Refrigeration and freezing lower temperature to slow down decaying reactions.

食品保鲜：冷藏和冷冻降低温度，从而减缓腐败反应。
Fireworks and explosives: These are designed to react extremely fast, releasing huge amounts of energy in a short time.

烟花和炸药：它们被设计成极快反应，在短时间内释放巨大能量。
Industrial synthesis: The Haber process uses high pressure, moderate temperature, and an iron catalyst to make ammonia efficiently.

工业合成：哈伯法使用高压、适温和铁催化剂来高效合成氨。
Medical diagnostics: Enzyme tests use catalytic activity to detect substances in blood or urine quickly.

医学诊断：酶测试利用催化活性快速检测血液或尿液中的物质。
Corrosion prevention: Painting or covering metals blocks oxygen and water, slowing down the rusting reaction.

防腐：涂漆或覆盖金属可隔绝氧气和水，减缓生锈反应。

By controlling concentration, pressure, surface area, temperature, and catalysts, scientists can speed up useful reactions and slow down unwanted ones.

通过控制浓度、压力、表面积、温度和催化剂，科学家可以加快有用反应，同时减缓不需要的反应。

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Rates of Reaction | 反应速率