📚 Rates of Reaction and Energy Changes | 反应速率与能量变化
In chemistry, controlling how fast reactions occur is just as important as knowing what products form. Some reactions need to be slowed down, while others must be accelerated for industrial or biological reasons.
在化学中，控制反应进行的速度与了解生成什么产物同样重要。有些反应需要减慢，而另一些则出于工业或生物原因必须加速。
1. What is Reaction Rate? | 什么是反应速率？
The reaction rate measures how quickly reactants are used up or products are formed per unit time. It is usually expressed as a change in concentration, mass, or volume per second.
反应速率衡量单位时间内反应物消耗或产物生成的快慢。它通常表示为单位时间内浓度、质量或体积的变化。
Average rate = Δ quantity of reactant or product ÷ Δ time
平均速率 = 反应物或产物的变化量 ÷ 时间变化量
For example, if 30 cm³ of hydrogen gas is produced in 60 seconds, the average rate is 0.5 cm³/s.
例如，如果 60 秒内产生 30 cm³ 氢气，平均速率为 0.5 cm³/s。
2. Collision Theory | 碰撞理论
Chemical reactions can only happen when particles collide with sufficient energy and with the correct orientation. This minimum energy required for a successful collision is called the activation energy.
化学反应只有在粒子以足够的能量和正确的方向碰撞时才会发生。这种成功碰撞所需的最低能量称为活化能。
If particles collide too gently or at the wrong angle, they simply bounce apart and no reaction occurs.
如果粒子碰撞太轻或角度不对，它们只会弹开，不会发生反应。
3. Effect of Temperature | 温度的影响
Increasing the temperature increases the kinetic energy of the particles. They move faster and collide more frequently, so the reaction rate increases.
升高温度会增加粒子的动能。粒子运动更快，碰撞更频繁，因此反应速率增大。
More importantly, a higher temperature means a greater proportion of particles have energy equal to or greater than the activation energy, leading to more successful collisions.
更重要的是，温度升高意味着更大比例的粒子具有等于或高于活化能的能量，从而产生更多有效碰撞。
4. Effect of Concentration and Pressure | 浓度与压强的影响
For solutions, increasing the concentration of a reactant means more particles per unit volume. This leads to more frequent collisions and a faster reaction.
对于溶液，增大反应物的浓度意味着单位体积内粒子更多。这导致碰撞更频繁，反应更快。
For gases, increasing the pressure has a similar effect by compressing the gas particles into a smaller volume, increasing their collision frequency.
对于气体，增大压强有类似效果，因为它将气体粒子压缩到更小的体积中，增加了碰撞频率。
5. Effect of Surface Area | 表面积的影响
For solid reactants, breaking a solid into smaller pieces increases its surface area. More particles are exposed to the other reactant, so collisions happen more often.
对于固体反应物，将固体破碎成更小的颗粒会增大其表面积。更多粒子暴露在另一种反应物中，因此碰撞更频繁发生。
A powdered solid reacts faster than large lumps because the powder has a much larger total surface area.
粉末状固体的反应速度比大块固体快得多，因为粉末的总表面积大得多。
6. Effect of Catalysts | 催化剂的影响
A catalyst is a substance that increases the rate of a reaction without being used up itself. It provides an alternative reaction pathway with a lower activation energy.
催化剂是能加快反应速率而自身不被消耗的物质。它提供了一条活化能更低的替代反应路径。
With a lower activation energy, a larger proportion of particles have enough energy to react, so the reaction speeds up. Catalysts are not consumed, so they can be reused.
由于活化能降低，更大比例的粒子有足够能量参与反应，因此反应加快。催化剂不会被消耗，所以可以重复使用。
7. Measuring Reaction Rates | 测量反应速率
Reaction rates can be followed by measuring changes in mass, volume of gas produced, colour, turbidity, or pH against time.
反应速率可以通过测量质量变化、气体产生体积、颜色、浊度或 pH 值随时间的变化来跟踪。
For example, the reaction between marble chips and hydrochloric acid can be monitored by measuring the loss in mass as carbon dioxide escapes.
例如，大理石碎片与盐酸的反应可以通过测量二氧化碳逸出时的质量损失来监测。
If the reaction produces a precipitate, the time taken for a mark to disappear can be used to compare relative rates.
如果反应生成沉淀，可以通过标记消失所需的时间来比较相对速率。
8. Exothermic and Endothermic Reactions | 放热与吸热反应
Reactions can also be classified by their energy changes. An exothermic reaction transfers thermal energy to the surroundings, causing a temperature rise.
反应还可以按能量变化分类。放热反应向周围传递热能，导致温度升高。
An endothermic reaction absorbs thermal energy from the surroundings, causing a temperature drop.
吸热反应从周围吸收热能，导致温度下降。
Combustion, respiration, and neutralisation are exothermic. Photosynthesis and thermal decomposition are endothermic.
燃烧、呼吸和中和反应是放热反应。光合作用和热分解是吸热反应。
9. Energy Profile Diagrams | 能量曲线图
Energy profile diagrams show the energy changes during a reaction. For an exothermic reaction, the products have less energy than the reactants, so the overall energy change ΔH is negative.
能量曲线图显示反应过程中的能量变化。对于放热反应，产物的能量低于反应物，因此总能量变化 ΔH 为负值。
For an endothermic reaction, the products have more energy than the reactants, so ΔH is positive. The peak of the curve represents the activation energy barrier.
对于吸热反应，产物的能量高于反应物，因此 ΔH 为正值。曲线的峰值代表活化能势垒。
10. Activation Energy and Catalysts | 活化能与催化剂
Activation energy is a barrier to reaction. Even exothermic reactions need some initial energy input to get started, such as a spark to ignite a fuel.
活化能是反应的障碍。即使是放热反应也需要一些初始能量来启动，例如点燃燃料需要火花。
Catalysts lower the activation energy, which means a greater fraction of particles have enough energy to overcome the barrier at the same temperature.
催化剂降低活化能，这意味着在相同温度下，有足够能量克服势垒的粒子比例更大。
11. Real-world Applications | 实际应用
Controlling reaction rates is vital in industry and daily life. Refrigeration slows down food spoilage by lowering the temperature, reducing the rate of bacterial reactions.
控制反应速率在工业和日常生活中至关重要。冷藏通过降低温度来减缓食物变质，降低细菌反应的速率。
In the Haber process for making ammonia, an iron catalyst speeds up the reaction so that a high yield can be achieved in a shorter time at moderate conditions.
在哈伯法制氨中，铁催化剂加快反应，使得在中等条件下较短时间即可获得高产率。
Catalytic converters in cars use platinum and rhodium to speed up the breakdown of harmful exhaust gases into less harmful substances.
汽车中的催化转化器使用铂和铑来加速有害废气分解为危害较小的物质。
12. Summary Table | 总结表
The key factors affecting reaction rate are summarised below.
影响反应速率的关键因素总结如下。
| Factor | Effect on rate | Explanation |
|---|---|---|
| Temperature increase | Increases | More particles exceed activation energy |
| Concentration / pressure increase | Increases | More frequent collisions |
| Surface area increase | Increases | More exposed particles to collide |
| Catalyst added | Increases | Lowers activation energy |
Understanding these principles helps chemists design safer, faster, and more efficient reactions for real-world uses.
理解这些原理有助于化学家设计更安全、更快速、更高效的实际应用反应。
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