📚 Catalysts and Reaction Rates | 催化剂与反应速率
Catalysts are substances that speed up chemical reactions without being consumed. They are essential to both industrial chemistry and biological life, and they appear frequently in the Edexcel IGCSE Science examination.
催化剂是能在自身不被消耗的情况下加速化学反应的物质。它们在工业化学和生物生命中都至关重要，并且在 Edexcel IGCSE Science 考试中频繁出现。
1. What is a Catalyst? | 什么是催化剂？
A catalyst is a substance that increases the rate of a chemical reaction by providing an alternative reaction pathway with lower activation energy. It remains chemically unchanged at the end of the reaction.
催化剂是一种通过提供活化能更低的替代反应途径来提高化学反应速率的物质。它在反应结束时化学性质保持不变化。
It is not used up during the reaction – it can be reused.
反应过程中不被消耗——可以重复使用。
It does not start a reaction; it only speeds up a reaction that is already thermodynamically possible.
不能启动反应；只能加速热力学上可能发生的反应。
It does not alter the final amounts of products.
不改变产物的最终总量。
2. How Catalysts Work: Activation Energy | 催化剂作用原理：活化能
The activation energy (Eₐ) is the minimum energy that particles need to collide in order to react. Catalysts lower Eₐ, making it easier for reactant particles to reach the transition state. In an energy profile diagram, the catalyst provides a second curve with a lower 'hump'.
活化能（Eₐ）是微粒碰撞发生反应所需的最小能量。催化剂降低 Eₐ，使反应物微粒更容易到达过渡态。在能量轮廓图中，催化剂提供了另一条更低“峰”的曲线。
Eₐ (uncatalysed reaction) > Eₐ (catalysed reaction)
The energy change of the reaction, ΔH, remains the same because the catalyst does not change the energy of the reactants or the products.
反应的能量变化 ΔH 保持不变，因为催化剂不改变反应物或产物的能量。
3. Characteristics of Catalysts | 催化剂的特征
Specificity: Different reactions require different catalysts.
专一性：不同的反应需要不同的催化剂。
Small quantity: A tiny amount of catalyst is often enough to catalyse a large amount of reactant.
用量极少：少量催化剂即可催化大量反应物。
Unchanged: The catalyst is chemically unchanged, although its physical form may change.
自身不变：催化剂化学上不变化，但其物理形态可能改变。
Equilibrium: In a reversible reaction, a catalyst speeds up both forward and backward reactions equally, so it does not shift the position of equilibrium.
平衡影响：在可逆反应中，催化剂同等地加速正逆反应，因此不移动平衡位置。
4. Types of Catalysts: Homogeneous and Heterogeneous | 均相与非均相催化剂
A homogeneous catalyst is in the same phase as the reactants, for example an acid catalyst in ester hydrolysis. A heterogeneous catalyst is in a different phase, for example iron in the Haber process.
均相催化剂与反应物处于同一相，例如酯水解反应中的酸催化剂。非均相催化剂与反应物处于不同相，例如哈伯法中的铁催化剂。
| Type | Same phase? | Example |
| Homogeneous | Yes | Acid in esterification |
| Heterogeneous | No | Iron in Haber process |
Heterogeneous catalysts often have a large surface area and can be poisoned by impurities that block the active sites.
非均相催化剂通常具有较大的表面积，并可能被杂质阻塞活性位点而中毒。
5. Measuring Reaction Rates | 测量反应速率的方法
Reaction rates can be measured by following the change in reactant or product amount over time. Common methods include collecting a gas, measuring mass loss, observing colour change, and monitoring pH.
反应速率可通过追踪反应物或产物随时间的变化来测量。常用方法包括收集气体、测量质量损失、观察颜色变化以及监测 pH。
Gas volume: Use a gas syringe or an inverted measuring cylinder to record the volume of gas produced at regular intervals.
气体体积法：使用气体注射器或倒扣的量筒定期记录产生的气体体积。
Mass loss: If a gas escapes during the reaction, the mass of the reaction flask decreases; record the mass at intervals.
质量损失法：如果气体在反应中逸出，反应锥形瓶的质量会减少；定期记录质量。
Turbidity: A precipitate forms in some reactions; measure the time taken for a cross drawn under the flask to disappear.
浊度法：某些反应生成沉淀，可用烧瓶下方十字标志消失所需的时间来测量。
6. Effect of Catalysts on Rate Graphs | 催化剂对速率曲线的影响
The graph of product volume against time for a catalysed reaction starts steeper than the uncatalysed one and levels off at the same final volume. This shows the catalyst increases the rate but does not change the amount of product.
在“产物体积—时间”图上，催化反应曲线比无催化反应曲线起始更陡，并于相同的最终体积处变平。这表示催化剂提高了反应速率，但不改变产物的量。
Rate = gradient of the tangent = change in product concentration ÷ change in time
Initial rate can be found from the gradient of a tangent drawn at time zero.
初始速率可通过在时间零点绘制切线并计算其斜率来得出。
7. Industrial Applications of Catalysts | 催化剂的工业应用
Haber process: N₂ + 3H₂ ⇌ 2NH₃ uses an iron catalyst to produce ammonia for fertilisers.
哈伯法：N₂ + 3H₂ ⇌ 2NH₃ 使用铁催化剂生产氨，用于制造肥料。
Contact process: 2SO₂ + O₂ ⇌ 2SO₃ uses vanadium(V) oxide as a catalyst to make sulfuric acid.
接触法：2SO₂ + O₂ ⇌ 2SO₃ 使用五氧化二钒作催化剂来制造硫酸。
Catalytic converters: They convert toxic carbon monoxide and nitrogen oxides into less harmful gases such as carbon dioxide and nitrogen.
催化转换器：将有毒的一氧化碳和氮氧化物转化为二氧化碳和氮气等危害较小的气体。
8. Enzymes: Biological Catalysts | 酶：生物催化剂
Enzymes are proteins that act as biological catalysts. They are extremely specific, meaning each enzyme works on a particular substrate. Their activity depends on temperature and pH; the optimum temperature for most human enzymes is around 37 °C, but enzymes from other organisms can have different optima.
酶是充当生物催化剂的蛋白质。它们高度专一，即每种酶只作用于特定的底物。它们的活性取决于温度和 pH；大多数人体酶的最适温度约为 37 °C，但来自其他生物的酶可能有不同的最适条件。
Using the lock-and-key model makes it easy to remember that the shape of the active site matches the substrate. At high temperatures, enzymes denature because the protein shape changes permanently.
利用“锁钥模型”容易记住：活性位点的形状与底物匹配。在高温下，酶会变性，因为蛋白质的形状发生永久性改变。
9. Comparing Catalysts with Other Rate Factors | 催化剂与其他速率因素的比较
Temperature, concentration, pressure, surface area and catalysts all affect the rate of a reaction. The table below summarises these effects.
温度、浓度、压强、表面积和催化剂都会影响反应速率。下表总结了这些影响。
| Factor | Effect on rate | Effect on equilibrium position? |
| Temperature | Increases rate | Yes |
| Concentration / pressure | Increases rate | Yes |
| Surface area | Increases rate | No |
| Catalyst | Increases rate | No |
A catalyst speeds up both forward and backward reactions equally, so it does not change the yield at equilibrium; it simply helps equilibrium to be reached faster.
催化剂同等地加快正逆反应，所以它不改变平衡产率；只是帮助更快地到达平衡。
10. Summary and Exam Tips | 总结与考试建议
To score well on questions about catalysts, always state that a catalyst 'lowers the activation energy' and 'is not used up'. Be ready to draw or interpret an energy profile diagram showing two curves with different Eₐ values.
要在催化剂相关题目中拿高分，请务必说明催化剂“降低活化能”且“不被消耗”。准备好绘制或解读具有两条不同 Eₐ 曲线的能量轮廓图。
Know the key industrial reactions and their catalysts.
牢记关键工业反应及其催化剂。
Understand that enzymes are proteins and change shape at high temperatures.
理解酶是蛋白质，并能在高温下改变形状。
In practical questions, state the intervals used for measurement and consider safety like using safety goggles.
在实验题中，说明测量间隔，并考虑戴护目镜等安全问题。
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