📚 Enzymes and Rate of Reaction | 酶与反应速率
In both chemistry and biology, the speed of change is described by the rate of reaction. For IGCSE Science, understanding how to measure and control reaction rates is a central skill. This article links chemical kinetics with enzyme biology, giving you the key ideas needed for Edexcel examinations.
在化学和生物学中，变化的速度都用反应速率来描述。对于 IGCSE 科学而言，理解如何测量和控制反应速率是一项核心技能。本文将化学动力学与酶生物学联系起来，为你提供 Edexcel 考试所需的关键概念。
1. What is Rate of Reaction? | 什么是反应速率？
The rate of a reaction is the change in the amount of a reactant or product per unit of time. It can be calculated as the amount of reactant used or product formed divided by the time taken.
反应速率是指单位时间内反应物或产物数量的变化量。其计算方法为：消耗的反应物量或生成的产物量除以所用时间。
Rate = Amount of reactant used or product formed ÷ Time
Common units are g/s (grams per second) or cm³/s (cubic centimetres per second). If concentration is used, the unit is mol/dm³/s.
常用单位是 g/s（克每秒）或 cm³/s（立方厘米每秒）。如果使用浓度，单位则为 mol/dm³/s。
For example, when magnesium reacts with hydrochloric acid, hydrogen gas is released. We can record the gas volume every 10 seconds and plot a graph.
例如，镁与盐酸反应会放出氢气。我们可以每隔 10 秒记录一次气体体积并绘制曲线图。
2. How to Measure Reaction Rates | 如何测量反应速率
There are several practical methods used in the laboratory. The choice depends on the observable property of the reaction.
实验室中有多种实用的测量方法，选择哪种取决于反应所显示的可观测性质。
| Method | Example Reaction | What Is Measured |
| Mass loss | Carbonate + acid | Decrease in mass as gas escapes |
| Gas volume | Metal + acid | Volume of gas collected per minute |
| Turbidity | Sodium thiosulfate + acid | Time for a cross to disappear |
| pH change | Acid–base titration | pH drop or rise over time |
In the disappearing cross experiment, the reaction mixture becomes cloudy because solid sulfur is produced. The rate is judged by how quickly the black cross on paper becomes invisible when viewed through the flask.
在“消失的十字”实验中，反应混合液因生成固体硫而变浑浊。我们从烧瓶上方观察纸上的黑色十字，记录它完全被遮挡所需的时间，从而判断反应快慢。
3. Collision Theory | 碰撞理论
For a chemical reaction to happen, reactant particles must collide with each other. Not all collisions lead to a reaction; the collision must have enough energy, at least the activation energy.
化学反应要发生，反应物粒子之间必须相互碰撞。但并非所有碰撞都能引发反应；碰撞必须具有足够的能量，至少达到活化能。
The activation energy is the minimum energy needed to break existing bonds and start the reaction. Successful collisions are called effective collisions.
活化能是断裂已有化学键并启动反应所需的最小能量。能够引起反应的碰撞称为有效碰撞。
energy of reacting particles ≥ activation energy → effective collision
According to collision theory, anything that increases the frequency of collisions or the proportion of particles with energy above the activation energy will increase the reaction rate.
根据碰撞理论，凡是能提高碰撞频率或增加能量高于活化能的粒子比例的因素，都会加快反应速率。
4. Factors That Affect Reaction Rate | 影响反应速率的因素
Five key factors are required for Edexcel IGCSE: temperature, concentration, pressure, surface area and catalysts.
Edexcel IGCSE 要求掌握五个关键因素：温度、浓度、压强、接触面积和催化剂。
Temperature: Particles gain kinetic energy and move faster, so collisions are more frequent and more energetic. A higher proportion of collisions have energy above the activation energy.
温度：粒子获得动能后运动加快，因此碰撞更频繁、更具能量。能量高于活化能的碰撞比例也随之增大。
Concentration: In a solution, a higher concentration means more reactant particles per unit volume. The collision frequency increases, so the rate rises.
浓度：在溶液中，浓度越高，单位体积内的反应物粒子越多。碰撞频率增加，速率随之上升。
Pressure: For gases, increasing pressure pushes the particles closer together. This increases the number of collisions per second.
压强：对于气体，增大压强使粒子靠得更近。这增加了每秒的碰撞次数。
Surface area: Breaking a solid into smaller pieces exposes more surface area. More particles are available at the surface to collide with other reactants.
接触面积：将固体粉碎成小块能暴露出更大的表面积，在表面可供碰撞的反应物粒子就更多。
Catalysts: A catalyst lowers the activation energy without being used up in the reaction, so more particles have sufficient energy for an effective collision.
催化剂：催化剂能降低活化能，而本身不会被消耗，因此更多粒子拥有发生有效碰撞所需的能量。
You should be able to sketch and interpret graphs showing the effect of each factor on the rate.
你应该能够画出并解读显示各因素对速率影响的曲线图。
5. Catalysts and Enzymes | 催化剂与酶
A catalyst is a substance that speeds up a chemical reaction but remains chemically unchanged at the end. In living organisms, most catalysts are proteins called enzymes.
催化剂是一种能加快化学反应而自身在反应结束时保持化学不变的物质。在生物体内，绝大多数催化剂是称为酶的蛋白质。
Enzymes are highly specific. Each enzyme catalyses only one type of reaction because of its unique active site. The reactant, called the substrate, binds to this active site.
酶具有高度的专一性。每种酶只催化一种类型的反应，这是由其独特的活性位点决定的。反应物称为底物，它与活性位点结合。
Enzyme + Substrate → Enzyme–Substrate Complex → Enzyme + Product
Because the enzyme is not consumed, a small amount of enzyme can process a large amount of substrate at a high speed.
由于酶不会被消耗，少量酶就能高速处理大量底物。
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