📚 Rates of Reaction | 反应速率
Chemical reactions happen at different speeds. The rate of a reaction tells us how quickly reactants are used up or products are formed.
化学反应以不同速度发生。反应速率告诉我们反应物被消耗或产物生成有多快。
In this revision guide, we will explore how to measure reaction rates, the collision theory, and the key factors that change the speed of a reaction, along with practical examples and exam tips for Edexcel IGCSE Science.
在本复习指南中，我们将探讨如何测量反应速率、碰撞理论，以及改变反应速度的关键因素，同时给出适用于 Edexcel IGCSE 科学考试的实践案例和考试技巧。
1. What is Rate of Reaction? | 什么是反应速率？
Rate of reaction is a measure of how fast a reaction occurs. It can be expressed as the change in concentration of a reactant or product per unit time.
反应速率是衡量反应发生快慢的物理量。它可以表示为反应物或产物浓度在单位时间内的变化量。
The equation for rate is often written as: rate = amount of reactant used or product formed ÷ time taken.
速率的常用公式为：速率 = 被消耗的反应物量或生成的产物量 ÷ 所用时间。
For example, if 5 g of magnesium reacts completely in 20 seconds, the average rate is 5 ÷ 20 = 0.25 g/s.
例如，5 克镁在 20 秒内完全反应，平均速率为 5 ÷ 20 = 0.25 克/秒。
2. Measuring Reaction Rates | 测量反应速率
There are several ways to follow the progress of a reaction. The method chosen depends on which observable change is easy to measure.
追踪反应进程有多种方法。选择哪种方法取决于哪种可观测变化最容易测量。
Common methods include measuring the volume of gas produced, the loss of mass, a change in colour, or the change in pH.
常见方法包括测量生成气体的体积、质量减少、颜色变化或 pH 值变化。
For a reaction that produces carbon dioxide, you can use a gas syringe to record the volume of gas at regular time intervals.
对于产生二氧化碳的反应，你可以使用气体注射器按固定时间间隔记录气体体积。
If gas escapes, the mass of the flask decreases. Plotting mass loss against time gives a curve showing the rate.
如果气体逸出，锥形瓶的质量会减少。绘制质量损失对时间的曲线，可以得到显示速率的曲线。
3. Collision Theory | 碰撞理论
For a chemical reaction to happen, particles must collide with each other with enough energy and the correct orientation.
化学反应要发生，粒子必须相互碰撞，并且具有足够的能量和正确的取向。
The minimum energy needed to start a reaction is called the activation energy, Eₐ.
引发反应所需的最小能量称为活化能，记作 Eₐ。
Not every collision is successful. Only collisions that meet the energy requirement are called effective collisions.
并非每次碰撞都能反应。只有满足能量要求的碰撞才称为有效碰撞。
Increasing the rate of reaction means increasing the frequency of effective collisions, not just the total number of collisions.
提高反应速率意味着增加有效碰撞的频率，而不仅仅是增加碰撞总次数。
4. Concentration | 浓度的影响
Increasing the concentration of a reactant means there are more particles in the same volume.
增加反应物浓度意味着相同体积内粒子数更多。
This leads to more frequent collisions between reactant particles, so the rate of reaction increases.
这导致反应物粒子之间碰撞更频繁，因此反应速率增大。
For example, dilute hydrochloric acid reacts more slowly with magnesium than concentrated hydrochloric acid does.
例如，稀盐酸与镁反应比浓盐酸与镁反应更慢。
In a graph of volume of gas against time, the higher concentration gives a steeper curve at the start and finishes sooner.
在气体体积对时间的图中，较高浓度在开始时曲线更陡，且更快结束。
5. Temperature | 温度的影响
Raising the temperature makes particles move faster, so they collide more often and with more energy.
升高温度使粒子运动更快，因此碰撞更频繁且具有更多能量。
More particles have energy greater than or equal to the activation energy, so the fraction of effective collisions increases dramatically.
更多粒子具有大于或等于活化能的能量，因此有效碰撞的比例显著增加。
As a rule of thumb, a 10 °C rise in temperature roughly doubles the rate of a reaction.
经验规则是，温度每升高 10 °C，反应速率大约加倍。
This is why food decays faster in warm weather and why we refrigerate perishable goods.
这就是为什么食物在温暖天气中变质更快，以及我们为什么要冷藏易腐食品。
6. Surface Area | 表面积的影响
For solid reactants, the reaction can only happen at the surface where particles collide with particles in solution or gas.
对于固体反应物，反应只能在表面发生，因为固体粒子与溶液或气体中的粒子在这里碰撞。
Breaking a solid into smaller pieces increases its total surface area to volume ratio.
将固体粉碎成更小颗粒会增加其总表面积与体积之比。
With a larger surface area, more reactant particles are exposed and collisions become more frequent, so the rate increases.
表面积更大时，更多反应物粒子暴露在外，碰撞更频繁，因此速率增大。
For example, small marble chips react with hydrochloric acid much faster than one large chip of the same mass.
例如，小块大理石与盐酸反应比同质量的一大块反应快得多。
7. Catalysts | 催化剂的影响
A catalyst is a substance that speeds up a chemical reaction without being used up in the process.
催化剂是一种能加快化学反应速率而自身不消耗的物质。
It works by lowering the activation energy, providing an alternative reaction pathway with a lower energy barrier.
它通过降低活化能、提供一条能量障碍更低的替代反应路径来发挥作用。
Catalysts are specific in action; different reactions need different catalysts.
催化剂具有专一性；不同反应需要不同的催化剂。
In biology, enzymes are biological catalysts that help digestion and control metabolic reactions.
在生物学中，酶是生物催化剂，有助于消化并控制代谢反应。
Catalysts are used industrially in the Haber process, the Contact process and in catalytic converters in cars.
工业上，催化剂用于哈伯法、接触法以及汽车催化转化器。
8. Rate Graphs | 速率图
In a typical rate graph, the amount of product (e.g. volume of gas) is plotted against time.
在典型的速率图中，产物量（如气体体积）对时间作图。
The gradient of the curve at any point gives the rate at that instant. The steepest gradient is at the start, then the curve levels off as reactants are used up.
曲线上任意点的斜率给出该瞬间的速率。开始时斜率最大，然后随着反应物被消耗，曲线逐渐变平。
When the line becomes horizontal, the reaction has stopped, either because a reactant has run out or an equilibrium has been reached.
当线变为水平时，反应停止，可能是因为某反应物耗尽或达到平衡。
Comparing two conditions, the curve that is steeper initially and finishes earlier has the higher rate.
比较两种条件时，初始更陡且更早结束的曲线对应更高的速率。
9. Calculating Mean Rate | 计算平均速率
The mean rate of a reaction over a period can be calculated from a graph using the total change divided by the total time.
一段时间内的平均速率可以通过总变化量除以总时间从图中计算得出。
For example, if 40 cm³ of gas is produced in 80 seconds, the mean rate = 40 ÷ 80 = 0.5 cm³/s.
例如，如果 80 秒内产生 40 cm³ 气体，平均速率 = 40 ÷ 80 = 0.5 cm³/s。
You can also calculate the rate at a specific time by drawing a tangent to the curve and finding its gradient.
你也可以通过在曲线上画切线并求其斜率来计算特定时刻的瞬时速率。
Tangent gradient = change in y ÷ change in x over the tangent line.
切线斜率 = 切线上 y 的变化量 ÷ x 的变化量。
10. Practical: Disappearing Cross | 实验：消失的十字
One classic experiment to measure rate uses the reaction between sodium thiosulfate and hydrochloric acid to produce sulfur.
一个经典的速率测量实验利用硫代硫酸钠与盐酸反应生成硫。
The sulfur makes the solution cloudy, so a cross drawn on paper underneath the flask gradually disappears.
生成的硫使溶液变浑浊，因此画在烧杯下方纸上的十字逐渐消失。
You measure the time from mixing until the cross is no longer visible. The shorter the time, the faster the reaction.
你需要测量从混合开始到十字完全看不见的时间。时间越短，反应越快。
Repeating with different concentrations of sodium thiosulfate shows how concentration affects rate. The experiment should be repeated to obtain reliable results.
使用不同浓度的硫代硫酸钠重复实验，可以显示浓度对速率的影响。实验应重复进行以获得可靠结果。
11. Applications of Reaction Rate | 反应速率的应用
Understanding reaction rates is vital in industry. Chemical manufacturers want to produce products quickly without wasting energy or materials.
理解反应速率在工业中至关重要。化学品制造商希望快速生产产品，同时不浪费能量或原料。
In the food industry, low temperatures slow down the rate of microbial growth, making refrigeration an important preservation method.
在食品工业中，低温会减缓微生物生长速率，因此冷藏是重要的保鲜方法。
In medicine, controlled-release drugs use knowledge of reaction rates to deliver medicine slowly over time.
在医学中，缓释药物利用反应速率知识在一段时间内缓慢释放药物。
Fireworks and explosives rely on very fast reactions, while rusting of iron is a slow reaction we try to slow down further.
烟花和炸药依赖非常快的反应，而铁的生锈是我们要减缓的慢反应。
12. Exam Tips | 考试技巧
When describing the effect of a factor, always link it to the frequency of effective collisions and, where relevant, the energy of collisions.
描述某一因素的影响时，始终将其与有效碰撞频率联系起来，并在相关时联系碰撞能量。
Use the correct vocabulary: particles, collision, activation energy, effective, concentration, catalyst. Avoid vague terms like ‘more collisions happen’ without explaining why.
使用正确词汇：粒子、碰撞、活化能、有效、浓度、催化剂。避免模糊说法，比如只说“碰撞更多”而不解释原因。
When interpreting graphs, quote the time at which the reaction finishes and compare initial gradients clearly.
解读图表时，要指出反应结束的时间，并清晰比较初始斜率。
Always include units in your calculations. A rate without units is incomplete.
计算时始终带单位。没有单位的速率是不完整的。
In practical questions, mention how to make the experiment fair: same temperature, same mass, same volume, and repeat to calculate a mean.
在实验题中，说明如何保证公平：相同温度、相同质量、相同体积，并重复实验计算平均值。
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