📚 Rate of Reaction: Speed, Collisions and Catalysis | 反应速率：速度、碰撞与催化
The rate of a chemical reaction describes how quickly reactants are converted into products. Some reactions, such as an explosion, finish in a fraction of a second. Others, like the rusting of iron, can take years. In the Edexcel IGCSE Science course, you must be able to describe and explain how factors such as concentration, temperature, surface area and catalysts affect this speed.
化学反应速率描述的是反应物转化为产物的快慢程度。有些反应，例如爆炸，在不到一秒钟内就完成了；而另一些反应，例如铁的生锈，则可能需要数年时间。在 Edexcel IGCSE 科学课程中，你必须能够描述并解释浓度、温度、表面积和催化剂等因素如何影响这一快慢。
The rate is measured as the change in the amount of a reactant or product per unit time. For example, in the reaction between magnesium and hydrochloric acid, hydrogen gas is released, so the rate can be expressed as the volume of gas collected per second.
速率以单位时间内反应物或产物数量的变化来衡量。例如，在镁与盐酸的反应中会释放氢气，因此速率可以表示为每秒收集到的气体体积。
Rate = Δamount / Δtime
速率 = 数量的变化量 ÷ 时间的变化量
1. What is Rate of Reaction? | 什么是反应速率？
Chemists measure how fast a reaction happens by following a measurable quantity, such as gas volume, mass lost, or the intensity of a colour. A fast reaction, such as magnesium dissolving in acid, shows a rapid change in these quantities. A slow reaction, such as the ageing of a painting, shows almost no change over a short time.
化学家通过追踪一个可测量的量，例如气体体积、质量损失或颜色强度，来判断反应发生的快慢。快速反应，如镁在酸中溶解，会显示出这些量的迅速变化。慢速反应，如油画的年代老化，在短时间内几乎看不出任何变化。
The rate can also be written using the word “per”: for example, grams per minute (g/min) or cubic centimetres per second (cm³/s). You should always include units when you quote a rate in an exam answer.
速率也可以用“每”来表达：例如，每分钟克数（g/min）或每秒立方厘米数（cm³/s）。在考试答案中引用速率时，你应当始终写明单位。
2. Measuring the Rate | 测量反应速率
Several practical techniques are used to measure reaction rates, and the best method depends on the reaction. For reactions that produce a gas, a gas syringe is a convenient tool. The plunger moves outward as gas collects, and you can record the volume every 10 seconds.
有几种实用技术可用于测量反应速率，最佳方法取决于反应本身。对于产生气体的反应，气体注射筒（gas syringe）是一种方便的工具。随着气体收集，活塞向外移动，你可以每 10 秒记录一次体积。
Alternatively, the flask can be placed on a digital balance. As gas escapes, the total mass of the flask and its contents falls. The readings drop quickly at first and then more slowly, giving you the data needed to calculate the rate.
另一种方法是将烧瓶放在数字天平上。随着气体逸出，烧瓶及内容物的总质量下降。读数一开始下降很快，随后变慢，为你提供计算速率所需的数据。
For reactions that form an insoluble solid, known as a precipitate, a different method is used. A piece of paper with a black cross is placed under the flask. The time taken for the cross to disappear as the mixture turns cloudy is recorded. This time is inversely related to the rate.
对于生成不溶性固体（即沉淀）的反应，则使用另一种方法。在烧瓶下方放置一张画有黑色十字的纸，记录混合物变浑浊至十字消失所需的时间。该时间与速率成反比。
Whichever method is used, a fair test requires you to keep all other variables constant. Only the factor being investigated should be changed, so that any difference in rate can be linked directly to that factor.
无论使用哪种方法，公平实验都要求你保持其他所有变量不变，只改变正在研究的因素，这样才能将速率的任何差异直接归因于该因素。
3. Collision Theory | 碰撞理论
Collision theory explains why reactions occur at different speeds. It states that particles must collide before a chemical change can happen. However, most collisions do not lead to a reaction; only successful collisions produce a chemical change.
碰撞理论解释了反应为何以不同速率发生。该理论指出，粒子必须先发生碰撞，化学变化才可能发生。然而，大多数碰撞并不会导致反应；只有有效碰撞（successful collisions）才能产生化学变化。
A successful collision has two requirements. First, the colliding particles must have enough kinetic energy, at least equal to the activation energy (Ea), to break existing bonds and start the reaction. Second, they must collide with the correct orientation, so that the reacting parts meet each other.
一次有效碰撞需要满足两个条件。第一，碰撞粒子的动能必须足够大，至少要达到活化能（Ea），才能打破原有化学键并引发反应。第二，它们必须以正确的方向碰撞，使彼此的反应部位能够接触。
Activation energy is the minimum energy that reacting particles must have for a reaction to occur. Only particles with energy equal to or greater than Ea can convert a collision into a reaction. Any factor that changes the number or energy of collisions will change the rate.
活化能是反应粒子发生反应所必须具备的最低能量。只有能量等于或高于 Ea 的粒子才能将碰撞转化为反应。任何改变碰撞次数或碰撞能量的因素，都会改变反应速率。
4. Concentration and Pressure | 浓度与压强
Increasing the concentration of a dissolved reactant increases the rate of reaction. This is because more particles are present in the same volume of solution, so the particles are closer together and collide more frequently. A higher frequency of collisions leads to a higher frequency of successful collisions each second.
增大溶液中反应物的浓度会加快反应速率。这是因为相同体积的溶液中粒子数量更多，粒子之间距离更近，碰撞更加频繁。更高的碰撞频率会使每秒的有效碰撞次数增加。
For gases, increasing the pressure squeezes the particles into a smaller volume, which has the same effect as increasing concentration. The particles become more crowded and collide more often, so the rate rises.
对于气体，增大压强会将粒子压缩到更小的体积中，其效果与增大浓度相同。粒子变得更加密集，碰撞更加频繁，因此速率升高。
It is important to note that changing concentration or pressure does not change the average energy of the particles. The activation energy stays the same. The rate increases simply because collisions happen more often, not because each collision is more powerful.
重要的是，改变浓度或压强并不会改变粒子的平均能量，活化能保持不变。速率提高只是因为碰撞发生得更频繁，而不是因为每次碰撞都更有威力。
5. Temperature | 温度
Temperature has a particularly strong effect on reaction rate. When the temperature rises, two things happen. First, particles move faster, so they collide more frequently. Second, and more importantly, the particles have more kinetic energy, so a much greater proportion of collisions exceed the activation energy.
温度对反应速率的影响尤其显著。温度升高时会发生两件事：第一，粒子运动加快，因此碰撞更加频繁；第二，这一点更为重要，粒子具有更大的动能，因此超过活化能的碰撞比例大大增加。
For many reactions, a rise of 10 °C roughly doubles the rate. This is why food spoils quickly in warm weather and why industrial reactions are often held in temperature-controlled vessels. The rate increase is far greater than what simple collision frequency alone would predict.
对许多反应而言，温度每升高 10 °C，速率大约翻倍。这就是为什么食物在温暖天气中很快变质，也是工业反应常在温控容器中进行的原因。这一速率上升远比单纯碰撞频率增加所能解释的要大得多。
The energy distribution of particles is shown by a Boltzmann distribution curve. The activation energy is marked as a vertical line on the right of the curve. At a higher temperature, the curve becomes flatter and wider, so a larger area lies to the right of the activation energy line. This larger area represents the much greater number of particles able to react.
粒子的能量分布可以用玻尔兹曼分布曲线表示，活化能在曲线右侧以一条竖线标出。在较高温度下，曲线变得更平坦、更宽，因此位于活化能线右侧的面积更大。这一更大的面积代表了能够反应的粒子数大大增加。
6. Surface Area | 表面积
Solids react only at their surface, where their particles are exposed to the other reactant. If a solid lump is crushed into smaller pieces, its total surface area increases dramatically, and more particles are available to collide at the same time.
固体只在其表面发生反应，因为只有表面的粒子才暴露于另一反应物。如果将一块大固体压碎成更小的碎片，其总表面积会大幅增加，同一时刻可供碰撞的粒子就更多了。
A powdered solid therefore reacts faster than one large lump of the same mass. The mass of solid is unchanged; only the surface area has increased. With a larger exposed area, the number of collisions per second between reactant particles rises, so the rate increases.
因此，粉末状固体比同样质量的一大块固体反应得更快。固体的质量没有改变，改变的只是表面积。由于暴露面积更大，每秒反应粒子之间的碰撞次数增多，速率随之上升。
If you plot the gas volume against time for the same mass of powder and lump, the powder gives a steeper initial slope. However, both lines finish at the same final gas volume, because the total amount of reactant is identical. The faster reaction simply reaches completion earlier.
如果对相同质量的粉末和大块固体绘制气体体积—时间曲线，粉末的起始斜率更陡。然而，两条线最终达到相同的气体总体积，因为反应物的总量相同。较快的反应只是更早达到终点而已。
7. Catalysts | 催化剂
A catalyst is a substance that speeds up a reaction while remaining chemically unchanged at the end. It works by offering an alternative reaction pathway with a lower activation energy. This means that a much larger fraction of the particles already has enough energy to react.
催化剂是一种能加快反应速率，而反应结束时自身化学性质不变的物质。它通过提供一条活化能更低的替代反应路径来起作用，这意味着已有更大比例的粒子拥有足够的能量去反应。
In an energy profile diagram, the catalysed pathway has a smaller “hump” than the uncatalysed pathway. Particles with moderate energy can now cross the lower barrier successfully. The rate increases although the individual particles have not gained any extra energy.
在能量曲线图中，催化路径的“小丘”比无催化路径的要低。具有中等能量的粒子现在能够成功越过较低的能量屏障。尽管单个粒子并没有获得额外能量，反应速率仍然增加了。
Catalysts are vital in industry because they reduce energy costs. For example, iron is used in the Haber process to manufacture ammonia, and manganese dioxide catalyses the decomposition of hydrogen peroxide in the school laboratory. Enzymes are biological catalysts that control reactions in living cells.
催化剂在工业中至关重要，因为它能降低能源成本。例如，铁在哈伯法中用于制造氨，而在学校实验室中，二氧化锰催化过氧化氢的分解。酶是控制活细胞中反应的生物催化剂。
A catalyst does not change the final amount of product. It only helps the product form faster. Because the catalyst is not used up, a small amount can speed up a vast quantity of reactants, and it can be recovered unchanged after the reaction.
催化剂不会改变最终产物的量，它只是帮助产物更快地形成。由于催化剂不会被消耗，少量催化剂就可以加快大量反应物的反应，而且反应后可以原样回收。
8. Interpreting Rate Graphs | 解读速率曲线图
Rate graphs usually show the volume of gas produced (y-axis) against time (x-axis). The curve is steepest at the very beginning, because the concentration of reactants is highest then, so the rate of reaction is greatest. As reactants are used up, the slope becomes gentler.
速率曲线图通常展示产生的气体体积（纵轴）随时间（横轴）的变化。曲线在最初阶段最陡，因为此时反应物浓度最高，反应速率最大。随着反应物逐渐消耗，斜率变得平缓。
When the curve becomes horizontal, the reaction has stopped. This happens when at least one reactant is completely used up. The final constant reading is the total volume of product formed. A faster reaction reaches this line sooner, not higher.
当曲线变为水平时，反应已经停止。这发生在至少一种反应物被完全消耗之时。最终的稳定读数就是产物形成的总体积。较快的反应会更快到达这条水平线，而不是到达更高的位置。
To find the rate at any single moment, draw a tangent to the curve at that point and calculate its gradient. The steeper the tangent, the faster the rate at that moment. The gradient at the start of the reaction is called the initial rate.
要计算某一瞬间的速率，可在该点作曲线的切线并计算其斜率。切线越陡，那一刻的速率越快。反应开始时的斜率称为初始速率。
You can also calculate the average rate of the whole reaction using the total product formed divided by the total time taken.
你还可以用生成产物的总量除以整个反应所用的总时间，计算整个反应的平均速率。
Average rate = total volume of gas / total time
平均速率 = 气体总体积 ÷ 总时间
9. Experiment Design | 实验设计
You may be asked to plan an experiment to investigate how a factor affects reaction rate. A classic Edexcel practical is the reaction between sodium thiosulfate and hydrochloric acid. The mixture becomes cloudy as solid sulfur is formed.
你可能会被要求设计
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