📚 5.13 Rates of Reaction: Factors and Graphs | 5.13 反应速率：影响因素与图表
In Edexcel IGCSE Science, the concept of rate of reaction is essential to understanding how chemical processes occur. This section explores how quickly reactants turn into products, which quantities can be measured, and how different conditions change the speed of a reaction.
在爱德思 IGCSE 科学中，反应速率的概念是理解化学反应如何进行的关键。本部分探讨反应物转化为产物的快慢、可以测量哪些量，以及不同条件如何改变反应的速度。
1. Defining Reaction Rate | 定义反应速率
The rate of a reaction is a measure of how quickly a reactant is used up or a product is formed. In quantitative terms, rate is the change in concentration of a reactant or product per unit time.
反应速率用于衡量反应物被消耗或产物生成的快慢。从定量角度看，速率是反应物或产物浓度在单位时间内的变化量。
rate = change in concentration / time
反应速率 = 浓度变化量 / 时间
The units for rate of reaction are usually mol dm⁻³ s⁻¹, g s⁻¹, or cm³ s⁻¹ depending on what is being measured.
反应速率的单位通常为 mol dm⁻³ s⁻¹、g s⁻¹ 或 cm³ s⁻¹，具体取决于测量的是什么量。
2. Measuring Reaction Rate | 测量反应速率
There are several laboratory methods to follow the progress of a reaction. The most common methods measure the volume of gas produced, the loss of mass, the formation of a precipitate, or changes in pH and colour.
实验室中有多种追踪反应进程的方法。最常用的方法包括测量产生气体的体积、质量减少、沉淀生成，以及 pH 值或颜色变化。
| Method | What you measure | Example reaction |
| Gas syringe | Volume of gas (cm³) over time | Magnesium + hydrochloric acid |
| Loss of mass | Mass lost as gas escapes (g) | Carbonate + acid |
| Precipitate formation | Cloudiness / time taken for cross to disappear | Sodium thiosulfate + acid |
Choosing the right method depends on whether a gas is given off, whether the solution becomes cloudy, or whether a colour change occurs.
选择合适的方法取决于反应是否放出气体、溶液是否变浑浊，或者是否发生颜色变化。
3. Collision Theory | 碰撞理论
In order for particles to react, they must collide with each other. Not every collision is successful. A successful collision requires enough energy to break existing bonds, and the particles must be arranged in a favourable orientation.
要使粒子发生反应，它们必须相互碰撞。但并非每次碰撞都能成功。一次成功碰撞需要足够的能量来断裂已有化学键，并且粒子需要以有利的方向相互接触。
The minimum energy needed for a successful collision is called the activation energy, often abbreviated as Eₐ.
成功碰撞所需的最低能量称为活化能，常简写为 Eₐ。
Increasing the frequency of successful collisions increases the rate of reaction. Any factor that makes collisions more frequent or more energetic will therefore speed up the reaction.
提高有效碰撞频率会增加反应速率。因此，任何能使碰撞更频繁或更具能量的因素都会加快反应速度。
4. Concentration and Pressure | 浓度和压力
When the concentration of a dissolved reactant is increased, there are more particles in the same volume. This leads to a higher frequency of collisions, so the rate of reaction increases.
当溶解反应物的浓度增加时，相同体积内有更多粒子，这导致碰撞频率更高，因此反应速率增大。
For gases, increasing the pressure has the same effect. The gas particles are pushed closer together, so they collide more often, increasing the rate of reaction.
对于气体，增加压力也有类似效果。气体粒子被压得更近，碰撞更频繁，从而加快反应速率。
higher concentration / pressure → more collisions per unit time → higher rate
更高浓度/压力 → 单位时间碰撞更多 → 速率更高
In exam answers, be precise: the increase in rate is due to increased collision frequency, not because every collision is successful.
在考试作答中，请务必表述准确：速率提高是因为碰撞频率增加，而不是因为每次碰撞都成功。
5. Temperature | 温度
Raising the temperature increases the average kinetic energy of particles. Particles move faster, so they collide more frequently. More importantly, a much greater proportion of the particles have energy equal to or greater than the activation energy.
升高温度会增加粒子的平均动能。粒子运动更快，碰撞更频繁。更重要的是，拥有等于或高于活化能能量的粒子比例会大幅增加。
This second effect is usually the dominant reason why reaction rates increase sharply with temperature. A common rule of thumb is that a 10 °C rise can roughly double the rate of many reactions.
第二个效应通常是温度升高使反应速率显著增加的主要原因。经验规律是，温度每升高 10 °C，许多反应的速率大约会翻倍。
Make sure you can explain this in terms of the Boltzmann–Maxwell energy distribution, not just by saying ‘particles move faster’.
请确保你能用玻尔兹曼-麦克斯韦能量分布解释这一点，而不仅仅说“粒子运动更快”。
6. Surface Area | 表面积
When a solid reactant is broken into smaller pieces, its total surface area increases. This means that more particles are exposed to the other reactant, providing more sites for collisions to occur.
当固体反应物被粉碎成更小的颗粒时，其总表面积增大。这意味着更多粒子暴露于另一种反应物，提供了更多可供碰撞的位置。
For example, a powdered substance reacts faster than a large lump of the same mass because the powder has a much larger surface area to volume ratio.
例如，粉末状物质比同质量的大块反应更快，因为粉末具有更大的表面积与体积比。
larger surface area → more exposed particles → higher collision frequency → faster reaction
更大表面积 → 更多暴露的粒子 → 碰撞频率更高 → 反应更快
7. Catalysts | 催化剂
A catalyst is a substance that increases the rate of a chemical reaction while being chemically unchanged at the end of the reaction. It works by providing an alternative reaction pathway with a lower activation energy.
催化剂是一种能提高化学反应速率，但反应结束后自身化学性质不变的物质。它通过提供一条活化能更低的新反应途径来起作用。
With a lower activation energy, a greater proportion of collisions become successful at the same temperature. Therefore, the reaction rate increases without the need for more energy input.
由于活化能降低，在相同温度下更大比例的碰撞成为有效碰撞，因此反应速率提高，而不需要额外输入更多能量。
Catalysts are important in industry because they reduce energy costs and increase production rates. Common examples include iron in the Haber process and platinum in catalytic converters.
催化剂在工业中非常重要，因为它们降低能量成本并提高生产效率。常见例子包括哈伯法中的铁以及催化转换器中的铂。
Remember that a catalyst does not change the position of equilibrium, nor does it increase the yield of a reversible reaction. It only helps the system reach equilibrium faster.
请记住，催化剂不会改变可逆反应的平衡位置，也不会提高产率，它只是帮助体系更快达到平衡。
8. Graphs and Tangent Calculations | 图表与切线计算
Reaction rates can be followed using graphs of volume, mass, or concentration against time. The steeper the curve, the faster the reaction rate. As reactants are used up, the curve becomes less steep and eventually flattens.
反应速率可以通过绘制体积、质量或浓度随时间变化的图表来追踪。曲线越陡，说明反应越快。随着反应物被消耗，曲线变缓，最终变得平坦。
To find the rate at a particular moment, draw a tangent to the curve at that point, and calculate its gradient:
要在某个特定时刻求反应速率，可以在曲线上该点作切线，然后计算其斜率：
rate = gradient of tangent = Δy / Δx
速率 = 切线斜率 = Δy / Δx
For example, if a tangent line passes through the points (20 s, 30 cm³) and (80 s, 70 cm³), then the rate is (70 – 30) / (80 – 20) = 40 / 60 = 0.67 cm³ s⁻¹.
例如，若一条切线经过 (20 s, 30 cm³) 和 (80 s, 70 cm³) 两点，则速率为 (70 – 30) / (80 – 20) = 40 / 60 = 0.67 cm³ s⁻¹。
Be sure to include units in your gradient calculation. In exams, a tangent drawn with a ruler must be long enough to read accurate coordinates.
在斜率计算中务必包含单位。在考试中，用直尺画的切线应足够长，以便准确读取坐标。
9. Experimental Methods in the Lab | 实验室实验方法
For reactions that produce a gas, such as hydrochloric acid reacting with calcium carbonate, you can collect the gas in a gas syringe and record the volume every 10 seconds.
对于产生气体的反应，比如盐酸与碳酸钙反应，可以用气体注射器收集气体，并每 10 秒记录一次体积。
For reactions that cause a colour change, you can use the ‘disappearing cross’ method. Place a conical flask on a paper cross, add reactants, and time how long it takes for the cross to disappear.
对于引起颜色变化的反应，可以使用“十字消失法”。将锥形瓶放在纸十字上，加入反应物，并计时直到十字看不见为止。
When comparing rates, you must keep all other variables constant. This includes temperature, total volume, concentration of other reactants, and the size of solid pieces.
在比较反应速率时，必须保持所有其他变量不变，包括温度、总体积、其他反应物的浓度以及固体颗粒的大小。
Repeated readings help reduce random error. Calculate a mean time for each set of conditions to make the conclusion more reliable.
重复读数有助于减小偶然误差。对每组条件计算平均时间，使结论更加可靠。
10. Common Misconceptions | 常见误区
A frequent mistake is saying that a catalyst ‘is used up’ in the reaction. In fact, a catalyst is chemically unchanged at the end, although it may participate in a physical process.
常见错误是认为催化剂在反应中被“消耗”了。实际上，催化剂在反应结束时化学上保持不变，尽管它可能参与物理过程。
Another misconception is that increasing concentration increases the energy of particles. Concentration only affects the number of particles per unit volume, not their energy.
另一个误区是认为增加浓度会增加粒子的能量。浓度只影响单位体积内的粒子数量，而不影响粒子能量。
Similarly, increasing pressure does not increase the speed of gas particles. It simply makes them closer together, which increases collision frequency.
同样，增加压力并不会增加气体粒子的运动速度，只是使它们靠得更近，从而增加碰撞频率。
Finally, ‘rate’ and ‘time’ are not the same. A shorter reaction time means a faster rate, but the rate is calculated as 1/time or as a change over time, depending on the method used.
最后，“速率”和“时间”不是同一个概念。较短的反应时间意味着较快的速率，但速率可计算为 1/时间 或随时间的变化量，具体取决于所用方法。
11. Exam Tips and Worked Example | 考试技巧与例题
When a question asks you to explain why a reaction speeds up, always mention collision theory. State what happens to the frequency or energy of collisions, and link this to the activation energy.
当题目要求解释反应为何加速时，一定要提到碰撞理论。说明碰撞频率或能量发生了什么变化，并将其与活化能联系起来。
Use the words ‘successful collisions’ rather than just ‘collisions’. A high rate of reaction requires more successful collisions per second, not merely more collisions.
使用“有效碰撞”而不是仅仅说“碰撞”。高反应速率需要每秒更多成功碰撞，而不只是更多碰撞。
Consider this worked example: 1.0 g of marble chips was added to 50 cm³ of 1.0 mol dm⁻³ hydrochloric acid at 25 °C. How would the rate change if the same mass of powdered marble was used at the same temperature?
看一个例题：25 °C 下，将 1.0 g 大理石碎片加入 50 cm³ 浓度为 1.0 mol dm⁻³ 的盐酸中。如果在相同温度下使用相同质量的粉末状大理石，反应速率会如何变化？
Answer: The powdered marble has a greater surface area than the chips. More calcium carbonate particles are exposed to hydrogen ions, so collisions between reactant particles are more frequent. Therefore, the rate of reaction increases.
答案：粉末状大理石比碎片有更大的表面积。更多碳酸钙粒子暴露于氢离子中，因此反应物粒子间的碰撞更频繁，反应速率增大。
In a data-response question, always read the scale carefully, use appropriate units, and state how you know the reaction has stopped, for example when the gas volume becomes constant.
在数据作答类题目中，务必仔细读数刻度，使用合适的单位，并说明如何判断反应已停止，例如气体体积不再增加时。
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