📚 Concentration and Reaction Rate | 浓度与反应速率
In KS3 and GCSE chemistry, understanding how concentration changes the speed of a reaction is a key skill. Concentration tells us how crowded the particles are in a solution, and reaction rate tells us how quickly reactants turn into products. This article explains the collision theory behind the effect, the classic experiments used to measure it, the graph skills examiners reward, the higher-level rate equation, and the real-world importance of concentration.
在 KS3 与 GCSE 化学中，理解浓度如何改变化学反应的快慢是一项关键技能。浓度告诉我们溶液中粒子有多拥挤，而反应速率告诉我们反应物转变为产物的快慢。本文将解释背后的碰撞理论、用于测量反应速率的经典实验、考官重视的图表技能、更高层次的速率方程，以及浓度在现实世界中的重要性。
1. What is Concentration? | 什么是浓度？
Concentration describes how much solute is dissolved in a certain volume of solvent or solution. A concentrated solution has more solute particles per cm³ than a dilute solution. For example, 2 mol/dm³ hydrochloric acid is twice as concentrated as 1 mol/dm³ hydrochloric acid, so every cubic decimetre of it contains twice as many HCl formula units and, once ionised, twice as many H⁺ ions.
浓度描述了一定体积的溶剂或溶液中溶解了多少溶质。浓溶液每立方厘米所含的溶质粒子比稀溶液更多。例如，2 mol/dm³ 的盐酸浓度是 1 mol/dm³ 盐酸的两倍，因此每立方分米中 HCl 化学式单位的数量是两倍，电离后 H⁺ 离子的数量也是两倍。
The common units are grams per cubic decimetre (g/dm³) or moles per cubic decimetre (mol/dm³). One cubic decimetre equals 1000 cm³, so 1 mol/dm³ is the same as 1 mol per 1000 cm³, or 0.001 mol per cm³. At KS3 you often simply compare volumes of acid, or use the labels "concentrated" and "dilute", but you should recognise the unit mol/dm³ because it appears in every rate experiment later on.
常用的单位有克每立方分米（g/dm³）或摩尔每立方分米（mol/dm³）。1 立方分米等于 1000 cm³，因此 1 mol/dm³ 就等于每 1000 cm³ 含 1 mol，或每立方厘米含 0.001 mol。在 KS3 阶段，你常常只需比较酸的体积，或使用"浓"与"稀"这样的标签，但你应该认识 mol/dm³ 这个单位，因为它会出现在之后每一个速率实验中。
concentration (mol/dm³) = amount of solute (mol) ÷ volume of solution (dm³)
浓度 (mol/dm³) = 溶质的物质的量 (mol) ÷ 溶液体积 (dm³)
Two practical points matter for experiments. First, when you dilute an acid you add water, so the number of moles of acid stays the same but the volume increases and the concentration falls. Second, changing concentration is not the same as changing amount: 50 cm³ of 1 mol/dm³ acid and 25 cm³ of 2 mol/dm³ acid contain exactly the same number of moles, but the second is more concentrated and will react faster at the start.
有两点实验上的要点很重要。第一，稀释酸是加水，所以酸的物质的量不变，但体积增大、浓度下降。第二，改变浓度不等于改变总量：50 cm³ 的 1 mol/dm³ 酸与 25 cm³ 的 2 mol/dm³ 酸含有完全相同的物质的量，但后者浓度更高，起始反应更快。
2. Why Concentration Matters: Collision Theory | 为什么浓度重要：碰撞理论
Reactions happen when reactant particles collide. However, not every collision causes a reaction. For a collision to be successful, the particles must collide with at least the activation energy and with the correct orientation. The activation energy, given the symbol Eₐ, is the minimum energy needed to break the bonds in the reactants and start the reaction.
反应发生的前提是反应物粒子发生碰撞。然而，并非每次碰撞都会引起反应。要使碰撞成功，粒子必须以不低于活化能的能量、并以正确的取向碰撞。活化能（符号为 Eₐ）是打断反应物中的化学键并引发反应所需的最低能量。
If you increase the concentration, you pack more reactant particles into the same volume. The particles are closer together, so they collide more often. In any given second there are more collisions, and therefore more successful collisions, so the rate of reaction increases. Temperature is different: heating makes particles move faster and gives them more energy, so it increases both the frequency of collisions and the proportion of collisions that reach Eₐ.
如果提高浓度，你就在相同体积中塞入了更多反应物粒子。粒子之间距离更近，因此碰撞更频繁。在任意一秒内碰撞次数更多，因此成功碰撞也更多，反应速率随之提高。温度的作用则不同：加热使粒子运动更快、能量更高，因此既提高了碰撞频率，也提高了达到活化能的碰撞所占的比例。
rate of reaction ∝ frequency of successful collisions
反应速率 ∝ 成功碰撞的频率
- r concentration → more particles per unit volume → more frequent collisions → faster rate.
- Lower concentration → fewer particles per unit volume → fewer collisions → slower rate.
- Concentration does not change the activation energy and does not change how much product can form.
- 浓度更高 → 单位体积内粒子更多 → 碰撞更频繁 → 速率更快。
- 浓度更低 → 单位体积内粒子更少 → 碰撞更少 → 速率更慢。
- 浓度不会改变活化能，也不会改变最终能生成多少产物。
3. Concentrated and Dilute Solutions Compared | 浓溶液与稀溶液对比
A useful picture is a crowded room compared with an almost empty one. In a crowded room, people bump into each other constantly; in an empty room, collisions are rare. Particles in a concentrated solution behave like the crowd, while particles in a dilute solution behave like the few people in an empty hall.
一个有用的比喻是拥挤的房间与几乎空无一人的房间。在拥挤的房间里，人们不停地互相碰撞；在空房间里，碰撞则很少发生。浓溶液中的粒子就像人群，而稀溶液中的粒子就像空荡大厅中稀疏的几个人。
| Feature 特征 | Dilute solution 稀溶液 | Concentrated solution 浓溶液 |
| Particles per cm³ 每 cm³ 粒子数 | Few 少 | Many 多 |
| Collisions per second 每秒碰撞次数 | Low 低 | High 高 |
| Initial rate 初始速率 | Slow 慢 | Fast 快 |
| Graph gradient 图像斜率 | Shallow 平缓 | Steep 陡峭 |
| Time to finish 反应完成时间 | Longer 更长 | Shorter 更短 |
Note carefully that the table compares the same volume of acid at different concentrations, not different amounts of acid. If the amount of acid also changes, the final yield changes as well, and that is a different variable.
请特别注意，上表比较的是相同体积、不同浓度的酸，而不是不同量的酸。如果酸的量也改变了，最终产率也会随之改变，那就是另一个变量了。
4. The Classic Experiment: Marble Chips and Acid | 经典实验：大理石与酸
The standard school experiment reacts marble chips (calcium carbonate) with dilute hydrochloric acid and measures the mass lost as carbon dioxide escapes. The equation is:
学校里的标准实验是用大理石碎块（碳酸钙）与稀盐酸反应，并测量二氧化碳逸出时损失的质量。方程式为：
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
The apparatus is simple: a conical flask containing the marble chips, a plug of cotton wool in the neck, and the whole flask standing on a top-pan balance. Acid is added, the timer starts, and the mass reading is recorded every 20 or 30 seconds. The cotton wool lets gas escape but stops spray from leaving the flask, which would cause a false mass loss.
装置很简单：一个装有石灰石碎块的锥形瓶，瓶口塞一团棉花，整个锥形瓶放在电子天平上。加入酸后开始计时，每隔 20 或 30 秒记录一次质量读数。棉花能让气体逸出，但阻止液雾飞溅离开锥形瓶，否则会造成虚假的质量损失。
To make it a fair test, you must keep everything constant except the concentration:
要使其成为公平试验，除浓度外，你必须保持其他一切条件不变：
- Same mass of marble chips, of the same size and surface area.
- Same volume of acid and same temperature.
- Same flask, same cotton wool, same balance.
- 大理石碎块的质量相同、大小与表面积相同。
- 酸的体积相同、温度相同。
- 使用同一个锥形瓶、同样的棉花、同一台天平。
If the marble chips are in excess, the total mass lost at the end is the same for every concentration, because the same amount of carbonate reacts and produces the same amount of CO₂. Only the speed differs. If the acid is in excess instead, the more concentrated acid produces more gas overall, so the final plateaus differ.
若大理石碎块过量，则最终损失的总质量对所有浓度都相同，因为参与反应的碳酸盐量相同、生成的 CO₂ 量也相同，只有快慢不同。反之若酸过量，则浓度更高的酸最终产生的气体更多，最终的平台位置也就不同。
5. The Sodium Thiosulfate and Cross Experiment | 硫代硫酸钠与"十字"实验
The second classic method reacts sodium thiosulfate solution with dilute hydrochloric acid. A pale yellow precipitate of sulfur slowly forms and makes the mixture cloudy:
第二种经典方法是用硫代硫酸钠溶液与稀盐酸反应。淡黄色的硫沉淀缓慢生成，使混合液逐渐变浑浊：
Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + S(s) + SO₂(g) + H₂O(l)
A black cross is drawn on a white tile or a piece of paper under the flask. You look down through the solution and time how long it takes for the cross to disappear from view. The faster the reaction, the sooner the sulfur hides the cross, so a shorter time means a faster rate.
在白瓷砖或白纸背后画一个黑十字，放在锥形瓶下方。你从上方透过溶液观察，记录十字从视线中消失所需的时间。反应越快，硫越早遮住十字，因此时间越短表示速率越快。
rate ∝ 1 ÷ time taken for the cross to disappear
速率 ∝ 1 ÷ 十字消失所需时间
Only the volume of water and the volume of thiosulfate change between runs, so the total volume stays the same and the concentration of thiosulfate changes while everything else is constant. Sulfur dioxide is toxic, so the flask should not be breathed over and the room should be well ventilated.
各次实验之间只改变水的体积与硫代硫酸钠溶液的体积，因此总体积保持不变，硫代硫酸钠的浓度发生变化，而其他条件恒定。二氧化硫有毒，不要俯身吸入瓶口气体，并保持室内通风良好。
6. Measuring Reaction Rate: Practical Methods | 反应速率的测量方法
Because concentration changes the rate, you need a reliable way to measure how fast a reaction is going. The method you choose depends on what changes during the reaction: a gas is given off, a solid disappears, or the colour and cloudiness change.
由于浓度会改变反应速率，你需要可靠的方法来测量反应进行得有多快。选择哪种方法取决于反应过程中发生了什么变化：放出气体、固体消失，还是颜色与浑浊度发生变化。
- Gas syringe: measure the volume of gas collected every 10 seconds. Fast, accurate, and gives a smooth volume-time curve.
- Mass loss: stand the flask on a balance and record the decreasing mass. Simple and safe, but small changes at the start are hard to read.
- Disappearing cross: measure the time for a precipitate to hide a mark. Cheap and visual, but the end point is judged by eye, so repeats are essential.
- Colorimetry: measure how strongly light is absorbed as a coloured species forms or fades. Very precise for A-level work.
- Electrical conductivity: useful when the number of ions in solution changes, such as in a titration-like reaction.
- 气体注射器：每 10 秒测量一次收集到的气体体积。快速、准确，能画出平滑的体积-时间曲线。
- 质量损失法：把锥形瓶放在天平上，记录不断减小的质量。简单安全，但起始阶段的质量变化很小，不易读数。
- 十字消失法：测量沉淀遮住标记所需的时间。成本低、直观，但终点靠眼睛判断，因此必须重复实验。
- 比色法：测量有色物质生成或消退时对光的吸收强度。对 A-level 实验而言非常精确。
- 电导率法：当溶液中离子数目发生变化时很有用，例如类滴定反应。
Whichever method you use, the rule for comparing rates is the same: a steeper initial curve means a faster reaction, and the rate at the very start is called the initial rate.
无论使用哪种方法，比较速率的规则都一样：起始曲线越陡，反应越快；起始瞬间的速率称为初始速率。
7. Reading Rate Graphs Correctly | 正确读取速率图表
Rate graphs usually plot the volume of gas produced, or the mass lost, against time. The gradient of the curve is the rate of reaction at that moment, and a tangent drawn at time zero gives the initial rate.
速率图表通常以生成的气体体积（或损失的质量）对时间作图。曲线在某点的斜率就是该时刻的反应速率，在时间为零处画切线即可得到初始速率。
- Steepest part at the beginning: the reactants are most concentrated, so collisions are most frequent.
- Curve flattens because reactants are being used up and their concentration falls.
- Horizontal plateau means the reaction has stopped, usually because one reactant has run out (the limiting reactant).
- Two curves with the same final plateau but different gradients show the same amount of product formed at different speeds.
- 起始处最陡：此时反应物浓度最高，碰撞最频繁。
- 曲线逐渐变平，是因为反应物被消耗，其浓度下降。
- 水平的平台表示反应已经停止，通常是因为某一种反应物耗尽（限量反应物）。
- 两条曲线最终平台相同但斜率不同，说明生成相同量的产物，但快慢不同。
| Acid concentration 酸的浓度 | Initial gradient 初始斜率 | Time to finish 完成时间 | Final gas volume 最终气体体积 |
| 0.5 mol/dm³ | Gentle 平缓 | Long 长 | Same 相同 |
| 1.0 mol/dm³ | Medium 中等 | Medium 中等 | Same 相同 |
| 2.0 mol/dm³ | Steep 陡峭 | Short 短 | Same 相同 |
In an exam, always describe what happens and then explain why, using the words particle, collide, frequency and successful collision. A full-mark answer for "why does a more concentrated acid react faster?" is: "There are more acid particles in the same volume, so the particles collide more frequently, so the frequency of successful collisions increases and the rate of reaction is faster."
在考试中，永远先描述现象，再解释原因，并使用粒子、碰撞、频率与成功碰撞这些词。对"为什么更浓的酸反应更快？"这一问题的满分答案是："相同体积中酸的粒子更多，所以粒子碰撞更频繁，成功碰撞的频率增加，反应速率因此更快。"
8. Concentration Compared with Other Factors | 浓度与其他影响因素的比较
Concentration is only one of several factors that change the rate. Comparing them helps you avoid mixing up the explanations.
浓度只是影响速率的若干因素之一。把它们放在一起比较，有助于你避免混淆不同的解释。
- Temperature: increases both collision frequency and the fraction of particles with energy greater than or equal to Eₐ, so it has a very large effect.
- Surface area: only affects reactions involving a solid. Smaller pieces expose more surface, so more collisions occur per second.
- Catalyst: provides an alternative pathway with a lower activation energy. It does not change collision frequency.
- Pressure: for gases only. Compressing a gas brings the particles closer, which is effectively the same idea as increasing concentration.
- Concentration: changes the number of particles per unit volume in a solution or gas, so it changes collision frequency only.
- 温度：既提高碰撞频率，也提高能量达到或超过 Eₐ 的粒子所占比例，因此影响非常大。
- 表面积：只影响涉及固体的反应。碎块越小，暴露的表面越大，每秒发生的碰撞越多。
- 催化剂：提供一条活化能更低的替代途径，它不改变碰撞频率。
- 压强：只对气体有效。压缩气体使粒子更靠近，本质上与提高浓度是同一个道理。
- 浓度：改变溶液或气体中单位体积的粒子数目，因此只改变碰撞频率。
Remember that all rate factors change only the speed. None of them changes the position of equilibrium or the total amount of product formed from a fixed amount of limiting reactant.
请记住，所有影响速率的因素都只改变快慢。它们都不会改变平衡的位置，也不会改变由一定量限量反应物所能生成的产物总量。
9. Higher Level: Rate Equation and Order | 进阶：速率方程与反应级数
At A-level, the effect of concentration is written as a rate equation. For a reaction between A and B, the rate is proportional to the concentrations raised to powers called orders:
在 A-level 阶段，浓度的影响被写成速率方程。对于 A 与 B 之间的反应，速率与浓度的若干次幂（称为级数）成正比：
rate = k [A]ᵐ [B]ⁿ
Here k is the rate constant, m is the order with respect to A, and n is the order with respect to B. The overall order is m + n. Orders are found by experiment, not by looking at the balanced equation, and they are usually 0, 1 or 2. Zero order means the rate does not depend on that concentration at all; first order means doubling the concentration doubles the rate; second order means doubling the concentration makes the rate four times larger. The units of k depend on the overall order, for example mol dm⁻³ s⁻¹ for a zero-order reaction and s⁻¹ for a first-order reaction.
其中 k 是速率常数，m 是对 A 的级数，n 是对 B 的级数，总级数为 m + n。级数由实验确定，不能只看配平方程式；通常为 0、1 或 2。零级表示速率完全不依赖于该浓度；一级表示浓度加倍则速率加倍；二级表示浓度加倍则速率变为原来的四倍。k 的单位取决于总级数，例如零级反应为 mol dm⁻³ s⁻¹，一级反应为 s⁻¹。
Two graph skills go with this. On a concentration-time graph, a straight line of constant negative gradient means zero order, a constant half-life curve means first order. On a rate-concentration graph, a horizontal line means zero order, a straight line through the origin means first order, and an upward curve means second order.
与此相关还有两项图表技能。在浓度-时间图上，斜率恒定的直线表示零级，半衰期恒定的曲线表示一级。在速率-浓度图上，水平线表示零级，过原点的直线表示一级，向上弯曲的曲线表示二级。
10. Common Mistakes and Exam Tips | 常见错误与考试技巧
Many marks are lost through small slips in wording rather than a lack of understanding. Read the following traps carefully before your next assessment.
很多失分源于措辞上的小疏忽，而不是理解错误。在下一次测验前请仔细阅读以下陷阱。
- Saying "more collisions" without saying "more frequent collisions per second". Rate depends on collisions per unit time, not on the total number of collisions.
- Confusing concentration with amount. Doubling the volume of acid doubles the amount but does not change the concentration.
- Claiming that a higher concentration gives more product. It gives the same product faster when the other reactant is in excess.
- Forgetting to control temperature. The reaction itself releases heat, so a very fast run may warm up.
- Not repeating the disappearing-cross experiment; the end point is subjective, so repeats and a mean are expected.
- Writing the rate as time instead of 1 ÷ time when the cross method is used.
- 只说"碰撞更多"，却没说"每秒碰撞更频繁"。速率取决于单位时间内的碰撞次数，而不是碰撞总数。
- 把浓度与物质的量混淆。酸的体积加倍会使物质的量加倍，但浓度不变。
- 声称浓度更高会生成更多产物。当另一种反应物过量时，它只是更快地生成相同量的产物。
- 忘记控制温度。反应本身会放热，非常快的一组实验可能会升温。
- 没有重复十字消失实验；终点判断具有主观性，因此应当重复实验并取平均值。
- 使用十字法时把速率写成时间，而不是 1 ÷ 时间。
11. Concentration in the Real World | 浓度在现实世界中的应用
Chemists control concentration every day, and the reason is often money as much as science. A factory that doubles the concentration of a reactant may finish a batch in half the time, but concentrated reagents are more expensive, more corrosive and more dangerous to handle.
化学家每天都在控制浓度，其理由往往既有科学因素，也有经济因素。工厂若把反应物浓度提高一倍，一批产品可能只需一半时间完成，但浓试剂更昂贵、腐蚀性更强、处理更危险。
- Medicine: the concentration of an active ingredient decides both the dose absorbed and how quickly a drug acts in the body.
- Water treatment: chlorine is added at a carefully controlled concentration - too low and bacteria survive, too high and the water is unsafe to drink.
- Food preservation: concentrated salt or sugar solutions slow down the growth of microorganisms.
- Car engines and industry: higher concentrations of oxygen or fuel give faster combustion; acid rain forms when concentrated pollutant gases react in the atmosphere.
- Laboratory safety: dilute acids are used for school experiments because they react at a controllable speed.
- 医药：有效成分的浓度既决定被吸收的剂量，也决定药物在体内起效的快慢。
- 水处理：氯被以严格控制的浓度加入——太低细菌存活，太高则水不适合饮用。
- 食品保鲜：高浓度的盐或糖溶液可减缓微生物的生长。
- 汽车发动机与工业：更高浓度的氧气或燃料带来更快的燃烧；当浓度较高的污染气体在大气中反应时，便形成酸雨。
- 实验室安全：学校实验使用稀酸，因为它们的反应速度可控。
12. Quick Recap and Practice Questions | 快速回顾与练习
Here is the whole topic in five sentences. Concentration measures how much solute is dissolved per unit volume. More concentrated solutions contain more particles per cm³, so particles collide more frequently and there are more successful collisions per second. Therefore the rate of reaction increases, which shows up as a steeper initial gradient on a rate graph. The same total amount of product still forms if the other reactant is in excess, and concentration never changes the activation energy.
用五句话概括整个主题。浓度衡量单位体积中溶解了多少溶质。浓度更高的溶液每 cm³ 含更多粒子，因此粒子碰撞更频繁，每秒的成功碰撞也更多。因此反应速率提高，在速率图上表现为起始斜率更陡。若另一种反应物过量，最终生成的产物总量仍然相同，而浓度从不改变活化能。
Try these questions: (1) Explain, in terms of particles and collisions,
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