📚 IGCSE CCEA Chemistry: Chemical Equilibrium Key Points | IGCSE CCEA 化学：化学平衡 考点精讲
Understanding chemical equilibrium is fundamental to explaining why some reactions do not go to completion and how industrial conditions are chosen to maximise yield. This article covers all the key concepts, definitions and applications required for the CCEA IGCSE Chemistry exam, including reversible reactions, dynamic equilibrium, Le Chatelier’s principle, and the Haber and Contact processes.
理解化学平衡是解释为什么有些反应不会进行到底以及如何选择工业条件以最大化产率的基础。本文涵盖了 CCEA IGCSE 化学考试所需的所有关键概念、定义和应用，包括可逆反应、动态平衡、勒夏特列原理以及哈伯法和接触法。
1. Reversible Reactions | 可逆反应
A reversible reaction is one in which the products can react together, under the same conditions, to re-form the original reactants. The reaction is represented using a double arrow (⇌) to show that both the forward and backward reactions are possible.
可逆反应是指产物在相同条件下可以重新反应生成原来的反应物的反应。该反应使用双箭头 (⇌) 表示，表明正反应和逆反应都可以发生。
For example, the thermal decomposition of ammonium chloride is reversible: NH₄Cl(s) ⇌ NH₃(g) + HCl(g). When heated, ammonium chloride decomposes into ammonia and hydrogen chloride gases; on cooling, the gases recombine to form solid ammonium chloride.
例如，氯化铵的热分解是可逆的：NH₄Cl(s) ⇌ NH₃(g) + HCl(g)。加热时，氯化铵分解为氨气和氯化氢气体；冷却时，气体重新结合形成固态氯化铵。
Another common example is the hydration of anhydrous copper(II) sulfate: CuSO₄(s) + 5H₂O(l) ⇌ CuSO₄·5H₂O(s). Adding water to white anhydrous copper(II) sulfate turns it blue, and heating the blue hydrated crystals drives the reaction in reverse.
另一个常见的例子是无水硫酸铜的水合：CuSO₄(s) + 5H₂O(l) ⇌ CuSO₄·5H₂O(s)。向白色无水硫酸铜中加水会使其变蓝，而加热蓝色水合晶体则会使反应逆向进行。
2. Dynamic Equilibrium | 动态平衡
Dynamic equilibrium is reached in a closed system when the rate of the forward reaction equals the rate of the backward reaction. At this point, the concentrations of reactants and products remain constant, but both reactions continue to occur at the molecular level.
在封闭系统中，当正反应速率等于逆反应速率时，即达到动态平衡。此时，反应物和产物的浓度保持不变，但在分子水平上两个反应仍在持续进行。
It is essential that the system is closed to prevent the escape of any gaseous reactants or products. If a gas is allowed to leave, equilibrium cannot be established because the backward reaction cannot occur fully.
系统必须是封闭的，以防止任何气态反应物或产物逸出，这一点至关重要。如果有气体逸出，就无法建立平衡，因为逆反应无法充分进行。
3. Characteristics of a System at Equilibrium | 平衡系统的特征
A system at dynamic equilibrium has several observable features: the macroscopic properties (such as colour, pressure and density) remain constant; the concentrations of all reactants and products are unchanged over time; and equilibrium can be approached from either direction.
处于动态平衡的系统有几个可观察的特征：宏观性质（如颜色、压强和密度）保持不变；所有反应物和产物的浓度不随时间变化；并且可以从任何一个方向达到平衡。
The equilibrium does not necessarily mean that the amounts of reactants and products are equal. The equilibrium position can favour either the reactants or the products, depending on the reaction conditions.
平衡并不一定意味着反应物和产物的量相等。平衡位置可以偏向反应物或产物，具体取决于反应条件。
4. Le Chatelier’s Principle | 勒夏特列原理
Le Chatelier’s principle states that if a system at dynamic equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will shift to oppose that change and restore a new equilibrium.
勒夏特列原理指出，如果处于动态平衡的系统受到浓度、压强或温度变化的影响，平衡位置将发生移动，以对抗这种变化，并建立新的平衡。
This principle allows chemists to predict how changing conditions will affect the yield of a reversible reaction. It is widely used in the chemical industry to optimise the production of important chemicals such as ammonia and sulfuric acid.
该原理使化学家能够预测条件变化如何影响可逆反应的产率。它在化学工业中被广泛应用，用以优化氨和硫酸等重要化学品的生产。
5. Effect of Concentration Changes | 浓度变化的影响
If the concentration of a reactant is increased, the equilibrium shifts to the right (in the forward direction) to reduce the concentration of that reactant by forming more products. Conversely, increasing the concentration of a product shifts the equilibrium to the left.
如果增加反应物的浓度，平衡会向右移动（正反应方向），通过生成更多产物来降低该反应物的浓度。反之，增加产物的浓度会使平衡向左移动。
For instance, in the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), adding more nitrogen shifts the equilibrium to the right, producing more ammonia. Removing ammonia as it forms also shifts the equilibrium to the right, increasing yield.
例如，在反应 N₂(g) + 3H₂(g) ⇌ 2NH₃(g) 中，增加氮气会使平衡向右移动，生成更多氨。在氨生成时将其移除也会使平衡向右移动，提高产率。
6. Effect of Pressure Changes | 压强变化的影响
Pressure changes only affect equilibria involving gases where the total number of gas molecules on each side of the equation is different. Increasing pressure shifts the equilibrium towards the side with fewer gas molecules, as this helps to reduce the pressure.
压强的变化只影响涉及气体的平衡，并且要求方程式两边气体分子总数不同。增加压强会使平衡向气体分子数较少的一侧移动，因为这会帮助降低压强。
In the Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), there are 4 moles of gas on the left and 2 moles on the right. High pressure (around 200 atm) shifts the equilibrium to the right, favouring ammonia production. However, if the number of gas molecules is equal on both sides, pressure has no effect on the equilibrium position.
在哈伯法中，N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，左边有 4 摩尔气体，右边有 2 摩尔。高压（约 200 atm）会使平衡向右移动，有利于氨的生成。但如果两边气体分子数相等，压强对平衡位置没有影响。
7. Effect of Temperature Changes | 温度变化的影响
Temperature changes affect the equilibrium position depending on whether the forward reaction is exothermic or endothermic. Increasing temperature favours the endothermic direction, because the system absorbs heat to oppose the rise in temperature. Decreasing temperature favours the exothermic direction.
温度的变化对平衡位置的影响取决于正反应是放热还是吸热。升高温度有利于吸热方向，因为系统吸收热量以对抗温度的升高。降低温度有利于放热方向。
For the Haber process, the forward reaction is exothermic: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹. Therefore, lowering the temperature shifts the equilibrium to the right, increasing the yield of ammonia. However, very low temperatures make the reaction uneconomically slow, so a compromise temperature of about 450 °C is used.
对于哈伯法，正反应是放热的：N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹。因此，降低温度会使平衡向右移动，提高氨的产率。然而，过低的温度会使反应速率过慢，在经济上不可行，因此采用约 450 °C 的折中温度。
8. Effect of a Catalyst | 催化剂的作用
A catalyst speeds up both the forward and backward reactions equally by providing an alternative reaction pathway with a lower activation energy. It does not change the position of equilibrium; it only helps the system reach equilibrium more quickly.
催化剂通过提供较低活化能的替代反应路径，同等程度地加快正反应和逆反应的速率。它不会改变平衡位置；它只是帮助系统更快地达到平衡。
In industry, catalysts are vital because they allow equilibrium to be reached at lower temperatures, saving energy and time. For example, iron is used as a catalyst in the Haber process, and vanadium(V) oxide (V₂O₅) is used in the Contact process. Neither catalyst alters the equilibrium yield, but they make the process economically viable.
在工业中，催化剂至关重要，因为它们使得在较低温度下即可达到平衡，从而节省能源和时间。例如，哈伯法中使用铁作为催化剂，接触法中使用五氧化二钒 (V₂O₅)。两种催化剂都不会改变平衡产率，但它们使工艺在经济上可行。
9. Summary of Factors Affecting Equilibrium | 影响平衡的因素总结
The following table summarises how concentration, pressure, temperature and catalysts influence the position of equilibrium and the rate at which equilibrium is attained. Remember that only temperature, concentration and pressure (for gases with different mole numbers) can shift the equilibrium position.
下表总结了浓度、压强、温度和催化剂如何影响平衡位置以及达到平衡的速率。请记住，只有温度、浓度和压强（对于气体分子数不同的反应）能够移动平衡位置。
| Change / 变化 | Effect on Equilibrium Position / 对平衡位置的影响 | Effect on Rate / 对速率的影响 |
|---|---|---|
| Increase reactant concentration / 增加反应物浓度 | Shifts to the right (forward) / 向右（正反应方向）移动 | Increases forward rate / 正反应速率加快 |
| Increase pressure (fewer gas moles on right) / 增加压强（右边气体分子数较少） | Shifts to the right / 向右移动 | Increases rate of both forward and backward reactions / 加快正逆反应速率 |
| Increase temperature (exothermic forward) / 升高温度（正反应放热） | Shifts to the left (endothermic direction) / 向左（吸热方向）移动 | Increases rate of both reactions / 加快两个反应的速率 |
| Add a catalyst / 加入催化剂 | No shift / 无移动 | Increases rate equally / 同等程度加快速率 |
10. Industrial Application: The Haber Process | 工业应用：哈伯法
The Haber process is the industrial manufacture of ammonia from nitrogen and hydrogen: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic (ΔH = −92 kJ mol⁻¹), and the number of gas molecules decreases from 4 to 2.
哈伯法是从氮气和氢气工业生产氨的方法：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)。正反应是放热的 (ΔH = −92 kJ mol⁻¹)，并且气体分子数从 4 减少到 2。
The typical conditions used are a temperature of about 450 °C, a pressure of 200 atm, and an iron catalyst. The high pressure shifts the equilibrium to the right, increasing ammonia yield. The moderate temperature is a compromise: lower temperatures would give a higher equilibrium yield, but the rate would be too slow. The iron catalyst speeds up the reaction without affecting the equilibrium position.
使用的典型条件是温度约 450 °C、压强 200 atm 以及铁催化剂。高压使平衡向右移动，提高氨的产率。适中的温度是一种折中：更低的温度会给出更高的平衡产率，但速率会太慢。铁催化剂加快了反应速率，而不影响平衡位置。
Unreacted nitrogen and hydrogen are recycled back into the reactor, and ammonia is continuously removed by cooling and liquefaction, which also helps drive the equilibrium to the right.
未反应的氮气和氢气被循环回反应器，氨通过冷却和液化被连续移除，这也有助于推动平衡向右移动。
11. The Contact Process | 接触法
The Contact process is used to manufacture sulfuric acid through the oxidation of sulfur dioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). This reaction is exothermic (ΔH = −196 kJ mol⁻¹) and involves a decrease in gas molecules from 3 to 2.
接触法通过氧化二氧化硫来生产硫酸：2SO₂(g) + O₂(g) ⇌ 2SO₃(g)。该反应是放热的 (ΔH = −196 kJ mol⁻¹)，并且气体分子数从 3 减少到 2。
The industrial conditions are a temperature of around 450 °C, a pressure of about 2 atm, and a vanadium(V) oxide (V₂O₅) catalyst. Unlike the Haber process, the pressure used is only slightly above atmospheric, because the equilibrium already lies far to the right at low pressure. The catalyst is essential to achieve a high rate at the moderate temperature.
工业条件是温度约 450 °C、压强约 2 atm 以及五氧化二钒 (V₂O₅) 催化剂。与哈伯法不同，所使用的压强仅略高于常压，因为在低压下平衡已经很偏向右边了。催化剂对于在适中温度下实现高反应速率至关重要。
The sulfur trioxide produced is then absorbed in concentrated sulfuric acid to form oleum, which is later diluted to produce concentrated sulfuric acid. Understanding the equilibrium principles behind this process helps explain the choice of reaction conditions.
生成的二氧化硫随后被浓硫酸吸收形成发烟硫酸，之后再稀释以生产浓硫酸。理解该过程背后的平衡原理有助于解释反应条件的选择。
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