📚 IGCSE Chemistry: Chemical Equilibrium Key Points | IGCSE化学：化学平衡考点精讲
Chemical equilibrium is a fundamental topic in IGCSE Chemistry that explains how reversible reactions reach a state of balance. Mastering the concepts of dynamic equilibrium, Le Chatelier’s principle, and the factors affecting the position of equilibrium is essential for exam success. This revision guide breaks down every key point with precise, paired English–Chinese explanations, designed to help you understand, recall, and apply the material confidently.
化学平衡是IGCSE化学的核心主题之一，它解释可逆反应如何达到平衡状态。掌握动态平衡、勒夏特列原理以及影响平衡位置的因素是通过考试的关键。本复习指南通过精准的中英对照讲解，逐一剖析每个考点，帮助你理解、记忆并自信地应用相关知识。
1. Reversible Reactions | 可逆反应
A reversible reaction is one where the products can react to re-form the original reactants. These reactions are indicated by the double arrow symbol ⇌ in chemical equations. In a closed system, both the forward and backward reactions occur simultaneously, and neither the reactants nor the products can escape.
可逆反应是指生成物可以再反应重新生成原来反应物的反应。这类反应在化学方程式中用双箭头符号 ⇌ 表示。在封闭系统中，正反应和逆反应同时发生，反应物和生成物都无法离开系统。
For example, the thermal decomposition of ammonium chloride is a classic reversible reaction: NH₄Cl(s) ⇌ NH₃(g) + HCl(g). When heated, ammonium chloride breaks down into ammonia and hydrogen chloride gases; when cooled, the gases recombine to form solid ammonium chloride.
例如，氯化铵的热分解就是一个典型的可逆反应：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，两种气体重新结合生成固体氯化铵。
2. Dynamic Equilibrium | 动态平衡
Dynamic equilibrium is the state reached in a reversible reaction when the rate of the forward reaction equals the rate of the backward reaction, and the concentrations of reactants and products remain constant. It is called ‘dynamic’ because both reactions are still occurring at the molecular level, even though macroscopic properties appear unchanged.
动态平衡是可逆反应中达到的一种状态，此时正反应速率等于逆反应速率，且反应物和生成物的浓度保持恒定。之所以称为“动态”，是因为在分子水平上两个方向的反应仍在进行，尽管宏观性质看似不变。
For a dynamic equilibrium to be established, the system must be closed, and the temperature must remain constant. In an open system, gaseous products may escape, preventing equilibrium from being achieved.
要建立动态平衡，系统必须是封闭的，且温度保持恒定。在开放系统中，气态生成物可能逸出，导致无法达到平衡。
3. Position of Equilibrium | 平衡位置
The position of equilibrium describes the relative amounts of reactants and products at equilibrium. If the equilibrium mixture contains more products than reactants, we say the position lies to the right. If it contains more reactants, the position lies to the left.
平衡位置描述了平衡时反应物和生成物的相对比例。如果平衡混合物中生成物多于反应物，我们说平衡位置偏向右侧；如果反应物更多，则偏向左侧。
The position of equilibrium is influenced by changes in concentration, pressure (for gases), and temperature. It is not affected by the addition of a catalyst, which only speeds up the rate at which equilibrium is attained without altering the equilibrium composition.
平衡位置受浓度、压强（对于气体）和温度变化的影响。加入催化剂不会改变平衡位置，催化剂只能加快达到平衡的速率，而不改变平衡组成。
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 conditions, the position of equilibrium will shift to counteract that change and restore a new equilibrium. This principle is a powerful tool for predicting how a reaction responds to external stresses.
勒夏特列原理指出，如果处于动态平衡的系统受到外界条件改变的影响，平衡位置将发生移动，以抵消这一改变并建立新的平衡。这个原理是预测反应如何应对外部扰动的重要工具。
The principle applies to changes in concentration, pressure, and temperature. Understanding its application allows chemists to manipulate industrial processes to maximise the yield of desired products.
该原理适用于浓度、压强和温度的变化。理解其应用可以帮助化学家调控工业过程，以最大化目标产物的产率。
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 (towards the products) to use up the added reactant. Similarly, increasing the concentration of a product shifts the equilibrium to the left. Removing a product drives the equilibrium to the right, favouring further product formation.
如果增大某种反应物的浓度，平衡将向右移动（朝向生成物方向）以消耗掉增加的反应物。同样，增大生成物的浓度会使平衡向左移动。移除生成物则驱动平衡向右移动，有利于生成更多产物。
For example, in the equilibrium reaction between iron(III) ions and thiocyanate ions: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) (blood-red). Adding more Fe³⁺ or SCN⁻ intensifies the red colour, showing a shift to the right. Adding a substance that removes Fe³⁺ (such as a complexing agent) reduces the colour intensity, shifting the equilibrium to the left.
例如，在铁(III)离子与硫氰酸根离子的平衡反应中：Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)（血红色）。加入更多Fe³⁺或SCN⁻会使红色加深，表明平衡向右移动。加入能去除Fe³⁺的物质（如配位剂）则会减弱颜色，使平衡向左移动。
6. Effect of Temperature Changes | 温度变化的影响
Temperature changes affect equilibrium according to the enthalpy change of the reaction. For an exothermic reaction (ΔH negative), increasing the temperature shifts the equilibrium to the left (towards the reactants), as the system tries to absorb the added heat by favouring the endothermic reverse reaction. Decreasing the temperature shifts the equilibrium to the right, favouring the exothermic forward reaction.
温度变化根据反应的焓变影响平衡。对于放热反应（ΔH为负值），升高温度会使平衡向左移动（朝向反应物），因为系统通过促进吸热的逆反应来吸收增加的热量。降低温度则使平衡向右移动，有利于放热的正反应。
For an endothermic reaction (ΔH positive), increasing the temperature shifts the equilibrium to the right (products side), while decreasing the temperature shifts it to the left. This behaviour is entirely consistent with Le Chatelier’s principle.
对于吸热反应（ΔH为正值），升高温度会使平衡向右移动（生成物一侧），而降低温度则使平衡向左移动。这一行为完全符合勒夏特列原理。
Consider the dimerisation of nitrogen dioxide: 2NO₂(g) ⇌ N₂O₄(g) (ΔH is negative, exothermic forward reaction). At room temperature, an equilibrium mixture appears brown due to NO₂. Heating the mixture turns it darker brown because the equilibrium shifts left to produce more NO₂; cooling it makes it paler as more colourless N₂O₄ forms.
考虑二氧化氮的二聚反应：2NO₂(g) ⇌ N₂O₄(g)（ΔH为负值，正反应放热）。室温下，平衡混合物因NO₂呈棕色。加热混合物会使其变为更深的棕色，因为平衡左移生成更多NO₂；冷却则颜色变浅，形成更多无色的N₂O₄。
7. Effect of Pressure Changes | 压强变化的影响
Pressure changes only affect equilibria involving gases, and only when there is a change in the total number of gas molecules between reactants and products. Increasing the pressure shifts the equilibrium towards the side with fewer gas molecules, reducing the pressure. Decreasing the pressure shifts the equilibrium towards the side with more gas molecules.
压强变化只影响涉及气体的平衡，且仅当反应物和生成物之间气体分子总数发生变化时才有效。增大压强会使平衡向气体分子数较少的一侧移动，从而降低压强。减小压强则使平衡向气体分子数较多的一侧移动。
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 favours the forward reaction, increasing the yield of ammonia. Conversely, for reactions where the number of gas molecules is the same on both sides, such as H₂(g) + I₂(g) ⇌ 2HI(g), pressure changes have no effect on the position of equilibrium.
在哈伯法中：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，左侧有4摩尔气体，右侧有2摩尔气体。高压有利于正反应，增加氨的产率。相反，对于两侧气体分子数相同的反应，例如H₂(g) + I₂(g) ⇌ 2HI(g)，压强变化对平衡位置没有影响。
8. Effect of a Catalyst | 催化剂的影响
A catalyst provides an alternative reaction pathway with a lower activation energy for both the forward and backward reactions. It increases the rate of both reactions equally, so the system reaches dynamic equilibrium faster. Crucially, a catalyst does not change the position of equilibrium or the equilibrium concentrations; it only shortens the time needed to attain equilibrium.
催化剂为正向和逆向反应都提供了一条活化能较低的反应途径。它同等地加快两个反应的速率，因此系统能更快地达到动态平衡。关键的是，催化剂不改变平衡位置或平衡浓度；它只是缩短了达到平衡所需的时间。
In the Contact process, vanadium(V) oxide (V₂O₅) catalyses the oxidation of SO₂ to SO₃ without affecting the equilibrium yield. Similarly, iron is used in the Haber process to lower the temperature required for a useful rate, but the equilibrium composition at a given temperature and pressure is dictated solely by thermodynamics.
在接触法中，五氧化二钒（V₂O₅）催化SO₂氧化为SO₃，但不影响平衡产率。同样，哈伯法中使用铁催化剂以降低获得有实用价值的反应速率所需的温度，但在给定温度和压强下的平衡组成完全由热力学决定。
9. The Haber Process – Ammonia Synthesis | 哈伯法——合成氨
The Haber process is the industrial manufacture of ammonia from nitrogen and hydrogen: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH = -92 kJ mol⁻¹, exothermic). The reaction is carried out under carefully chosen compromise conditions: a pressure of 200 atm, a temperature of around 450°C, and an iron catalyst.
哈伯法是从氮气和氢气工业制造氨的方法：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)（ΔH = -92 kJ mol⁻¹，放热）。该反应在精心选择的折衷条件下进行：压强200 atm，温度约450°C，并使用铁催化剂。
A high pressure favours the forward reaction because it reduces the number of gas molecules, but extremely high pressures are expensive and hazardous. A lower temperature would give a higher equilibrium yield of ammonia, but at a very slow rate. The chosen temperature of 450°C is a compromise: it allows the reaction to proceed at an acceptable rate while still maintaining a reasonable equilibrium yield. The iron catalyst lowers the activation energy so that the forward and backward reactions both speed up, allowing equilibrium to be reached more quickly without altering the yield.
高压有利于正反应（因为气体分子数减少），但过高的压强成本高且危险。较低的温度会提高氨的平衡产率，但反应速率极慢。选用的450°C是一个折衷：它使反应以可接受的速率进行，同时保持合理的平衡产率。铁催化剂降低活化能，使正逆反应都加速，从而更快达到平衡而不改变产率。
10. The Contact Process – Sulfuric Acid Production | 接触法——硫酸生产
The Contact process involves the reversible oxidation of sulfur dioxide to sulfur trioxide: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (ΔH = -197 kJ mol⁻¹, exothermic). This step is crucial for producing sulfuric acid, one of the most important industrial chemicals.
接触法涉及二氧化硫可逆氧化为三氧化硫的反应：2SO₂(g) + O₂(g) ⇌ 2SO₃(g)（ΔH = -197 kJ mol⁻¹，放热）。这一步对于生产最重要的工业化学品之一硫酸至关重要。
The reaction is typically carried out at a pressure of 1–2 atm, a temperature of about 450°C, and with a vanadium(V) oxide (V₂O₅) catalyst. Because there are 3 moles of gas on the left and 2 moles on the right, high pressure would favour the forward reaction, but even a modest pressure gives a high conversion, making high pressures unnecessary. The exothermic nature means a low temperature favours a high equilibrium yield, but again a moderate temperature is used to ensure a fast enough rate. The catalyst V₂O₅ does not alter the equilibrium position but enables the process to run efficiently.
该反应通常在1-2 atm压强、约450°C温度下进行，并使用五氧化二钒（V₂O₅）催化剂。由于左侧有3摩尔气体，右侧有2摩尔，高压有利于正反应，但即使在常压或微加压下也能获得很高的转化率，因此无需高压。放热特性意味着低温有利于高平衡产率，但同样采用中等温度以保证足够快的速率。催化剂V₂O₅不改变平衡位置，但使得过程高效运行。
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