📚 Chemical Equilibrium | 化学平衡
Chemical equilibrium is a vital concept in OCR GCSE Chemistry. It explains how reversible reactions can reach a state of balance in a closed system, and how we can alter conditions to favour the formation of desired products. Mastering equilibrium is essential for understanding industrial processes such as the Haber process and the Contact process. This article breaks down all the key points you need to know, with clear explanations, examples, and exam-focused tips.
化学平衡是 OCR GCSE 化学中的一个重要概念。它解释了可逆反应如何在封闭系统中达到平衡状态，以及我们如何改变条件来促进所需产物的生成。掌握平衡对于理解哈伯法、接触法等工业过程至关重要。本文通过清晰的解释、实例和考点提示，梳理了你需要掌握的所有要点。
1. What is a Reversible Reaction? | 什么是可逆反应？
In most reactions you see at GCSE, reactants turn into products and the reaction goes to completion. However, some reactions can go in both directions. These are called reversible reactions. We use the symbol ⇌ to show that the reaction can proceed forwards and backwards.
在 GCSE 阶段见到的大多数反应中，反应物变为产物，反应进行到底。但有些反应可以双向进行，称为可逆反应。我们用符号 ⇌ 表示反应可以正向和逆向进行。
A classic example is the heating of hydrated copper(II) sulfate. Blue CuSO₄·5H₂O crystals can be heated to form white anhydrous CuSO₄ and steam. When water is added to the white solid, the blue crystals form again.
一个经典例子是水合硫酸铜(II)的加热。蓝色的 CuSO₄·5H₂O 晶体加热后形成白色无水 CuSO₄ 和水蒸气。当向白色固体加水时，蓝色晶体会再次生成。
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)
Another readily demonstrated reversible reaction is the thermal decomposition of ammonium chloride. Solid NH₄Cl breaks down into ammonia and hydrogen chloride gases on heating. When these gases cool, they recombine to form a white solid of ammonium chloride.
另一个常用来演示的可逆反应是氯化铵的热分解。固体 NH₄Cl 在加热下分解成氨气和氯化氢气体。当这些气体冷却时，它们重新结合形成白色氯化铵固体。
NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
2. Dynamic Equilibrium | 动态平衡
When a reversible reaction takes place in a closed system, the forward and reverse reactions eventually reach the same rate. This state is called dynamic equilibrium. At equilibrium, the concentrations of reactants and products remain constant, but the reactions do not stop – both forward and reverse reactions continue to occur at the molecular level.
当可逆反应在封闭系统中发生时，正向和逆向反应最终速率相等。这种状态称为动态平衡。在平衡状态下，反应物和产物的浓度保持不变，但反应并未停止——在分子水平上，正向和逆向反应都在持续进行。
The term 'dynamic' emphasises that the system is not static; it is a continuous process where molecules are constantly being converted in both directions. Equilibrium can only be reached in a closed system where no substances can escape.
“动态”一词强调系统不是静止的，而是一个连续过程，分子不断在两个方向上转化。平衡只能在封闭系统中达到，即没有任何物质可以逸出。
3. Characteristics of Equilibrium | 平衡的特征
At dynamic equilibrium, several features are always true: the rates of the forward and reverse reactions are equal; the concentrations of all reactants and products remain constant; the system must be closed; and the reaction must be reversible. Macroscopically, no observable changes occur, but microscopically the reactions are proceeding.
在动态平衡下，几个特征始终成立：正逆反应速率相等；所有反应物和产物的浓度保持恒定；系统必须是封闭的；反应必须是可逆的。宏观上没有可观察到的变化，但在微观上反应仍在进行。
Equilibrium can be approached from either direction. Starting with only reactants, the forward reaction dominates until equilibrium is established. Starting with only products, the reverse reaction dominates until the same equilibrium state is reached.
平衡可以从两个方向趋近。如果从纯反应物开始，正向反应占主导直至建立平衡。如果从纯产物开始，逆向反应占主导直至达到相同的平衡状态。
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, temperature, or pressure, the position of equilibrium will shift to oppose that change. This principle helps us predict how equilibrium yields can be altered.
勒夏特列原理指出，如果处于动态平衡的系统受到浓度、温度或压力的变化，平衡位置将发生移动以抵消这种变化。这一原理帮助我们预测如何改变平衡产率。
The 'position of equilibrium' refers to the relative amounts of reactants and products. If the equilibrium shifts to the right, more products are formed. If it shifts to the left, more reactants are formed. The next sections examine each factor in detail.
“平衡位置”指的是反应物和产物的相对含量。如果平衡向右移动，则生成更多产物；如果向左移动，则生成更多反应物。以下各节将详细讨论各影响因素。
5. Effect of Changing Concentration | 浓度变化的影响
If the concentration of a reactant is increased, the equilibrium shifts to the right to produce more product and reduce the concentration of the added substance. Conversely, increasing the concentration of a product shifts the equilibrium to the left, favouring the reformation of reactants.
如果增加某一反应物的浓度，平衡将向右移动，生成更多产物以降低所加物质的浓度。相反，如果增加某一产物的浓度，平衡向左移动，促使重新生成反应物。
A colourful illustration is the equilibrium between iron(III) ions and thiocyanate ions: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq). The product is blood-red. Adding Fe³⁺ or SCN⁻ intensifies the red colour, while adding a substance that removes Fe³⁺ (such as fluoride ions) causes the colour to fade.
一个色彩鲜明的实例是铁(III)离子与硫氰酸根离子的平衡：Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)。该产物呈血红色。添加 Fe³⁺ 或 SCN⁻ 会使红色加深，而添加去除 Fe³⁺ 的物质（如氟离子）则会使颜色变浅。
This principle is used industrially to drive a reaction towards completion by continuously removing a product, thereby shifting the equilibrium to the right.
这一原理在工业上被用于通过连续移走产物，使平衡不断向右移动，从而驱动反应进行到底。
6. Effect of Changing Temperature | 温度变化的影响
Temperature changes affect equilibrium depending on whether the forward reaction is exothermic or endothermic. If the forward reaction is exothermic (releases heat, ΔH negative), increasing the temperature favours the endothermic reverse reaction, shifting the equilibrium to the left.
温度变化对平衡的影响取决于正向反应是放热还是吸热。如果正向反应放热（释放热量，ΔH 为负），升高温度有利于吸热的逆向反应，使平衡向左移动。
Decreasing the temperature of an exothermic equilibrium shifts the position to the right, increasing the yield of products. For an endothermic forward reaction, the opposite is true: heating favours the products, while cooling favours the reactants.
对于放热反应，降低温度使平衡向右移动，增加产物产率。如果正向反应是吸热的，情况则相反：加热有利于产物，冷却有利于反应物。
A classic example is the dimerisation of nitrogen dioxide: 2NO₂(g) ⇌ N₂O₄(g), where the forward reaction is exothermic. Brown NO₂ is converted to colourless N₂O₄. Cooling the mixture makes the colour paler (equilibrium shifts right
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