📚 Chemical Equilibrium | 化学平衡
Chemical equilibrium is a cornerstone topic in IGCSE AQA Chemistry, describing the delicate balance achieved when the rates of the forward and reverse reactions become equal in a closed system. A sound understanding of reversible reactions, dynamic equilibrium, and Le Chatelier’s principle enables you to predict how changes in concentration, temperature, and pressure affect the position of equilibrium, and to explain the compromises made in industrial processes such as the Haber process.
化学平衡是 IGCSE AQA 化学的一个基石主题，描述在封闭体系中当正、逆反应速率相等时所达到的微妙平衡。透彻理解可逆反应、动态平衡和勒夏特列原理，能让你预测浓度、温度和压力的变化如何影响平衡位置，并能解释哈伯法等工业流程中所做的折衷选择。
1. Reversible Reactions and Dynamic Equilibrium | 可逆反应与动态平衡
Many chemical reactions are reversible, meaning that the products formed can, under suitable conditions, react together to re-form the original reactants. A classic example is the thermal decomposition of ammonium chloride: NH₄Cl(s) ⇌ NH₃(g) + HCl(g).
许多化学反应是可逆的，意味着生成的产物在合适条件下可以重新反应形成原来的反应物。一个经典实例是氯化铵的热分解：NH₄Cl(s) ⇌ NH₃(g) + HCl(g)。
When a reversible reaction takes place in a closed system (where no matter enters or leaves), it eventually reaches a state known as dynamic equilibrium. At this point, the forward reaction and the backward reaction are still ongoing, but they proceed at exactly the same rate.
当一个可逆反应在封闭体系（没有物质进出）中进行时，它最终会达到一个称为动态平衡的状态。此时，正反应和逆反应仍在继续进行，但它们以完全相同的速率进行。
Because the rates are equal, the macroscopic concentrations of all reactants and products remain constant over time. It is essential to remember that equilibrium is dynamic, not static – molecules continue to react at the molecular level without any net change.
由于速率相等，所有反应物和产物的宏观浓度随时间保持不变。必须记住，平衡是动态的而非静止的——分子在微观层面持续反应，但没有净变化。
2. Characteristics of Dynamic Equilibrium | 动态平衡的特征
Dynamic equilibrium can only be achieved in a closed system. If the system is open and matter is allowed to escape, the reaction can never reach a true equilibrium state.
动态平衡只能在封闭体系中达到。如果体系开放且物质可以逸出，反应永远无法达到真正的平衡状态。
At equilibrium, the forward and reverse reaction rates are equal, meaning there is no net change in the amounts of reactants and products. However, this does not imply that the concentrations are numerically equal; the equilibrium mixture will have a specific ratio that depends on the reaction and the conditions.
在平衡时，正反应和逆反应的速率相等，意味着反应物和产物的量没有净变化。但这并不意味着两者的浓度数值相等；平衡混合物会有一个取决于反应和条件的特定比例。
Equilibrium is stable but sensitive – it can be disturbed by changing conditions such as concentration, temperature, or pressure. The system then responds to counteract the change and re-establishes a new equilibrium position, a principle we explore next.
平衡是稳定但敏感的——改变浓度、温度或压力等条件可使其发生扰动。随后体系会作出响应以抵消变化，并重新建立新的平衡位置，这就是我们接下来要探讨的原理。
3. Le Chatelier’s Principle: An Introduction | 勒夏特列原理简介
Le Chatelier’s principle states: if a system at equilibrium is subjected to a change in concentration, temperature or pressure, the equilibrium position shifts in the direction that tends to counteract the change. This powerful idea allows chemists to predict the outcome of disturbing an equilibrium.
勒夏特列原理指出：如果处于平衡态的体系受到浓度、温度或压力的变化，平衡位置会向着削弱该变化的方向移动。这一强大思想使化学家能够预测干扰平衡所导致的结果。
For instance, if we add more reactant, the equilibrium shifts to produce more product, using up the excess reactant. The principle applies qualitatively and is instrumental in optimising industrial chemical yields.
例如，如果加入更多反应物，平衡会向着生成更多产物的方向移动，以消耗多余的反应物。该原理应用于定性判断，并在优化工业化学产率方面起到关键作用。
It is also important to note that a catalyst has no effect on the position of equilibrium; it only speeds up the rate at which equilibrium is attained.
同样重要的是要注意，催化剂对平衡位置没有影响；它只加快达到平衡的速率。
4. Effect of Concentration Changes | 浓度变化的影响
If the concentration of a reactant is increased, the system shifts the equilibrium to the right (towards products) to lower the concentration of that reactant. Conversely, increasing the concentration of a product shifts the equilibrium to the left (towards reactants).
如果增加某一反应物的浓度，体系会使平衡向右（朝向产物）移动，以降低该反应物浓度。相反，增加某一产物的浓度会使平衡向左（朝向反应物）移动。
Removing a product as it is formed is a common technique to drive an equilibrium further to the right, increasing the yield. In the Haber process, ammonia is continuously liquefied and removed, pulling the equilibrium towards more ammonia production.
移除正在生成的产物是一种常用技术，可以推动平衡进一步向右移动，增加产率。在哈伯法中，氨被持续液化并移出，将平衡拉向生成更多氨的方向。
If a reactant is removed, the equilibrium shifts to the left to replenish it. These concentration effects are direct applications of Le Chatelier’s principle.
如果移除某一反应物，平衡向左移动以补充它。这些浓度效应是勒夏特列原理的直接应用。
5. Effect of Temperature Changes | 温度变化的影响
The effect of temperature on equilibrium depends on the enthalpy change (ΔH) of the forward reaction. For an exothermic reaction (ΔH negative, heat released), increasing the temperature favours the endothermic reverse reaction, shifting the equilibrium to the left. For an endothermic reaction (ΔH positive, heat absorbed), a temperature rise favours the forward reaction, shifting the equilibrium to the right.
温度对平衡的影响取决于正反应的焓变（ΔH）。对于放热反应（ΔH为负，释放热量），升高温度有利于吸热的逆反应，平衡向左移动；对于吸热反应（ΔH为正，吸收热量），升温有利于正反应，平衡向右移动。
Thus, exothermic reactions give a higher yield of products at lower temperatures, while endothermic reactions are favoured at higher temperatures. In practice, industrial processes often use a compromise temperature because a very low temperature would make the reaction too slow.
因此，放热反应在较低温度下产率更高，而吸热反应则在较高温度下更有利。实际工业过程中往往采用折衷温度，因为温度过低会使反应过慢。
It is useful to think of heat as a substance: adding heat shifts the equilibrium away from the heat source, and removing heat draws the equilibrium towards its production.
把热量看作一种物质会很有帮助：加入热量使平衡远离热源，移除热量则使平衡朝向其生成的方向移动。
6. Effect of Pressure Changes | 压力变化的影响
Pressure changes only affect equilibria involving gaseous reactants or products, and only when there is a difference in the total number of gas moles on each side of the equation. Increasing the pressure favours the side with the smaller number of gas molecules, as this reduces the overall pressure.
压力变化只影响涉及气态反应物或产物的平衡，并且仅当方程式两边气体总摩尔数不同时才起作用。增加压力有利于气体分子总数较少的一侧，因为这样可以降低总压力。
Decreasing the pressure favours the side with more moles of gas. If both sides have the same number of gas moles (e.g., H₂(g) + I₂(g) ⇌ 2HI(g)), a pressure change has no effect on the position of equilibrium.
减小压力有利于气体摩尔数较多的一侧。如果两边气体摩尔数相同（如 H₂(g) + I₂(g) ⇌ 2HI(g)），压力变化对平衡位置无影响。
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
In this reaction, there are 4 moles of gas on the left and 2 moles on the right. High pressure therefore shifts the equilibrium to the right, increasing the yield of ammonia. Extremely high pressures are expensive and dangerous, so a moderate pressure of around 200 atm is used industrially.
在该反应中，左侧有4摩尔气体，右侧有2摩尔。因此高压使平衡右移，增加氨的产率。极端高压既昂贵又危险，所以工业上采用约200 atm的中等压力。
7. Effect of a Catalyst | 催化剂的影响
A catalyst provides an alternative pathway with a lower activation energy for both the forward and reverse reactions, causing both rates to increase equally. Consequently, a catalyst does not alter the position of equilibrium; it only reduces the time needed to reach equilibrium.
催化剂为正、逆反应都提供了一条活化能较低的替代路径，使两个反应速率均等增加。因此，催化剂不改变平衡位置；它只缩短达到平衡所需的时间。
In the Haber process, finely divided iron is used as a catalyst. Without it, the reaction between nitrogen and hydrogen would be impractically slow at the operating temperature, even though the equilibrium yield would remain unchanged.
哈伯法使用细碎的铁作为催化剂。如果没有催化剂，即使在操作温度下氮气与氢气的反应也会过慢而不具备实用价值，尽管平衡产率不会改变。
Remember: catalysts are kinetic factors, not thermodynamic ones. They change how fast equilibrium is reached, not where the equilibrium lies.
记住：催化剂是动力学因素，而非热力学因素。它们改变达到平衡的快慢，而不改变平衡的位置。
8. The Haber Process: A Case Study | 哈伯法案例分析
The Haber process synthesises ammonia from nitrogen and hydrogen, and is one of the most important industrial chemical reactions. The balanced equation is exothermic:
哈伯法用氮气和氢气合成氨，是最重要的工业化学反应之一。该方程式为放热反应：
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹
Raw materials are obtained from air (nitrogen) and natural gas (hydrogen). The reaction is reversible, so the yield of ammonia is heavily influenced by the chosen temperature and pressure.
原料来自空气（氮气）和天然气（氢气）。反应是可逆的，因此氨的产率深受所选温度和压力的影响。
Typical industrial operating conditions are: a temperature of about 450 °C, a pressure of around 200 atmospheres, and an iron catalyst. These conditions represent a compromise between rate, yield, and economic costs.
典型的工业操作条件为：温度约450 °C、压力约200大气压、以及铁催化剂。这些条件代表了速率、产率和经济成本之间的折衷。
9. Applying Le Chatelier’s Principle to the Haber Process | 应用勒夏特列原理分析哈伯法
Let us rationalise the Haber process conditions using Le Chatelier’s principle. The forward reaction is exothermic, so a lower temperature would drive the equilibrium towards more ammonia. However, a very low temperature makes the rate too slow to be economical. The moderate 450 °C is a compromise that gives an acceptable rate while still yielding a reasonable amount of ammonia.
我们运用勒夏特列原理来解释哈伯法的条件。正反应放热，所以较低温度会使平衡向生成更多氨的方向移动。但温度过低会使速率过慢，在经济上不可行。适中的450 °C是一个折衷，既能提供可接受的速率，又能得到合理的氨产率。
There are fewer gas moles on the product side (4 → 2), so high pressure favours the forward reaction. Although even higher pressures would increase the yield further, they require extremely sturdy and expensive equipment. The chosen pressure of 200 atm balances yield improvement with plant costs.
产物侧气体摩尔数较少（
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