GCSE AQA Chemistry: Chemical Equilibrium | GCSE AQA 化学：化学平衡考点精讲

📚 GCSE AQA Chemistry: Chemical Equilibrium | GCSE AQA 化学：化学平衡考点精讲

Chemical equilibrium is a core topic in AQA GCSE Chemistry, focusing on reversible reactions and how systems respond to changes in conditions. Understanding equilibrium helps explain industrial processes like the Haber process and many natural phenomena.

化学平衡是AQA GCSE化学的核心主题，重点在于可逆反应以及系统如何应对条件的变化。理解平衡有助于解释工业过程（如哈伯法）和许多自然现象。

1. Reversible Reactions | 可逆反应

A reversible reaction is one in which the products can react together to re-form the original reactants. This is shown using the symbol ⇌ in equations. In a closed system, both forward and backward reactions occur.

可逆反应是指生成物可以重新反应生成原来的反应物的反应。在方程式中用符号 ⇌ 表示。在封闭系统中，正向反应和逆向反应都会发生。

For example, the thermal decomposition of ammonium chloride is reversible: NH₄Cl(s) ⇌ NH₃(g) + HCl(g). Upon cooling, the ammonia and hydrogen chloride 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)。冷却时，氨和氯化氢重新结合生成固态氯化铵。

Another common example is the hydration and dehydration of copper(II) sulfate: CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l). Heating the hydrated crystals drives the forward reaction, producing white anhydrous copper sulfate and water.

另一个常见例子是硫酸铜的水合与脱水：CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)。加热含水晶体促使正向反应，生成白色无水硫酸铜和水。

2. Dynamic Equilibrium | 动态平衡

Dynamic equilibrium is reached when the forward and reverse reactions occur at exactly the same rate in a closed system. The concentrations of reactants and products remain constant, but reactions are still happening at the molecular level.

动态平衡是在封闭系统中，当正向反应和逆向反应速率完全相等时达到的状态。反应物与生成物的浓度保持不变，但分子层面的反应仍在进行。

It is important to recognise that equilibrium is dynamic, not static. Both reactions continue, but there is no net change in the amounts of substances. This can only happen in a closed system so that no matter escapes.

重要的是要认识到平衡是动态的，而非静止的。两个方向的反应都在继续，但物质的总量没有净变化。这只有在封闭系统中才能实现，以免物质逸出。

3. Le Chatelier’s Principle | 勒夏特列原理

Le Chatelier’s principle states: If a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts to oppose the change. This helps predict how equilibrium responds to concentration, temperature, or pressure changes.

勒夏特列原理表述为：如果平衡体系受到条件变化的影响，平衡位置会向减弱这种变化的方向移动。这有助于预测平衡如何响应浓度、温度或压力的变化。

The principle allows us to work out whether the forward or reverse reaction is favoured after a disturbance. It does not tell us how fast the new equilibrium is reached, only the direction of the shift.

该原理使我们能判断平衡受到干扰后，正向反应还是逆向反应会占优。它并不告诉我们达到新平衡的快慢，只给出移动的方向。

4. 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 use up the added substance and produce more product. Conversely, increasing the concentration of a product shifts equilibrium to the left.

如果增加反应物的浓度，平衡向右（正向）移动，以消耗加入的物质并生成更多产物。反之，增加生成物的浓度会使平衡向左移动。

A classic demonstration uses the equilibrium between iron(III) ions and thiocyanate ions: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq). The FeSCN²⁺ complex is blood-red. Adding more Fe³⁺ or SCN⁻ deepens the colour as equilibrium shifts right. Adding a product-removing reagent reduces the colour.

一个经典演示实验利用铁(III)离子与硫氰酸根离子之间的平衡：Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)。FeSCN²⁺络合物呈血红色。加入更多 Fe³⁺ 或 SCN⁻ 会使颜色加深，因为平衡向右移动。加入某种去除产物的试剂可使颜色变浅。

5. Effect of Temperature Changes | 温度变化的影响

Temperature changes shift equilibrium depending on whether the forward reaction is exothermic or endothermic. An increase in temperature favours the endothermic direction to absorb the extra heat energy.

温度变化改变平衡，取决于正向反应是放热还是吸热。升高温度有利于吸热方向，以吸收额外的热量。

For example, the formation of ammonia is exothermic: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹. Increasing the temperature shifts equilibrium to the left (endothermic direction), decreasing the yield of ammonia.

例如，氨的生成是放热的：N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹。升高温度使平衡向左（吸热方向）移动，降低氨的产率。

Conversely, if the forward reaction is endothermic, e.g. N₂(g) + O₂(g) ⇌ 2NO(g) ΔH = +180 kJ mol⁻¹, raising the temperature shifts equilibrium to the right, producing more NO.

相反，如果正向反应是吸热的，如 N₂(g) + O₂(g) ⇌ 2NO(g) ΔH = +180 kJ mol⁻¹，升高温度会使平衡向右移动，生成更多 NO。

6. Effect of Pressure Changes | 压力变化的影响

Pressure changes only affect equilibria involving gases. An increase in pressure favours the side of the reaction with fewer gas molecules (moles of gas), as this helps reduce the pressure.

压力变化只影响涉及气体的平衡。增加压力有利于气体分子数较少的那一侧，因为这有助于降低压力。

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 shifts equilibrium to the right, increasing ammonia yield. Conversely, reducing pressure shifts it left.

在哈伯法中，N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，左边有 4 mol 气体，右边有 2 mol。高压使平衡向右移动，提高氨的产率。反之，降低压力平衡向左移动。

If both sides have the same number of gas molecules, e.g. H₂(g) + I₂(g) ⇌ 2HI(g), changing pressure has no effect on the position of equilibrium.

如果两侧气体分子数相等，如 H₂(g) + I₂(g) ⇌ 2HI(g)，改变压力对平衡位置没有影响。

7. Effect of a Catalyst on Equilibrium | 催化剂对平衡的影响

Adding a catalyst speeds up both the forward and reverse reactions equally. It lowers the activation energy for both directions, so equilibrium is reached faster, but the position of equilibrium remains unchanged.

加入催化剂同等程度地加快正向和逆向反应。它降低了两个方向的活化能，因此平衡能更快达到，但平衡位置保持不变。

This means a catalyst does not increase the yield of a reversible reaction; it simply allows the system to reach the same equilibrium composition more quickly. In industry, catalysts are essential for economic production rates.

这意味着催化剂不会提高可逆反应的产率；它只是让系统更快地达到相同的平衡组成。在工业上，催化剂对实现经济的生产速率至关重要。

8. The Haber Process: A Case Study | 哈伯法：案例分析

The Haber process is used to manufacture ammonia from nitrogen and hydrogen: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). Since the forward reaction is exothermic and results in fewer gas molecules, the yield of ammonia is favoured by low temperature and high pressure.

哈伯法用于从氮气和氢气生产氨：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)。由于正向反应是放热的，且气体分子数减少，低温高压有利于氨的产率。

However, operating at very low temperatures makes the reaction too slow, even with a catalyst. Very high pressures are expensive and require stronger, costlier equipment. Therefore, a compromise is made: typical conditions are about 450 °C, 200 atmospheres, and an iron catalyst.

然而，在极低温度下操作会使反应过慢，即使有催化剂也不行。非常高的压力成本高昂，需要更坚固、更昂贵的设备。因此需要采取折衷方案：典型条件约为 450 °C、200 个大气压，并使用铁催化剂。

The reaction mixture leaving the reactor contains only about 15–20% ammonia. The unreacted N₂ and H₂ are recycled back into the reactor, improving overall efficiency.

离开反应器的混合物中只含约15–20%的氨。未反应的 N₂ 和 H₂ 会被循环送回反应器，从而提高整体效率。

9. Choosing Compromise Conditions in Industry | 工业上妥协条件的选择

Industrial processes must balance equilibrium yield, reaction rate, and economic factors. The Haber process is a perfect example: a low temperature gives a higher equilibrium yield but a slow rate; a high pressure gives a higher yield but increases plant costs.

工业过程必须在平衡产率、反应速率和经济因素之间取得平衡。哈伯法就是一个完美的例子：低温可获得更高的平衡产率，但速率慢；高压可提高产率，但增加工厂成本。

Similarly, in the Contact process for sulfuric acid: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −197 kJ mol⁻¹. A temperature of around 450 °C is used with a vanadium(V) oxide catalyst, and a pressure only slightly above atmospheric (1–2 atm) because the equilibrium already lies far to the right.

类似地，在硫酸的接触法生产中：2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −197 kJ mol⁻¹。使用约 450 °C 的温度和五氧化二钒催化剂，压力仅略高于常压（1–2 atm），因为平衡已大大偏向右侧。

These compromises ensure that the process is both chemically efficient and economically viable.

这些折衷方案确保了过程既具有化学效率，又具有经济可行性。

10. Key Summary and Common Pitfalls | 要点总结与常见误区

Equilibrium is dynamic, not static – forward and reverse reactions continue at equal rates. Adding a catalyst does not alter the equilibrium position; it only speeds up the attainment of equilibrium. Le Chatelier’s principle describes the shift that opposes the change, but the change is not completely cancelled.

平衡是动态的，而非静止的——正逆反应以相等速率持续进行。加入催化剂不改变平衡位置，只加快达到平衡的速度。勒夏特列原理描述了向减弱变化方向的移动，但变化并未被完全抵消。

Always check whether the forward reaction is exothermic or endothermic before predicting the effect of temperature. For pressure changes, count only the moles of gas on each side; if equal, pressure has no effect on position. Concentration changes always shift equilibrium to consume the added substance.

在预测温度影响前，务必先确认正向反应是放热还是吸热。对于压力变化，只计算每侧气体的摩尔数；若相等，则压力对位置无影响。浓度变化总是使平衡向消耗加入物质的方向移动。

Common exam questions ask you to predict the direction of shift, explain colour changes, or suggest why industrial conditions are chosen. Support each answer with the appropriate principle or reasoning, mentioning both equilibrium and rate considerations.

常见的考题会要求你预测移动方向、解释颜色变化，或说明为何选择特定的工业条件。回答时要使用适当的原理并解释理由，同时提及平衡和速率两方面的考虑。

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GCSE AQA Chemistry: Chemical Equilibrium | GCSE AQA 化学：化学平衡 考点精讲