Le Chatelier’s Principle | 勒夏特列原理考点精讲

📚 Le Chatelier’s Principle | 勒夏特列原理考点精讲

Le Chatelier’s principle is a cornerstone of GCSE Chemistry that allows us to predict how a reversible reaction at equilibrium responds to changes in concentration, pressure and temperature. Mastering this principle is essential not only for exam success but also for understanding the design of important industrial processes like the Haber and Contact processes. This revision guide breaks down every key point, provides clear examples and avoids common pitfalls.

勒夏特列原理是 GCSE 化学的基石，它使我们能够预测处于平衡状态的可逆反应如何应对浓度、压强和温度的变化。掌握这一原理不仅对考试成功至关重要，而且有助于理解哈伯法和接触法等重要工业过程的设计。本复习指南将分解每个要点，提供清晰示例，并避免常见误区。

1. Dynamic Equilibrium Recap | 动态平衡复习

In a reversible reaction, reactants form products as the forward reaction proceeds, but products can also break down to re-form reactants in the backward reaction. When the rate of the forward reaction equals the rate of the backward reaction, the system has reached dynamic equilibrium. At this point, the concentrations of all reactants and products remain constant – not because the reactions have stopped, but because both forward and backward reactions continue at exactly the same speed.

在可逆反应中，反应物通过正反应生成产物，而产物也可以通过逆反应重新生成反应物。当正反应速率等于逆反应速率时，系统达到动态平衡。此时，所有反应物和产物的浓度保持恒定——这并不是因为反应已经停止，而是因为正反应和逆反应仍以完全相同的速率继续进行。

A classic example is the synthesis of ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). In a closed container, this reaction will eventually reach a state where the amounts of N₂, H₂ and NH₃ no longer change, even though both the forward and backward reactions keep occurring.

一个经典例子是合成氨：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)。在密闭容器中，该反应最终会达到 N₂、H₂ 和 NH₃ 的量不再变化的状态，尽管正逆反应仍在不断进行。

2. Statement of Le Chatelier’s Principle | 勒夏特列原理的陈述

Le Chatelier’s principle states: 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. The key word is “oppose” – the equilibrium moves in a direction that tends to reduce the effect of the disturbance, but it does not completely cancel it out. A new equilibrium is established with a different mixture of reactants and products.

勒夏特列原理指出：如果处于动态平衡的系统受到浓度、压强或温度的变化，平衡位置将发生移动，以削弱这种改变。关键词是“削弱”——平衡会向着减小干扰影响的方向移动，但不会完全抵消干扰。系统会建立一个新的平衡，其中反应物和产物的组成与原来不同。

For example, if you add more of a reactant to an equilibrium mixture, the equilibrium shifts towards the products (to the right) to use up the added substance. This does not return the system to the original reactant concentration; instead, the concentrations of all species adjust until a new equilibrium composition is reached.

例如，如果你向平衡混合物中添加更多的某种反应物，平衡将向生成物方向移动（向右移动），以消耗掉加入的物质。这并不会使系统回到原来的反应物浓度；相反，所有物种的浓度将重新调整，直至达到新的平衡组成。

3. Effect of Concentration Changes | 浓度变化的影响

Changing the concentration of a reactant or product disturbs the equilibrium. According to Le Chatelier’s principle, the equilibrium shifts to counteract the change. If the concentration of a reactant is increased, the system shifts towards products (to the right) to reduce the concentration of that reactant. If the concentration of a product is increased, the equilibrium shifts towards reactants (to the left) to remove some of the added product. Conversely, removing a substance causes the equilibrium to shift to replace it.

改变反应物或产物的浓度会破坏平衡。根据勒夏特列原理，平衡会发生移动以对抗这种改变。如果增加反应物的浓度，系统会向产物方向移动（向右移动），以降低该反应物的浓度。如果增加产物的浓度，平衡会向反应物方向移动（向左移动），以去除部分添加的产物。反之，移除某种物质会使平衡向补充该物质的方向移动。

The table below summarises the effects of concentration changes at equilibrium.

下面的表格总结了浓度变化对平衡的影响。

In exam questions, you may be asked to predict the colour change of a solution. For instance, adding more Fe³⁺ ions to the equilibrium Fe³⁺ + SCN⁻ ⇌ FeSCN²

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