Le Chatelier's Principle – WJEC IGCSE Chemistry Exam Focus | IGCSE WJEC 化学：勒夏特列原理考点精讲

📚 Le Chatelier’s Principle – WJEC IGCSE Chemistry Exam Focus | IGCSE WJEC 化学：勒夏特列原理考点精讲

Le Chatelier’s Principle is a cornerstone of equilibrium chemistry. For WJEC IGCSE candidates, understanding how a system at equilibrium responds to changes in concentration, pressure and temperature is essential for both multiple-choice questions and structured long-answer problems. This article distils the principle into clear rules, practical industrial examples (Haber and Contact processes) and common exam traps, equipping you with the knowledge to explain shifts in equilibrium position confidently and accurately.

勒夏特列原理是化学平衡的基石。对于 WJEC IGCSE 考生而言，理解平衡体系如何应对浓度、压强和温度的变化，是选择题与结构化长答题的关键。本文将原理提炼为清晰的规则，结合实际工业案例（哈伯法和接触法）与常见考试陷阱，帮助你自信且准确地解释平衡位置的移动，掌握得分要领。

1. What is Le Chatelier’s Principle? | 什么是勒夏特列原理？

Le Chatelier’s Principle states that if a system at dynamic equilibrium experiences a change in concentration, pressure or temperature, the position of equilibrium shifts to oppose the change and a new equilibrium is established.

勒夏特列原理指出：如果一个处于动态平衡的体系受到浓度、压强或温度变化的干扰，平衡位置将向着减弱这种改变的方向移动，并建立起新的平衡。

It is important to remember that the principle only applies to closed systems in dynamic equilibrium. The rates of the forward and reverse reactions become equal again once the shift is complete, but the concentrations of reactants and products will have changed permanently.

必须牢记，该原理仅适用于处于动态平衡的封闭体系。一旦移动完成，正逆反应速率会再次相等，但反应物和产物的浓度将发生永久性改变。

2. 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 products) to use up the extra substance added, thereby reducing the imposed change.

如果增大一种反应物的浓度，平衡会向右移动（向产物方向），以消耗掉额外加入的物质，从而减弱这种改变。

Conversely, if a product is removed from the system, the equilibrium shifts to the right to replace the product that has been taken away. Removing a reactant shifts the position to the left.

反之，如果从体系中移除一种产物，平衡会向右移动以补充被移走的产物。移除反应物则会导致平衡向左移动。

You can think of the equilibrium system as ‘trying’ to maintain a steady composition. Adding a substance to one side forces a shift away from that side; taking a substance away pulls the equilibrium towards that side.

可以这样理解：平衡体系会“试图”维持稳定的组成。在一侧加入物质，就会把平衡推向远离该侧的方向；从一侧取走物质，平衡就会被拉向该侧。

3. Effect of Pressure Changes in Gaseous Systems | 气体体系中压强变化的影响

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.

压强变化只影响有气体参与的平衡，且只有当反应前后气体分子总数发生变化时才会产生影响。

If the pressure is increased, the equilibrium shifts towards the side with fewer moles of gas molecules to lower the pressure. If the pressure is decreased, the position shifts towards the side with more moles of gas to raise the pressure again.

若增大压强，平衡会向着气体分子总物质的量较小的一方移动，以降低压强。若减小压强，平衡则向着气体分子总数较多的一方移动，以求升高压强。

For example, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) has 3 moles of gas on the left and 2 moles on the right. High pressure favours the forward reaction and increases the yield of SO₃.

例如，2SO₂(g) + O₂(g) ⇌ 2SO₃(g) 反应中，左边有 3 mol 气体，右边有 2 mol 气体。高压有利于正反应，提高 SO₃ 的产率。

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

Temperature is the only condition that changes the value of the equilibrium constant, Kc. The direction of shift depends on whether the forward reaction is exothermic (ΔH negative) or endothermic (ΔH positive).

温度是唯一能改变平衡常数 Kc 值的条件。移动方向取决于正反应是放热（ΔH 为负）还是吸热（ΔH 为正）。

If the temperature is increased, the equilibrium shifts in the endothermic direction to absorb the extra heat. If the temperature is decreased, the equilibrium shifts in the exothermic direction to release heat.

若升高温度，平衡向吸热方向移动，以吸收多余的热量。若降低温度，平衡向放热方向移动，以释放热量。

It is a very common WJEC question to ask you to predict the yield of a product at different temperatures, given the sign of ΔH. For an exothermic forward reaction, lower temperatures give a higher equilibrium yield, but in practice a compromise temperature is often used to maintain a viable reaction rate.

WJEC 考试中非常常见的一类问题是：给定 ΔH 的符号，要求你预测不同温度下的产物产率。对于正反应放热的反应，低温能获得更高的平衡产率，但实际操作中往往选择折中的温度，以保持可接受的反应速率。

5. The Role of a Catalyst | 催化剂的作用

Adding a catalyst has absolutely no effect on the position of equilibrium. It speeds up both the forward and reverse reactions equally, so the equilibrium composition remains unchanged.

加入催化剂对平衡位置完全没有影响。它同等程度地加快正反应和逆反应的速率，因此平衡组成保持不变。

The only benefit of a catalyst in a reversible reaction is to allow the system to reach equilibrium faster. In industry, this means a lower temperature can be used without sacrificing too much rate, which can be important for exothermic processes where low temperature favours high yield.

在可逆反应中，催化剂的唯一好处是让体系更快地达到平衡。在工业上，这意味着可以在不牺牲太多速率的情况下使用较低的温度，这对于低温有利于高产率的放热过程尤为重要。

A classic WJEC exam trap is to say ‘a catalyst increases the yield of product’. This is incorrect. Always state clearly: a catalyst does not change the equilibrium position or the yield.

WJEC 考试中的经典陷阱就是说“催化剂提高产物产率”。这是错误的。一定要明确说明：催化剂不改变平衡位置，也不改变产率。

6. The Haber Process – Applying the Principle | 哈伯法——原理的应用

The Haber process synthesises ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = –92 kJ mol⁻¹. The forward reaction is exothermic and produces fewer moles of gas (4 moles → 2 moles).

哈伯法合成氨的反应为：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，ΔH = –92 kJ mol⁻¹。正反应放热且气体分子数减少（4 mol → 2 mol）。

According to Le Chatelier’s Principle, high pressure shifts the equilibrium to the right, increasing the yield of NH₃. A typical operating pressure is around 200 atm.

根据勒夏特列原理，高压会使平衡向右移动，提高 NH₃ 的产率。典型的操作压强约为 200 atm。

Low temperature also shifts the equilibrium to the right because the forward reaction is exothermic. However, a temperature of about 450 °C is used instead of room temperature to achieve a reasonable rate of reaction, even though this reduces the equilibrium yield slightly.

低温同样使平衡向右移动，因为正反应放热。然而，为了获得合理的反应速率，实际采用的是约 450 °C，尽管这会使平衡产率略有下降。

An iron catalyst is used to speed up the attainment of equilibrium. The unreacted N₂ and H₂ are recycled to improve overall efficiency.

使用铁催化剂来加速达到平衡。未反应的 N₂ 和 H₂ 会被循环使用，以提高总效率。

7. The Contact Process – Applying the Principle | 接触法——原理的应用

The Contact process manufactures sulfuric acid via the exothermic equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = –197 kJ mol⁻¹.

接触法通过放热反应生产硫酸：2SO₂(g) + O₂(g) ⇌ 2SO₃(g)，ΔH = –197 kJ mol⁻¹。

A high pressure (typically close to atmospheric but slightly raised) favours the forward reaction because there are 3 moles of gas on the left and 2 moles on the right. However, the equilibrium constant is already large enough to give a high yield without using very high pressures, making the process economically safer.

高压（通常接近常压但略高）有利于正反应，因为左边有 3 mol 气体，右边有 2 mol 气体。但由于平衡常数已经足够大，无需使用极高压力就能获得高产率，这使得该过程在经济上更安全。

A vanadium(V) oxide catalyst, V₂O₅, is used and the temperature is kept around 450 °C. As with the Haber process, a compromise temperature ensures a good rate without shifting the equilibrium unduly to the left.

使用五氧化二钒 V₂O₅ 作为催化剂，温度保持在 450 °C 左右。与哈伯法类似，折中的温度确保了良好的速率，同时不会过分使平衡向左移动。

8. Common Misconceptions and WJEC Pitfalls | 常见误区与 WJEC 易错点

One common error is to state that ‘the equilibrium shifts to the right to increase the rate of the forward reaction’. The rate changes are a consequence, not the cause. Always frame your answer in terms of opposing the imposed change.

一个常见错误是说“平衡向右移动是为了增加正反应速率”。速率的改变是移动的结果而非原因。答题时务必从“对抗外界改变”的角度来阐述。

Another mistake is to suggest that adding an inert gas at constant volume changes the equilibrium position. An inert gas like argon increases total pressure but does not change the partial pressures of reacting gases, so equilibrium does not shift.

另一个错误是认为在体积不变的条件下加入惰性气体会改变平衡位置。像氩这样的惰性气体增加了总压，但并未改变反应气体的分压，因此平衡不发生移动。

Students often forget that pressure only affects an equilibrium if the number of gas molecules is different on the two sides. When both sides have the same number of moles of gas, e.g. H₂(g) + I₂(g) ⇌ 2HI(g), pressure changes have no effect.

学生常常忘记：压强只对气体分子数在两侧不同的平衡产生影响。若两侧气体分子总数相同，例如 H₂(g) + I₂(g) ⇌ 2HI(g)，压强变化便没有影响。

9. WJEC Exam-Style Language and Command Words | WJEC 考试常用术语与指令词

When a question asks ‘explain, in terms of Le Chatelier’s Principle’, you must explicitly name the principle and then state the change, the shift, and the reason linked to opposing that change.

当题目要求“从勒夏特列原理的角度解释”时，你必须明确点出原理的名称，然后依次陈述施加的改变、平衡移动的方向，以及这是由于对抗该改变而发生的。

Use precise phrases such as ‘the position of equilibrium shifts to the right/left’ rather than ‘the equilibrium moves to the products/reactants’. WJEC mark schemes reward clear, scientific terminology.

使用准确的表述，例如“平衡位置向右/左移动”，而不是“平衡移向产物/反应物”。WJEC 评分标准对清晰、科学的术语会很加分。

In ‘suggest and explain’ questions on optimum conditions, always weigh the equilibrium yield argument against the rate argument. A condition that gives a slightly lower yield might still be chosen for a much faster rate, so long as unreacted materials are recycled.

在关于最佳条件的“建议并解释”类题目中，始终要权衡平衡产率与速率两方面的论点。如果能够循环使用未反应物料，那么某种条件即使略微降低产率，只要能大幅提高速率，也可能被选择。

10. Worked Example – Applying Le Chatelier to a New Reaction | 例题精讲——将勒夏特列原理应用于新反应

Question: Consider the equilibrium: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g), ΔH = +124 kJ mol⁻¹. Predict and explain the effect on the yield of PCl₅ of (a) increasing temperature, (b) increasing pressure, (c) adding more Cl₂.

题目：考虑平衡：PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)，ΔH = +124 kJ mol⁻¹。预测并解释以下操作对 PCl₅ 产率的影响：(a) 升高温度，(b) 增大压强，(c) 加入更多 Cl₂。

(a) The forward reaction is endothermic. Increasing temperature favours the endothermic direction, so the equilibrium shifts to the right. This decreases the amount of PCl₅, lowering its yield.

(a) 正反应吸热。升高温度有利于吸热方向，因此平衡向右移动。这减少了 PCl₅ 的量，降低了其产率。

(b) The forward reaction produces 2 moles of gas from 1 mole. Increasing pressure favours the side with fewer gas molecules, so the equilibrium shifts left, increasing the yield of PCl₅.

(b) 正反应由 1 mol 气体生成 2 mol 气体。增大压强有利于气体分子数较少的一侧，因此平衡向左移动，提高了 PCl₅ 的产率。

(c) Adding Cl₂ increases the concentration of a product. The system shifts left to oppose this increase, so more PCl₅ is formed and its yield rises.

This structured answer directly addresses the ‘predict and explain’ demand typical of WJEC 6-mark questions, combining the direction of shift with yield consequence and the principle’s reasoning.

这种结构化的答案直接回应了 WJEC 6 分题中常见的“预测并解释”要求，将移动方向、产率后果和原理推理有机结合起来。

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Le Chatelier’s Principle – WJEC IGCSE Chemistry Exam Focus | IGCSE WJEC 化学：勒夏特列原理 考点精讲