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 Edexcel Chemistry, helping us predict how reversible reactions respond to changes in conditions. This revision guide breaks down the key ideas, from concentration and pressure to temperature, using clear explanations and real-world examples like the Haber and Contact processes. Whether you are preparing for a written paper or tackling application questions, understanding this principle will give you a reliable strategy for explaining equilibrium shifts.

勒夏特列原理是 GCSE Edexcel 化学的核心内容，帮助我们预测可逆反应如何应对条件变化。本篇精讲将逐层揭示浓度、压力、温度等因素的影响，并辅以哈伯法、接触法等真实工业案例。无论你是在准备笔试还是应对应用题，掌握这一原理都能为你提供解释平衡移动的可靠策略。

1. Introduction to Reversible Reactions and Dynamic Equilibrium | 可逆反应与动态平衡简介

Many chemical reactions are reversible. In a closed system, the forward and reverse reactions can reach a state of dynamic equilibrium. At equilibrium, the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant – not because the reactions have stopped, but because they continue at the same speed.

许多化学反应是可逆的。在封闭系统中，正反应和逆反应可以达到动态平衡状态。在平衡时，正逆反应速率相等，反应物和产物的浓度保持不变——这并不是因为反应停止了，而是因为它们以相同的速率继续进行。

A common GCSE example is the hydration of copper(II) sulfate: CuSO₄·5H₂O (s) ⇌ CuSO₄ (s) + 5H₂O (g). In a sealed container, steam can rehydrate the white powder even as heating drives off water. Dynamic equilibrium only applies to closed systems where no matter escapes.

GCSE 常见的可逆反应例子是硫酸铜水合与脱水：CuSO₄·5H₂O (s) ⇌ CuSO₄ (s) + 5H₂O (g)。在密闭容器中，加热时水蒸气逸出，白色粉末也可以重新吸水变蓝。动态平衡仅适用于物质无法逸散的封闭系统。

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

Le Chatelier’s Principle states: if a system at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will shift to oppose that change. In other words, the system tries to ‘undo’ the disturbance.

勒夏特列原理指出：如果处于平衡状态的系统受到浓度、压力或温度的改变，平衡位置会发生移动，以对抗这种改变。换句话说，系统会尝试“抵消”扰动的影响。

This principle does not explain why the shift occurs at a molecular level; it simply summarises what will happen. It is a prediction tool. You must always relate the direction of shift to either the forward or reverse reaction’s endothermic or exothermic nature, or the side with more or fewer gas moles.

这一原理并不是在分子层面解释平衡移动的原因，而是一个现象总结。它是一种预测工具。在应用时，你始终需要结合正反应或逆反应是吸热还是放热、哪一侧气体分子数更多来分析移动方向。

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

If the concentration of a reactant is increased, the equilibrium shifts to the product side to use up the added reactant. Conversely, if a product is removed, the equilibrium shifts to replace it, favouring the forward reaction. Adding a catalyst or an inert solid (that is not part of the equilibrium) does not affect the position of equilibrium.

若增加反应物的浓度，平衡会向产物方向移动，以消耗掉加入的反应物。反之，若移除产物，平衡会向生成该产物的方向移动，有利于正反应。加入催化剂或不参与平衡的惰性固体不会影响平衡位置。

For the equilibrium: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) (pale yellow + colourless ⇌ blood-red), adding more Fe³⁺ ions intensifies the red colour as equilibrium shifts right. Adding a few drops of NaF solution removes Fe³⁺ by forming colourless complex [FeF₆]³⁻, shifting equilibrium left and reducing red colour.

以平衡 Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)（浅黄 + 无色 ⇌ 血红色）为例，增加 Fe³⁺ 离子会使平衡右移，红色加深。加入少量 NaF 溶液会因形成无色配离子 [FeF₆]³⁻ 而降低 Fe³⁺浓度，平衡左移，红色变浅。

4. Effect of Pressure Changes (for Gases) | 压力变化的影响（针对气体）

Changing pressure only affects equilibria involving gases where the total number of gaseous moles on each side is different. If pressure is increased, the equilibrium shifts to the side with fewer gas molecules to reduce the pressure. If pressure is decreased, it shifts to the side with more gas molecules.

改变压力只影响那些两侧气体分子总数不同的气态平衡。增大压力，平衡向气体分子数较少的一侧移动，以降低压力。减小压力，平衡则向气体分子数较多的一侧移动。

For the reaction: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), there are 3 moles on the left and 2 moles on the right. Increasing pressure favours the forward reaction, yielding more SO₃. For H₂(g) + I₂(g) ⇌ 2HI(g), there are 2 moles on each side, so pressure changes have no effect on the position of equilibrium.

反应 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) 中，左侧 3 mol 气体，右侧 2 mol。增大压力有利于正向反应，生成更多 SO₃。而对于 H₂(g) + I₂(g) ⇌ 2HI(g)，两侧均为 2 mol 气体，压力变化不影响平衡位置。

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

Temperature changes affect equilibrium depending on whether the forward reaction is exothermic or endothermic. If the temperature is increased, the equilibrium shifts in the endothermic direction to absorb the extra heat. If the temperature is decreased, it shifts in the exothermic direction to release heat.

温度对平衡的影响取决于正反应是放热还是吸热。升高温度，平衡向吸热方向移动，以吸收多余的热量。降低温度，平衡向放热方向移动，以释放热量。

For the reversible reaction: N₂O₄(g) ⇌ 2NO₂(g) (ΔH = +58 kJ mol⁻¹). The forward reaction is endothermic. Heating the mixture turns the colour darker brown as equilibrium shifts right, producing more brown NO₂. Cooling favours the reverse exothermic reaction, shifting left and forming colourless N₂O₄, so the colour pales.

对于可逆反应 N₂O₄(g) ⇌ 2NO₂(g)（ΔH = +58 kJ mol⁻¹），正向吸热。加热混合物，颜色变为深棕色，因为平衡右移生成更多棕色的 NO₂。降温则有利于逆向放热反应，平衡左移生成无色的 N₂O₄，颜色变浅。

Remember: only temperature changes alter the value of the equilibrium constant Kc. Concentration and pressure changes shift the position but not the constant.

注意：只有温度变化会改变平衡常数 Kc 的值。浓度和压力的变化只移动平衡位置，不改变平衡常数。

6. Effect of Catalysts on Equilibrium | 催化剂对平衡的影响

Adding a catalyst speeds up both the forward and reverse reactions equally. Therefore, a catalyst does not change the position of equilibrium. It simply allows the system to reach equilibrium faster. This is crucial in industrial processes where time and energy are expensive.

加入催化剂会同等程度地加快正逆反应速率。因此，催化剂不会改变平衡位置，只是让系统更快地达到平衡。这在时间和能源成本高昂的工业流程中至关重要。

In the Haber process, an iron catalyst is used to accelerate the formation of ammonia. It does not increase the yield of ammonia at equilibrium, but it makes the process economically viable at moderate temperatures. Without a catalyst, the reaction would be too slow to achieve a workable rate.

在哈伯法中，使用铁催化剂加速氨的合成。它不会提高平衡时氨的产率，但使得过程在中等温度下具有经济可行性。若无催化剂，反应速率太慢，难以实现工业化生产。

7. Applying Le Chatelier’s Principle to the Haber Process | 勒夏特列原理在哈伯法中的应用

The Haber process manufactures ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (ΔH = −92 kJ mol⁻¹). The forward reaction is exothermic and produces fewer gas molecules (4 mol → 2 mol). According to Le Chatelier’s Principle, high pressure and low temperature would maximise yield.

哈伯法用于制造氨：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)（ΔH = −92 kJ mol⁻¹）。正向反应放热且气体分子数减少（4 mol → 2 mol）。根据勒夏特列原理，高压和低温将最大化产率。

High pressure (typically 200 atm) shifts equilibrium to the right because the product side has fewer moles. Low temperature shifts equilibrium right because the forward reaction is exothermic; however, if the temperature is too low, the rate becomes too slow. A compromise temperature of about 450 °C is used together with an iron catalyst to achieve a reasonable rate with acceptable yield.

高压（通常 200 atm）使平衡向右移动，因为产物侧分子数更少。低温也使平衡右移，因为正反应放热；然而温度太低则反应速率过慢。实际采用约 450 °C 的折中温度，并借助铁催化剂，以获得可接受的产率和合理速率。

8. Compromise Conditions in Industry | 工业中的折中条件

Real industrial processes rarely use theoretically perfect equilibrium conditions. A compromise must be struck between yield, rate, safety, and cost. In the Haber process, a pressure much higher than 200 atm would give a better equilibrium yield, but the construction and energy costs for stronger vessels would outweigh the gain.

真实工业流程极少使用理论上完美的平衡条件。产率、速率、安全与成本之间必须取得折中。在哈伯法中，远高于 200 atm 的压力会带来更好的平衡产率，但建造高压反应容器的成本和能耗会超过收益。

Condition / 条件	Effect on yield / 对产率影响	Industrial choice / 工业选择
Pressure / 压力	Higher gives more NH₃ / 更高产率	200 atm (compromise)
Temperature / 温度	Lower gives more NH₃ / 更低温度高产率	450 °C (compromise)
Catalyst / 催化剂	No effect on position / 不影响位置	Iron catalyst / 铁催化剂

These compromise conditions are assessed in Edexcel exam questions requiring evaluation of yield versus rate and cost.

这些折中条件正是 Edexcel 考试中要求评价产率与速率、成本之间关系的重要考点。

9. Other Industrial Examples: Contact Process | 其他工业实例：接触法制硫酸

The Contact process for sulfuric acid production involves the equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) (ΔH = −197 kJ mol⁻¹). Again, the forward reaction is exothermic and reduces the number of gas particles from 3 to 2.

接触法制硫酸涉及平衡：2SO₂(g) + O₂(g) ⇌ 2SO₃(g)（ΔH = −197 kJ mol⁻¹）。正向反应放热且气体粒子数从 3 降至 2。

Using Le Chatelier’s Principle, high pressure would shift equilibrium right to produce more SO₃. However, high pressure is not used because the reaction already proceeds well at 1–2 atm with a vanadium(V) oxide catalyst at about 450 °C. Only a slight pressure above atmospheric is needed, which keeps costs low.

根据勒夏特列原理，高压会使平衡右移生成更多 SO₃。但实际并未采用高压，因为在 1–2 atm 和约 450 °C 下，借助五氧化二钒催化剂，反应已经进行得相当充分。只需略高于常压即可，这降低了成本。

The Contact process demonstrates that not all equilibria are optimised for maximum yield; economic factors often dominate.

接触法表明并非所有平衡都追求最大产率；经济因素往往占主导地位。

10. Predicting Direction of Shift – Worked Examples | 预测平衡移动方向 – 例题解析

Let’s work through a typical exam-style question: Consider the system PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) (ΔH = +93 kJ mol⁻¹). Predict the effect of (a) increasing temperature; (b) adding chlorine gas; (c) decreasing pressure.

我们来看一道典型考题：考虑体系 PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)（ΔH = +93 kJ mol⁻¹）。预测 (a) 升高温度；(b) 添加氯气；(c) 降低压力的影响。

(a) Forward reaction is endothermic. Increasing temperature shifts equilibrium to the right, absorbing extra heat. The mixture contains more PCl₃ and Cl₂. (b) Adding Cl₂ increases product concentration; equilibrium shifts left to use up some Cl₂, forming more PCl₅. (c) Reactant side has 1 mol, product side has 2 mol of gas. Decreasing pressure shifts equilibrium to the side with more gas moles – to the right, increasing PCl₃ and Cl₂ yields.

(a) 正反应吸热。升高温度，平衡右移以吸收额外热量，混合物中 PCl₃ 和 Cl₂ 增加。(b) 添加 Cl₂ 增加了产物浓度，平衡左移消耗部分 Cl₂，生成更多 PCl₅。(c) 反应物侧 1 mol 气体，产物侧 2 mol。降低压力平衡向气体分子数多的一侧移动——右移，PCl₃ 和 Cl₂ 产率增加。

11. Common Misconceptions and Exam Tips | 常见误解与应试技巧

Misconception: Catalysts shift the equilibrium. Catalysts only speed up attainment of equilibrium; they do not change the position. Edexcel examiners frequently test this.
误解：催化剂会移动平衡。 催化剂只加快达到平衡的速度，不改变平衡位置。Edexcel 考官常考这一点。
Misconception: Adding an inert gas at constant volume shifts equilibrium. Adding an inert gas at constant volume does not change the partial pressures of reacting gases, so the equilibrium position stays the same.
误解：恒容下加入惰性气体会使平衡移动。 恒容加入惰性气体不会改变反应气体的分压，因此平衡位置不变。
Misconception: A change in pressure always affects equilibrium. If there are equal numbers of gaseous moles on both sides, pressure has no effect on the position of equilibrium.
误解：压力变化总影响平衡。 如果两侧气体摩尔数相等，压力对平衡位置没有影响。
Exam tip: Always state that equilibrium shifts to ‘oppose’ the change, and link your answer to the exothermic or endothermic direction, or to the side with fewer/more gas moles.
应试技巧： 作答时必须说明平衡移动是为了“对抗”变化，并联系放热/吸热方向或气体分子数多/少的一侧。

12. Summary and Key Takeaways | 总结与要点回顾

Le Chatelier’s Principle: equilibrium shifts to oppose changes in concentration, pressure, or temperature.
勒夏特列原理：平衡会移动以对抗浓度、压力或温度的改变。
Concentration: increase reactant → shift to products; remove product → shift to more products.
浓度：增大反应物 → 向产物移动；移除产物 → 向更多产物移动。
Pressure: increase → shift to fewer gas moles; decrease → shift to more gas moles.
压力：增大 → 向气体摩尔数少的方向移动；减小 → 向气体摩尔数多的方向移动。
Temperature: increase → endothermic direction; decrease → exothermic direction.
温度：升高 → 吸热方向；降低 → 放热方向。
Catalysts do not alter the equilibrium position.
催化剂不改变平衡位置。
Industrial processes like Haber and Contact use compromise conditions balancing yield, rate, and cost.
哈伯法和接触法等工业流程采用折中条件，平衡产率、速率和成本。

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Le Chatelier’s Principle | 勒夏特列原理考点精讲