Le Chatelier’s Principle: Exam Focused Revision for IB and Edexcel Chemistry | 勒夏特列原理：IB与Edexcel化学考点精讲

📚 Le Chatelier’s Principle: Exam Focused Revision for IB and Edexcel Chemistry | 勒夏特列原理：IB与Edexcel化学考点精讲

Le Chatelier’s principle is a cornerstone of equilibrium chemistry, stating that a system at dynamic equilibrium, when disturbed by a change in concentration, pressure or temperature, will shift its position to oppose the disturbance. It appears in almost every IB and Edexcel Chemistry examination, from multiple‑choice questions to extended responses on industrial processes. Understanding its predictive power – and its limitations – is essential for top marks.

勒夏特列原理是平衡化学的基石，它指出处于动态平衡的体系在受到浓度、压强或温度变化的干扰时，会移动平衡位置以减弱这种干扰。从选择题到关于工业过程的扩展题，该原理几乎出现在每一次 IB 与 Edexcel 化学考试中。理解它的预测能力——以及它的局限性——是获取高分的关键。

1. Statement and Dynamic Equilibrium | 原理表述与动态平衡

Le Chatelier’s principle can be stated as: ‘If a system at equilibrium is subjected to a change in concentration, pressure or temperature, the equilibrium position shifts in the direction that tends to reduce the effect of that change.’ Importantly, it only applies to closed systems at dynamic equilibrium, where the rates of the forward and reverse reactions are equal and non‑zero.

勒夏特列原理可以表述为：“处于平衡状态的体系如果受到浓度、压强或温度的变化，平衡位置会朝减弱该变化的方向移动。”重要的是，它只适用于动态平衡的封闭体系，此时正、逆反应速率相等且不为零。

The shift does not completely cancel the applied change; rather, it partially offsets it, leading to a new equilibrium composition. For example, adding a reactant will cause the system to consume some of that extra reactant, but the final concentration will still be higher than before the addition.

平衡移动并不能完全抵消外加的变化，而是部分抵消，生成新的平衡组成。例如，加入一种反应物，体系会消耗掉部分新增的反应物，但该物质的最终浓度仍会比加入前高。

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

Changes in concentration directly alter the value of the reaction quotient Q, which then drives the shift to restore equilibrium (Q = Kc). Adding a reactant or removing a product favours the forward reaction; adding a product or removing a reactant favours the reverse reaction.

浓度变化直接改变反应商 Q 的值，从而推动平衡移动以重建平衡（Q = Kc）。增加反应物或移走生成物有利于正反应；增加生成物或移走反应物有利于逆反应。

Change	Equilibrium shift	变化	平衡移动
Increase [reactant]	Shifts right (forward)	增加反应物浓度	向右(正向)移动
Decrease [product]	Shifts right (forward)	减少生成物浓度	向右(正向)移动
Increase [product]	Shifts left (reverse)	增加生成物浓度	向左(逆向)移动
Decrease [reactant]	Shifts left (reverse)	减少反应物浓度	向左(逆向)移动

A classic demonstration involves the equilibrium between iron(III) thiocyanate ions: Fe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq). Adding Fe³⁺ or SCN⁻ intensifies the blood‑red colour (forward shift); adding a reagent that removes Fe³⁺ (e.g., F⁻ forming [FeF₆]³⁻) fades the colour (reverse shift).

一个经典演示涉及铁(III)‑硫氰酸根离子平衡：Fe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq)。加入 Fe³⁺ 或 SCN⁻ 会使血红色加深（正向移动）；加入能移除 Fe³⁺ 的试剂（如 F⁻ 形成 [FeF₆]³⁻）则使颜色变淡（逆向移动）。

3. Effect of Pressure Changes | 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 moles (Δn ≠ 0). Increasing pressure favours the side with fewer gas molecules; decreasing pressure favours the side with more gas molecules.

压强变化仅影响有气体参与的平衡，而且只有当气体总摩尔数发生变化（Δn ≠ 0）时才有效。增压有利于气体分子数较少的一侧；减压有利于气体分子数较多的一侧。

Pressure change	Effect on gases	压强变化	对气体的影响
Increase total pressure	Shifts to side with fewer gas molecules	总压增大	向气体分子数少的一侧移动
Decrease total pressure	Shifts to side with more gas molecules	总压减小	向气体分子数多的一侧移动
Add inert gas at constant volume	No shift (partial pressures unchanged)	恒容加入惰性气体	不移动（分压不变）

For the equilibrium 2NO₂(g) ⇌ N₂O₄(g), brown NO₂ and colourless N₂O₄, an increase in pressure shifts the position to the right (1 mole of N₂O₄ vs 2 moles of NO₂), causing the brown colour to fade. If Δn = 0, e.g., H₂(g) + I₂(g) ⇌ 2HI(g), pressure changes have no effect on the position of equilibrium.

对于平衡 2NO₂(g) ⇌ N₂O₄(g)，棕色的 NO₂ 和无色的 N₂O₄，增大压强会使平衡位置右移（1 mol N₂O₄ 对 2 mol NO₂），棕色变浅。若 Δn = 0，如 H₂(g) + I₂(g) ⇌ 2HI(g)，压强变化对平衡位置没有影响。

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

Temperature is the only factor that changes the equilibrium constant Kc (or Kp). An increase in temperature shifts the equilibrium in the endothermic direction (ΔH > 0), absorbing heat; a decrease favours the exothermic direction (ΔH < 0), releasing heat.

温度是唯一能改变平衡常数 Kc（或 Kp）的因素。升高温度使平衡向吸热方向（ΔH > 0）移动，吸收热量；降低温度则有利于放热方向（ΔH < 0），释放热量。

Temperature change	Effect on exothermic reaction (ΔH < 0)	Effect on endothermic reaction (ΔH > 0)
Increase T	Shifts left (reverse), Kc decreases	Shifts right (forward), Kc increases
Decrease T	Shifts right (forward), Kc increases	Shifts left (reverse), Kc decreases

For the cobalt(II) chloride equilibrium: [Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l), ΔH > 0. Heating turns the solution from pink to blue (endothermic forward reaction favoured); cooling restores the pink colour.

以氯化钴(II)平衡为例：[Co(H₂O)₆]²⁺(aq) + 4Cl⁻(aq) ⇌ [CoCl₄]²⁻(aq) + 6H₂O(l)，ΔH > 0。加热使溶液由粉红变蓝（有利于吸热正反应）；冷却则恢复粉红色。

5. Role of a Catalyst | 催化剂的作用

A catalyst provides an alternative pathway with a lower activation energy for both the forward and reverse reactions equally. It does not alter the position of equilibrium, the equilibrium composition, or the value of Kc. It merely speeds up the rate at which equilibrium is attained.

催化剂为正逆反应同等地提供活化能更低的替代途径。它不改变平衡位置、平衡组成，也不改变 Kc 值。它只加速达到平衡的速率。

In an exam, always state: ‘A catalyst increases the rate of both forward and reverse reactions equally, so the equilibrium position remains unchanged. It is used industrially to increase the rate, not to increase the yield.’

考试中务必答出：“催化剂同等程度地加快正、逆反应速率，因此平衡位置不变。工业上使用催化剂是为了加快速率，而不是提高产率。”

6. Le Chatelier’s Principle and the Haber Process | 勒夏特列原理与哈伯法

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

哈伯法合成氨：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 → 2）。

By Le Chatelier’s principle, a low temperature favours the exothermic forward reaction, giving a higher equilibrium yield of NH₃. However, low temperatures slow the rate, so a compromise temperature of 400–450 °C is used. High pressure shifts the equilibrium to the right (fewer gas molecules), increasing yield; 200 atm is selected to balance yield against equipment cost. An iron catalyst is used to speed up the reaction without affecting the equilibrium yield.

根据勒夏特列原理，低温有利于放热的正反应，提高氨的平衡产率。但低温使速率减慢，因此选择 400–450 °C 的折中温度。高压使平衡右移（气体分子数少），提高产率；选用 200 atm 以兼顾产率与设备成本。使用铁催化剂加快反应，但不影响平衡产率。

7. Contact Process and Other Applications | 接触法及其他应用

The Contact process produces sulfuric acid via: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), ΔH = −196 kJ mol⁻¹. The forward reaction is exothermic and reduces gas moles (3 → 2).

接触法生产硫酸：2SO₂(g) + O₂(g) ⇌ 2SO₃(g)，ΔH = −196 kJ mol⁻¹。正反应放热，气体分子数减少（3 → 2）。

A moderately high temperature (400–450 °C) is used to balance rate and yield; a catalyst V₂O₅ accelerates the reaction. Interestingly, the process operates at only 1–2 atm because Kp for this equilibrium is already very large – a high conversion is achieved even without extreme pressure, reducing costs.

使用中等偏高的温度（400–450 °C）以平衡速率与产率；催化剂 V₂O₅ 加速反应。有趣的是，该过程仅使用 1–2 atm 的压强，因为该反应的 Kp 已经非常大——无需高压即可获得高转化率，从而降低成本。

Other applications include esterification (removing water shifts equilibrium to the right, increasing ester yield), and the blood‑oxygen equilibrium (

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