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

In GCSE WJEC Chemistry, understanding reversible reactions and dynamic equilibrium is fundamental. Le Chatelier’s principle allows us to predict how the position of equilibrium will shift when a system at equilibrium is disturbed. This article breaks down every key concept you need to know for the exam, with paired English–Chinese explanations designed for revision.

在GCSE WJEC化学中，理解可逆反应和动态平衡是基础。勒夏特列原理能让我们预测当处于平衡的系统受到扰动时，平衡位置将如何移动。本文分解了考试中你需要掌握的每个关键概念，配有英中双语对照解释，便于复习。

1. Introduction to Dynamic Equilibrium | 动态平衡简介

Many chemical reactions are reversible, meaning they can proceed in both the forward and backward directions. In a closed system, when the rate of the forward reaction equals the rate of the backward reaction, dynamic equilibrium is established. At this point, the concentrations of reactants and products remain constant.

许多化学反应是可逆的，意味着它们可以正向和逆向进行。在一个封闭系统中，当正向反应速率与逆向反应速率相等时，就建立了动态平衡。此时，反应物和产物的浓度保持恒定。

It is crucial to understand that equilibrium can only be reached in a closed system – no substances are allowed to enter or leave. If products or reactants escape, the mixture never settles into a state of balance, and Le Chatelier’s principle cannot be applied.

非常重要的是要明白，平衡只能在封闭系统中达到——不允许有任何物质进入或离开。如果产物或反应物逸散，混合物永远无法达到平衡状态，勒夏特列原理也就无法应用。

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

Le Chatelier’s principle states: “If a system at dynamic equilibrium is subjected to a change in concentration, temperature or pressure, the position of equilibrium will shift to partially oppose the change.” This means the equilibrium will favour either the forward or the backward reaction to minimise the disturbance.

勒夏特列原理表述为：“如果一个处于动态平衡的系统受到浓度、温度或压强的改变，平衡将朝部分抵消该变化的方向移动。” 这意味着平衡将倾向于正向反应或逆向反应，以减小扰动。

It is important to note that the principle only predicts the direction of shift, not the rate at which the new equilibrium is reached. The shift always tries to counteract the stress applied, but the equilibrium constant K changes only with temperature.

需要注意的是，该原理只预测移动的方向，并不预测达到新平衡的速率。这种移动总是试图抵消施加的应力，但平衡常数K只随温度变化。

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

If the concentration of a reactant is increased, the system will shift equilibrium to the right (towards products) so that more of the added reactant is consumed. Similarly, if a product’s concentration is increased, equilibrium shifts left, favouring the reverse reaction.

如果增加一种反应物的浓度，系统会将平衡向右（产物方向）移动，以便消耗更多添加的反应物。同样地，如果增加一种产物的浓度，平衡将向左移动，倾向于逆向反应。

Removing a product as it forms (e.g., by condensing it or letting it react elsewhere) continuously shifts equilibrium to the right, driving the forward reaction. This is a common strategy in industrial processes to maximise yield.

在产物生成时将其移出（例如通过冷凝或使其在其他地方反应）会持续将平衡向右推动，促进正向反应。这是工业过程中提高产率的常用策略。

For example, in the equilibrium: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) (red colour), adding more Fe³⁺ solution intensifies the red colour, confirming the shift to the right.

例如，在平衡：Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)（红色）中，加入更多的Fe³⁺溶液会使红色加深，证实了平衡向右移动。

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

Temperature change affects the equilibrium position depending on whether the forward reaction is exothermic (releases heat) or endothermic (absorbs heat). If the temperature is increased, equilibrium shifts in the endothermic direction to absorb the extra heat. Lowering the temperature favours the exothermic direction.

温度变化对平衡位置的影响取决于正向反应是放热（释放热量）还是吸热（吸收热量）。如果温度升高，平衡向吸热方向移动以吸收额外热量。降低温度则有利于放热方向。

Consider the reversible decomposition of hydrogen iodide: 2HI(g) ⇌ H₂(g) + I₂(g). The forward reaction is endothermic. Heating the mixture will shift equilibrium to the right, increasing the purple iodine vapour. Cooling favours the recombination of HI.

考虑碘化氢的可逆分解：2HI(g) ⇌ H₂(g) + I₂(g)。正向反应是吸热的。加热混合物将使平衡向右移动，紫色碘蒸气增多。冷却则有利于HI的重新生成。

Always refer to the sign of ΔH given in the question. For an exothermic reaction, ΔH is negative; increasing temperature shifts equilibrium left. For endothermic, ΔH is positive; increasing temperature shifts equilibrium right.

一定要参考题目中给出的ΔH符号。对于放热反应，ΔH为负；升高温度使平衡左移。对于吸热反应，ΔH为正；升高温度使平衡右移。

5. Effect of Pressure Changes (Gaseous Systems) | 压力变化的影响（气体体系）

Pressure changes only affect equilibria involving gases. An increase in pressure shifts the position of equilibrium towards the side with fewer gas molecules (the smaller total number of moles of gas). Conversely, decreasing pressure favours the side with more gas molecules.

压力变化仅影响涉及气体的平衡。增加压力会使平衡位置向气体分子总数较少的一侧移动。相反，降低压力则有利于气体分子数较多的一侧。

Look at the balanced equation to count moles of gas on each side. For instance: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). There are 4 moles of gas on the left and 2 moles on the right. High pressure therefore favours the forward reaction, producing more ammonia.

观察配平方程式，计算每侧的气体摩尔数。例如：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)。左侧有4摩尔气体，右侧有2摩尔。因此，高压有利于正向反应，生成更多的氨。

If the number of gas molecules is the same on both sides, as in H₂(g) + I₂(g) ⇌ 2HI(g), a change in pressure has no effect on the equilibrium position.

如果两侧气体分子总数相同，如在 H₂(g) + I₂(g) ⇌ 2HI(g) 中，压力变化对平衡位置没有影响。

6. Effect of Catalysts | 催化剂的影响

A catalyst does not alter the position of equilibrium. It increases the rate of both the forward and backward reactions equally, so dynamic equilibrium is achieved more quickly. Catalysts provide an alternative reaction pathway with lower activation energy for both directions.

催化剂不会改变平衡位置。它同等程度地加快正向和逆向反应的速率，因此能更快地达到动态平衡。催化剂为两个方向提供了活化能更低的替代反应路径。

In an exam, do not confuse the role of a catalyst with shifting equilibrium. Adding a catalyst improves the rate of reaching equilibrium, but the final yield depends only on temperature and pressure conditions set by Le Chatelier’s principle.

考试中，不要把催化剂的作用与平衡移动混淆。添加催化剂可以提高达到平衡的速率，但最终产率只取决于由勒夏特列原理决定的温度和压力条件。

7. Application: The Haber Process | 应用：哈伯法合成氨

The Haber process manufactures ammonia from nitrogen and hydrogen:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = –92 kJ mol⁻¹

According to Le Chatelier, high pressure shifts equilibrium right (2 moles vs 4 moles gas) and low temperature favours the exothermic forward reaction. However, a compromise temperature of about 450 °C is used because low temperature makes the reaction too slow, even with an iron catalyst.

哈伯法由氮气和氢气生产氨：

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = –92 kJ mol⁻¹

根据勒夏特列原理，高压使平衡右移（右侧2摩尔气体对左侧4摩尔），低温有利于放热正向反应。但是，实际采用的是约450°C的折中温度，因为低温会使反应速度过慢，即使使用了铁催化剂也是如此。

A pressure of about 200 atmospheres is chosen. Higher pressures would shift the equilibrium further right but are expensive and require stronger, thicker-walled equipment, increasing safety risks and costs. The iron catalyst simply speeds up the attainment of equilibrium.

选择的压力约为200个大气压。更高的压力虽然能使平衡进一步向右移动，但成本高昂且需要更坚固的厚壁设备，增加安全风险和成本。铁催化剂仅加速平衡的达到。

8. Application: The Contact Process | 应用：接触法制硫酸

The Contact process produces sulfur trioxide, a key step in sulfuric acid manufacture:

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = –197 kJ mol⁻¹

Le Chatelier’s principle suggests high pressure (3 moles of gas on left, 2 on right) and low temperature (exothermic forward reaction) for maximum yield. In practice, a pressure of 1–2 atm is sufficient because the equilibrium already lies far to the right at moderate temperatures.

接触法生产三氧化硫，是制造硫酸的关键步骤：

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = –197 kJ mol⁻¹

勒夏特列原理建议采用高压（左侧3摩尔气体，右侧2摩尔）和低温（放热正向反应）以获得最大产率。实际上，1-2个大气压就足够了，因为在适中温度下平衡已经大幅偏向右侧。

A vanadium(V) oxide catalyst, V₂O₅, is used, and a compromise temperature of about 450 °C ensures a rapid rate without seriously reducing the equilibrium yield. The catalyst has no effect on the equilibrium position itself.

使用五氧化二钒催化剂V₂O₅，并且约450°C的折中温度确保反应速率快而不严重降低平衡产率。催化剂本身对平衡位置没有影响。

9. Predicting Colour Changes in Equilibria | 预测平衡中的颜色变化

WJEC GCSE exams often include colour changes in equilibrium mixtures. A classic example is the dichromate–chromate equilibrium:

2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l)
yellow orange

Adding acid (H⁺) increases the concentration of a reactant, so equilibrium shifts right to form more orange dichromate ions. Adding alkali removes H⁺ (as water forms), so equilibrium shifts left, restoring the yellow chromate colour.

WJEC GCSE考试经常涉及平衡混合物的颜色变化。一个经典例子是重铬酸盐–铬酸盐平衡：

2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l)
黄色橙色

加酸（H⁺）增加了反应物浓度，因此平衡向右移动，生成更多的橙色重铬酸根离子。加碱会移除H⁺（生成水），所以平衡左移，恢复黄色铬酸根的颜色。

Another common system is the nitrogen dioxide–dinitrogen tetroxide equilibrium: 2NO₂(g) ⇌ N₂O₄(g). NO₂ is brown, N₂O₄ is colourless. Cooling favours the forward exothermic reaction, making the colour fade; heating shifts equilibrium to the endothermic reverse reaction, deepening the brown colour.

另一个常见体系是二氧化氮–四氧化二氮平衡：2NO₂(g) ⇌ N₂O₄(g)。NO₂为棕色，N₂O₄无色。冷却有利于正向放热反应，使颜色变浅；加热使平衡向吸热的逆向移动，棕色加深。

10. Exam Tips for WJEC Le Chatelier Questions | WJEC勒夏特列原理考题技巧

When answering WJEC questions, always state the direction of shift clearly – left/right or towards reactants/products – and justify using the principle. Use correct chemical terminology: “position of equilibrium shifts to the right because the forward reaction is exothermic and decreasing temperature favours exothermic direction.”

回答WJEC考题时，始终要清晰陈述移动方向——向左/向右或朝向反应物/产物——并用原理进行解释。使用正确的化学术语：“平衡位置向右移动，因为正向反应放热，降低温度有利于放热方向。”

Prepare for data-based questions by practising with tables of colour changes, volume of gas produced, or concentration variations. Remember that only temperature changes alter the equilibrium constant; concentration and pressure changes shift the position but keep K constant (at the same temperature).

针对基于数据的问题，练习分析颜色变化、气体产量或浓度变化的表格。记住，只有温度变化才会改变平衡常数；浓度和压力变化只改变平衡位置，而K保持不变（同一温度下）。

Change	Shift of equilibrium	Reason (applying principle)
Increase [reactant]	To the right (products)	System opposes increase by consuming reactant
Increase temperature (exothermic)	To the left (reactants)	System opposes extra heat by favouring endothermic reverse reaction
Increase pressure (fewer gas molecules on right)	To the right (fewer moles)	System opposes higher pressure by shifting to side with fewer gas molecules

Always link your answer back to the stated change – “oppose the increase/decrease” – to show full understanding of Le Chatelier’s principle.

始终将答案与所述变化联系起来——”抵消增加/减少”——以充分展示对勒夏特列原理的理解。

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Le Chatelier’s Principle | 勒夏特列原理考点精讲