Le Chatelier's Principle: IGCSE CIE Chemistry Key Points | 勒夏特列原理考点精讲

📚 Le Chatelier’s Principle: IGCSE CIE Chemistry Key Points | 勒夏特列原理考点精讲

Le Chatelier’s Principle is a fundamental concept in chemical equilibrium that predicts how a system at equilibrium responds to external changes. It states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts to counteract the change and restore a new equilibrium. This principle is essential for understanding industrial processes, predicting colour changes, and answering exam questions confidently. In IGCSE CIE Chemistry, you must be able to apply it to concentration, pressure, temperature changes, and catalysts, as well as explain specific examples like the Haber and Contact processes.

勒夏特列原理是化学平衡中的一个基本概念，它预测处于平衡状态的体系如何应对外部变化。该原理指出：如果改变影响动态平衡的条件，平衡位置会向着减弱这种改变的方向移动，并建立新的平衡。这一原理对于理解工业过程、预测颜色变化以及自信地回答考试题目至关重要。在IGCSE CIE化学中，你必须能够将其应用于浓度、压强、温度的变化以及催化剂的影响，并解释哈伯法和接触法等具体实例。

1. Introduction to Le Chatelier’s Principle | 勒夏特列原理简介

Le Chatelier’s Principle applies only to systems in dynamic equilibrium, where the rate of the forward reaction equals the rate of the backward reaction and the concentrations of reactants and products remain constant. The principle can be summarised as: ‘If a system at equilibrium is subjected to a change in concentration, pressure or temperature, the equilibrium shifts in the direction that tends to oppose the change.’

勒夏特列原理仅适用于动态平衡体系，此时正反应速率等于逆反应速率，反应物和产物的浓度保持恒定。该原理可概括为：“如果改变影响平衡的一个条件（如浓度、压强或温度），平衡就向能够减弱这种改变的方向移动。”

It is important to remember that the equilibrium position moves, but the equilibrium constant Kc only changes with temperature. A catalyst does not alter the position of equilibrium; it merely speeds up the rate at which equilibrium is reached. Exam questions often ask you to predict the direction of shift and explain your reasoning in terms of opposing the change.

需要记住的是，平衡位置会移动，但平衡常数Kc只随温度改变。催化剂不会改变平衡位置，仅仅加快达到平衡的速率。考试题常要求你预测移动方向，并从“减弱改变”的角度解释原因。

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 the products) to use up the added reactant. Conversely, if the concentration of a product is increased, the equilibrium shifts to the left (towards the reactants) to consume the extra product. Removing a substance shifts the equilibrium in the direction that produces more of that substance.

如果增大反应物的浓度，平衡会向右（产物方向）移动，以消耗添加的反应物。相反，如果增大产物的浓度，平衡会向左（反应物方向）移动，以消耗多余的产物。移除某种物质会使平衡向生成更多该物质的方向移动。

For example, consider the equilibrium: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq). The solution is blood-red due to FeSCN²⁺. Adding more Fe³⁺ or SCN⁻ shifts the equilibrium to the right, intensifying the red colour. Adding a substance that removes Fe³⁺ (e.g. by precipitation) shifts the equilibrium to the left, and the red colour fades.

例如，考虑平衡：Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)。溶液因FeSCN²⁺而呈血红色。加入更多Fe³⁺或SCN⁻会使平衡向右移动，红色加深。加入能移除Fe³⁺的物质（如通过沉淀）则使平衡向左移动，红色变浅。

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 difference in the number of gas molecules on each side of the equation. Increasing the pressure shifts the equilibrium to the side with fewer gas molecules, because this reduces the pressure. Decreasing the pressure favours the side with more gas molecules.

压强的改变只影响有气体参与的平衡，并且仅当方程式两边气体分子数不同时才产生影响。增大压强，平衡向气体分子数较少的一方移动，因为这样可以降低压强。减小压强则有利于气体分子数较多的一方。

Consider the equilibrium: 2NO₂(g) ⇌ N₂O₄(g). On the left, there are 2 moles of gas; on the right, 1 mole. An increase in pressure shifts the equilibrium to the right, favouring the formation of colourless N₂O₄, so the brown colour of NO₂ fades. A decrease in pressure shifts the equilibrium to the left, increasing the brown colour. If the number of gas molecules is equal on both sides (e.g., H₂(g) + I₂(g) ⇌ 2HI(g)), changing pressure has no effect on the position of equilibrium.

考虑反应：2NO₂(g) ⇌ N₂O₄(g)。左边有2 mol气体，右边有1 mol。增大压强，平衡向右移动，有利于无色的N₂O₄生成，因此NO₂的棕色变浅。减小压强则平衡向左移动，棕色加深。如果两边气体分子数相等（如H₂(g) + I₂(g) ⇌ 2HI(g)），改变压强则不影响平衡位置。

Change	Effect on 2NO₂ ⇌ N₂O₄
Increase pressure	Shifts right (forward), fewer gas molecules
Decrease pressure	Shifts left (backward), more gas molecules

变化	对2NO₂ ⇌ N₂O₄的影响
增大压强	向右（正向）移动，气体分子数减少
减小压强	向左（逆向）移动，气体分子数增多

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

Temperature is the only external condition that changes the value of the equilibrium constant Kc. To apply Le Chatelier’s Principle, you must know whether the forward reaction is exothermic (releases heat, ΔH negative) or endothermic (absorbs heat, ΔH positive). Increasing the temperature favours the endothermic direction because it absorbs the added heat. Decreasing the temperature favours the exothermic direction.

温度是唯一能改变平衡常数Kc数值的外界条件。应用勒夏特列原理时，你必须清楚正反应是放热（ΔH为负）还是吸热（ΔH为正）。升高温度有利于向吸热方向移动，因为这样可以吸收添加的热量。降低温度则有利于向放热方向移动。

For example, the formation of N₂O₄ from NO₂ is exothermic: 2NO₂(g) ⇌ N₂O₄(g) ΔH = -57 kJ mol⁻¹. If the temperature increases, the equilibrium shifts to the left (endothermic direction) to absorb heat, so more brown NO₂ is formed and the mixture darkens. If the temperature decreases, the equilibrium shifts to the right (exothermic direction), making the mixture paler. This colour change is a classic observation in the lab.

例如，由NO₂生成N₂O₄是放热反应：2NO₂(g) ⇌ N₂O₄(g) ΔH = -57 kJ mol⁻¹。温度升高，平衡向左（吸热方向）移动以吸收热量，因此生成更多棕色的NO₂，混合气体颜色变深。温度降低，平衡向右（放热方向）移动，颜色变浅。这一颜色变化是实验室中的经典观察。

5. Effect of a Catalyst | 催化剂的影响

Adding a catalyst has no effect on the position of equilibrium. A catalyst increases the rate of both the forward and backward reactions equally, so the equilibrium mixture is achieved faster but the relative amounts of reactants and products at equilibrium remain unchanged. In terms of Le Chatelier’s Principle, a catalyst does not change the concentrations, pressure, or temperature, so there is no ‘opposing’ shift.

加入催化剂不会影响平衡位置。催化剂同等程度地加快正反应和逆反应的速率，因此能更快地达到平衡，但平衡时反应物和产物的相对量保持不变。根据勒夏特列原理，催化剂不改变浓度、压强或温度，因此没有“减弱”移动。

In industrial applications, a catalyst is vital not for increasing yield, but for allowing the reaction to reach equilibrium rapidly at a lower temperature than would otherwise be possible, saving time and energy. For example, iron in the Haber process and vanadium(V) oxide in the Contact process are used to speed up the reaction without affecting the equilibrium composition.

在工业应用中，催化剂的关键作用不是提高产率，而是使反应在较低温度下快速达到平衡，从而节省时间和能源。例如，哈伯法中的铁和接触法中的五氧化二钒，都用于加快反应而不影响平衡组成。

6. The Haber Process – A Classic Application | 哈伯法——经典应用

The Haber process synthesises ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹. The reaction is exothermic in the forward direction, and the number of gas molecules decreases from 4 moles on the left to 2 moles on the right. These features determine the choice of industrial conditions through Le Chatelier’s Principle.

哈伯法合成氨：N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = -92 kJ mol⁻¹。正反应放热，气体分子数从左侧4摩尔减少到右侧2摩尔。这些特征通过勒夏特列原理决定了工业条件的选择。

Pressure: High pressure shifts the equilibrium to the right (fewer gas molecules), increasing ammonia yield. In practice, a pressure of around 200 atm is used. Going much higher would increase equipment costs and safety risks without a proportional gain in yield. Temperature: Since the forward reaction is exothermic, low temperature favours ammonia production. However, at low temperatures the rate of reaction is too slow. A compromise temperature of about 450 °C is used, which gives a reasonable rate while maintaining an acceptable yield. Catalyst: Finely divided iron is used to speed up the attainment of equilibrium. Unreacted nitrogen and hydrogen are recycled to improve overall efficiency.

压强：高压使平衡向右（气体分子数少的方向）移动，提高氨的产率。实际生产中采用约200 atm的压强，因为过高压强会增加设备成本和安全风险，而产率提高不成比例。温度：由于正反应放热，低温有利于氨的生成。但低温下反应速率太慢。因此采用约450 °C的折中温度，既能获得合理的速率，又能保持可接受的产率。催化剂：使用细铁粉加快到达平衡。未反应的氮气和氢气循环使用以提高总效率。

7. The Contact Process – Another Industrial Example | 接触法——又一个工业实例

The Contact process produces sulfuric acid via the equilibrium: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = -197 kJ mol⁻¹. Forward reaction is exothermic, and gas molecules decrease from 3 to 2. Le Chatelier’s Principle indicates that high pressure and low temperature would favour SO₃ yield. However, industrial conditions are chosen differently.

接触法通过以下平衡生产硫酸：2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = -197 kJ mol⁻¹。正反应放热，气体分子数从3个减少到2个。勒夏特列原理指出高压和低温有利于SO₃产率，但实际工业条件有所不同。

Pressure: A pressure just above atmospheric (1-2 atm) is employed. Although high pressure shifts equilibrium to the right, the equilibrium already lies far to the right under normal conditions, giving a very high conversion. The additional yield from very high pressures does not justify the extra cost. Temperature: A compromise temperature of around 450 °C is used. Lower temperatures would increase yield but slow the rate, and the catalyst V₂O₅ requires this temperature to work effectively. Catalyst: Vanadium(V) oxide (V₂O₅) accelerates the reaction without shifting equilibrium.

压强：采用略高于常压的压强（1-2 atm）。虽然高压能使平衡右移，但在常压下平衡已强烈偏向右侧，转化率很高。过高压强带来的额外产率无法抵消额外成本。温度：采用约450 °C的折中温度。更低的温度虽能提高产率但会减慢速率，且V₂O₅催化剂需要在该温度下才有效。催化剂：五氧化二钒（V₂O₅）加速反应而不移动平衡。

8. Predicting Shifts in Equilibrium: A Summary | 预测平衡移动：总结

The following table summarises how concentration, pressure and temperature changes shift the position of equilibrium for a general reversible reaction. Always identify whether the change affects reactants, products, or the entire system.

下表总结了浓度、压强和温度的变化如何移动一般可逆反应的平衡位置。务必先确定变化影响的是反应物、产物还是整个体系。

Change	Equilibrium Shift
Increase reactant concentration	To the right (forward)
Increase product concentration	To the left (backward)
Increase pressure (fewer gas molecules on right)	To the right
Decrease pressure (fewer gas molecules on right)	To the left
Increase temperature (exothermic forward)	To the left (endothermic)
Decrease temperature (exothermic forward)	To the right (exothermic)

变化	平衡移动方向
增大反应物浓度	向右（正向）
增大产物浓度	向左（逆向）
增大压强（右边气体分子数少）	向右
减小压强（右边气体分子数少）	向左
升高温度（正向放热）	向左（吸热方向）
降低温度（正向放热）	向右（放热方向）

When a question involves a graph showing concentration or rate against time, use the initial change and the gradual re-establishment of equilibrium to deduce the shift. Remember that if the forward reaction is endothermic, an increase in temperature shifts the equilibrium to the right; if exothermic, it shifts to the left.

遇到图表题时，利用初始变化和平衡的逐渐重建来推断移动方向。记住，若正反应吸热，升高温度平衡右移；若正反应放热，则左移。

9. Colour Changes and Equilibrium Shifts | 颜色变化与平衡移动

Many chemical equilibria exhibit distinct colour changes when conditions are altered, providing a visible demonstration of Le Chatelier’s Principle. Besides the NO₂/N₂O₄ system, another important example is the chromate/dichromate equilibrium: 2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l). Chromate ions are yellow; dichromate ions are orange.

许多化学平衡在条件改变时显示出明显的颜色变化，直观演示了勒夏特列原理。除了NO₂/N₂O₄体系，另一个重要例子是铬酸盐/重铬酸盐平衡：2CrO₄²⁻(aq) + 2H⁺(aq) ⇌ Cr₂O₇²⁻(aq) + H₂O(l)。铬酸根离子呈黄色，重铬酸根离子呈橙色。

Adding an acid (increasing H⁺ concentration) shifts the equilibrium to the right to reduce H⁺, so the solution turns orange. Adding an alkali (removing H⁺) shifts the equilibrium to the left to replace H⁺, and the solution turns yellow. Temperature can also affect this equilibrium. For the CoCl₄²⁻/Co(H₂O)₆²⁺ equilibrium: Co(H₂O)₆²⁺(aq) + 4Cl⁻(aq) ⇌ CoCl₄²⁻(aq) + 6H₂O(l). This is endothermic in the forward direction; Co(H₂O)₆²⁺ is pink, and CoCl₄²⁻ is blue. Heating shifts the equilibrium to the right, turning the solution blue; cooling favours the left, turning it pink.

加酸（增大H⁺浓度）使平衡右移以消耗H⁺，溶液变橙色；加碱（移除H⁺）使平衡左移以补充H⁺，溶液变黄色。温度也能影响此平衡。对于CoCl₄²⁻/Co(H₂O)₆²⁺平衡：Co(H₂O)₆²⁺(aq) + 4Cl⁻(aq) ⇌ CoCl₄²⁻(aq) + 6H₂O(l)，正向吸热，Co(H₂O)₆²⁺呈粉色，CoCl₄²⁻呈蓝色。加热使平衡右移，溶液变蓝；冷却使平衡左移，变粉。

10. Common Misconceptions and Exam Tips | 常见误区与应试技巧

Misconception: Catalysts increase yield. A catalyst does not change the equilibrium position; it only speeds up the rate. Yield at equilibrium is unaffected.

误区：催化剂能提高产率。催化剂不改变平衡位置，只加快速率。平衡产率不受影响。
Misconception: Pressure changes always shift equilibrium. Pressure only matters when gases are present and the total number of gas molecules differs on each side. For reactions in solution or with equal gas molecules, pressure has no effect.

误区：压强变化总会移动平衡。只有存在气体且两边气体分子数不等时压强才重要。对于溶液中的反应或气体分子数相等的情况，压强无影响。
Misconception: Temperature changes have a simple ‘add/remove heat’ rule. You must know whether the forward reaction is exothermic or endothermic. Do not assume that adding heat always favours products.

误区：温度变化可用简单的“加热/去热”规则。必须明确正反应是放热还是吸热。不要假设加热总有利于产物。

Exam tip: When explaining a shift, always use the phrase ‘to oppose the change’ or ‘to counteract the increase/decrease’. State clearly the direction of shift (left or right, forward or backward) and link it to the specific factor. For questions about industrial conditions, explain the compromise between rate, yield and cost.

应试技巧：解释移动时，始终使用“为减弱这一变化”或“抵消增加/减少”的表述。明确说明移动方向（向左或向右，正向或逆向），并将其与具体因素相联系。对于工业条件问题，要解释速率、产率和成本之间的折中。

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Le Chatelier’s Principle: IGCSE CIE Chemistry Key Points | 勒夏特列原理考点精讲