Le Chatelier's Principle and Equilibrium Shifts | 勒夏特列原理与平衡移动

📚 Le Chatelier’s Principle and Equilibrium Shifts | 勒夏特列原理与平衡移动

Chemical equilibrium is a central concept in IB Chemistry HL and SL. When a system at equilibrium is disturbed, it responds in a predictable way governed by Le Chatelier’s Principle.

化学平衡是IB化学HL和SL的核心概念。当处于平衡的系统受到扰动时，它会以可预测的方式响应，这一行为由勒夏特列原理控制。

1. Dynamic Equilibrium | 动态平衡

In a closed system, a reversible reaction reaches dynamic equilibrium when the rate of the forward reaction equals the rate of the reverse reaction. At this point, the concentrations of all species remain constant, but the reaction has not stopped.

在封闭体系中，当正反应速率等于逆反应速率时，可逆反应达到动态平衡。此时所有物质的浓度保持不变，但反应并未停止。

Equilibrium is only possible in a closed system; if products can escape, equilibrium will never be established. Dynamic means that both forward and reverse reactions continue to occur simultaneously at equal rates.

平衡只有在封闭体系中才可能建立；如果产物能够逸出，平衡就永远不会达到。所谓动态，指的是正逆反应仍在同时进行，且速率相等。

2. The Principle | 勒夏特列原理的核心

Le Chatelier’s Principle states that if a dynamic equilibrium is disturbed by changing conditions, the position of equilibrium shifts to counteract the change and restore a new equilibrium.

勒夏特列原理指出：如果动态平衡因条件改变而受到扰动，平衡位置会向抵消该改变的方向移动，从而建立新的平衡。

This principle allows chemists to predict how a system responds to changes in concentration, pressure, temperature, and the addition of an inert gas or catalyst. It is a qualitative tool; the quantitative approach uses the reaction quotient Q and the equilibrium constant K.

该原理使化学家能够预测体系如何响应浓度、压力、温度以及惰性气体或催化剂的加入。它是一种定性工具；定量方法则使用反应商Q和平衡常数K。

3. Concentration Changes | 浓度变化

Increasing the concentration of a reactant shifts the equilibrium to the right, favouring the forward reaction and consuming some of the added reactant. Conversely, decreasing the concentration of a product also shifts the equilibrium to the right.

增加反应物浓度会使平衡向右移动，有利于正反应，并消耗部分加入的反应物。相反，减少产物浓度也会使平衡向右移动。

For example, in the reaction: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq), adding more Fe³⁺ intensifies the blood-red colour of FeSCN²⁺, showing a shift to the right.

例如，在反应 Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) 中，加入更多Fe³⁺会使FeSCN²⁺的血红色加深，表明平衡向右移动。

Adding a reactant → shift to products → more product forms.
Removing a product → shift to products → more product forms.
Adding a product → shift to reactants.

增加反应物 → 向产物方向移动 → 生成更多产物。
移除产物 → 向产物方向移动 → 生成更多产物。
增加产物 → 向反应物方向移动。

4. Pressure and Volume Changes | 压力与体积变化

Changing pressure or volume only affects equilibrium systems that contain gases. According to Le Chatelier’s Principle, increasing pressure (by decreasing volume) shifts the equilibrium toward the side with fewer moles of gas.

改变压力或体积只影响含有气体的平衡体系。根据勒夏特列原理，增大压力（通过减小体积）会使平衡向气体物质的量较少的一侧移动。

Consider the reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The left side has 4 moles of gas, while the right side has 2 moles. Increasing pressure favours the forward reaction, producing more ammonia.

考虑反应：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)。左侧有4摩尔气体，右侧有2摩尔。增大压力有利于正反应，生成更多氨。

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

If the number of moles of gas is equal on both sides, a change in pressure will not shift the equilibrium. Adding an inert gas at constant volume does not change the partial pressures of the reacting species, so the equilibrium is unaffected.

如果两侧气体物质的量相等，则改变压力不会使平衡移动。在恒容条件下加入惰性气体，不会改变反应物的分压，因此平衡不受影响。

5. Temperature Changes | 温度变化

Temperature affects the value of the equilibrium constant itself. When the temperature is increased, the equilibrium shifts in the endothermic direction to absorb the added heat. When temperature is decreased, it shifts in the exothermic direction.

温度会影响平衡常数本身。升高温度时，平衡向吸热方向移动以吸收额外热量；降低温度时，平衡向放热方向移动。

For the exothermic formation of ammonia, increasing temperature decreases the yield of NH₃ because the reverse endothermic reaction is favoured. However, industrial conditions often use a compromise temperature of around 450 °C to achieve a reasonable rate.

对于放热的合成氨反应，升高温度会降低NH₃的产率，因为逆向吸热反应被促进。然而，工业条件常采用约450 °C的折中温度，以获得合理的反应速率。

6. Catalysts and Equilibrium | 催化剂与平衡

Catalysts increase the rate of both the forward and reverse reactions equally, so they do not shift the position of equilibrium. They only help the system reach equilibrium faster.

催化剂同等程度地加快正反应和逆反应的速率，因此不会改变平衡位置。它们只是帮助体系更快地达到平衡。

In an equilibrium graph, a catalyst causes the concentrations to level off sooner, but the final equilibrium concentrations remain identical to those without a catalyst.

在平衡曲线图中，催化剂使浓度更早趋于平稳，但最终的平衡浓度与没有催化剂时完全相同。

Eₐ(forward) and Eₐ(reverse) both decrease by the same amount.

正反应和逆反应的活化能Eₐ降低相同的量。

7. Reaction Quotient Q vs Equilibrium Constant K | 反应商Q与平衡常数K

The reaction quotient Q is calculated using the same expression as K but with initial or non-equilibrium concentrations. Comparing Q with K tells us the direction of shift needed to reach equilibrium.

反应商Q用与K相同的表达式计算，但代入的是初始或非平衡浓度。比较Q与K可以判断达到平衡所需移动的方向。

For a general reaction: aA + bB ⇌ cC + dD,

对于一般反应：aA + bB ⇌ cC + dD，

K = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ)

If Q < K, the reaction proceeds forward to form more products. If Q > K, it proceeds in reverse. If Q = K, the system is at equilibrium.

如果Q < K，反应正向进行以生成更多产物。如果Q > K，反应逆向进行。如果Q = K，体系处于平衡状态。

8. Applying Le Chatelier’s Principle: Haber Process | 应用：哈伯法合成氨

The Haber process is the industrial synthesis of ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), with ΔH = -92 kJ mol⁻¹. Conditions are chosen to balance yield, rate, and cost.

哈伯法是工业合成氨的过程：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，ΔH = -92 kJ mol⁻¹。条件的选择需要在产率、速率和成本之间取得平衡。

High pressure (200 atm) favours the side with fewer moles of gas, increasing NH₃ yield.
Low temperature favours the exothermic forward reaction, but slows the rate; ~450 °C is a compromise.
Iron catalyst speeds up the reaction without affecting equilibrium position.

高压（200 atm）有利于气体物质的量较少的一侧，提高NH₃产率。
低温有利于放热正反应，但会降低速率；约450 °C是折中方案。
铁催化剂加快反应速率，但不影响平衡位置。

The table below summarises the effect of each condition on the equilibrium yield of ammonia.

下表总结了各条件对氨平衡产率的影响。

Condition	Effect on NH₃ yield	Reason
Increase pressure	Increase	Shifts to fewer gas moles
Increase temperature	Decrease	Endothermic reverse favoured
Add catalyst	No change	Both rates increased equally

9. Common Mistakes and Exam Tips | 常见错误与考试提示

IB examiners often report that students confuse the effects of pressure and concentration, or forget that catalysts do not shift equilibrium. Another common error is applying Le Chatelier’s Principle to solids and pure liquids incorrectly.

IB考官经常报告学生混淆压力和浓度的影响，或忘记催化剂不会移动平衡。另一个常见错误是错误地将勒夏特列原理应用于固体和纯液体。

Remember: changing the amount of a solid or pure liquid does not affect equilibrium because their activities are constant.
When comparing Q and K, always check the stoichiometric coefficients in the equilibrium expression.
For temperature changes, K changes; for concentration or pressure changes, K remains constant at constant temperature.

记住：改变固体或纯液体的量不影响平衡，因为它们的活度为常数。
比较Q和K时，务必检查平衡表达式中的化学计量系数。
温度改变会使K变化；在恒温下，浓度或压力改变不会改变K。

10. Summary | 总结

Le Chatelier’s Principle is a powerful framework for predicting equilibrium shifts. By understanding how concentration, pressure, volume, temperature, and catalysts affect a system, you can analyse industrial processes and exam questions with confidence.

勒夏特列原理是预测平衡移动的有力框架。通过理解浓度、压力、体积、温度和催化剂如何影响体系，你可以自信地分析工业过程和考试题目。

Pair this qualitative reasoning with the quantitative tool of Q vs K, and you will have a complete toolkit for tackling equilibrium problems in IB Chemistry.

将这种定性推理与Q与K的定量工具相结合，你就拥有了解决IB化学平衡问题的完整工具包。

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Le Chatelier’s Principle and Equilibrium Shifts | 勒夏特列原理与平衡移动