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 dynamic state in which the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of reactants and products remain constant over time. Le Chatelier’s principle allows us to predict how a system at equilibrium responds to external disturbances, making it one of the most powerful tools in chemistry.

化学平衡是一种动态状态：正反应速率与逆反应速率相等，因此反应物和产物的浓度随时间保持不变。勒夏特列原理帮助我们预测平衡体系如何应对外界扰动，是化学中最有力的工具之一。

1. Dynamic Equilibrium | 动态平衡概述

In a closed system, a reversible reaction reaches dynamic equilibrium when the forward and reverse reactions occur at the same rate. At equilibrium, macroscopic properties such as concentration, pressure, and color are constant, but microscopic processes continue.

在封闭体系中，当一个可逆反应的正、逆反应速率相等时，就达到了动态平衡。此时浓度、压力、颜色等宏观性质保持不变，但微观过程仍在持续进行。

Equilibrium can only be reached in a closed system where no substances can escape.

只有在封闭体系中，物质无法逸出时，才能达到平衡。
The equilibrium position can be expressed by the equilibrium constant Kc or Kp.

平衡位置可以用平衡常数 Kc 或 Kp 表示。

2. What Is Le Chatelier’s Principle? | 勒夏特列原理是什么？

Le Chatelier’s principle states: If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change, so as to restore a new equilibrium.

勒夏特列原理指出：如果改变条件使动态平衡受到扰动，平衡位置会向抵消该变化的方向移动，从而建立新的平衡。

“If a stress is applied to a system at equilibrium, the system shifts to relieve that stress.”

“当平衡体系受到外界应力时，体系会向减轻该应力的方向移动。”

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

Increasing the concentration of a reactant favours the forward reaction, consuming some of the added reactant. Decreasing the concentration of a product also favours the forward reaction, because the system tries to produce more product.

增大反应物浓度会使平衡向正反应方向移动，消耗部分加入的反应物；降低产物浓度同样会使平衡向正反应方向移动，因为体系会努力生成更多产物。

For the reaction: N₂ + 3H₂ ⇌ 2NH₃

对于反应：N₂ + 3H₂ ⇌ 2NH₃

Adding N₂ or H₂ shifts equilibrium to the right, producing more NH₃.

加入 N₂ 或 H₂，平衡右移，生成更多 NH₃。
Removing NH₃ as it forms also shifts equilibrium to the right.

不断移走生成的 NH₃，也能使平衡右移。

4. Effect of Pressure and Volume Changes | 压力与体积变化的影响

Pressure changes affect equilibrium only when the number of moles of gaseous reactants differs from that of gaseous products. Increasing pressure shifts equilibrium toward the side with fewer moles of gas; decreasing pressure shifts it toward the side with more moles of gas.

只有当气态反应物与气态产物的物质的量不同时，压力变化才会影响平衡。增大压力平衡向气体分子数较少的方向移动；减小压力则向气体分子数较多的方向移动。

Example: 2SO₂ + O₂ ⇌ 2SO₃

示例：2SO₂ + O₂ ⇌ 2SO₃

Condition	Moles before (left)	Moles after (right)	Shift direction
Increase pressure	3 mol	2 mol	Right (forward)
Decrease pressure	3 mol	2 mol	Left (reverse)

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

Temperature changes alter the value of the equilibrium constant itself. For an exothermic forward reaction (ΔH < 0), increasing temperature favours the reverse reaction; decreasing temperature favours the forward reaction. The opposite is true for an endothermic forward reaction (ΔH > 0).

温度变化会改变平衡常数的数值。对于正向放热反应（ΔH < 0），升高温度使平衡向逆反应方向移动；降低温度使平衡向正反应方向移动。对于正向吸热反应（ΔH > 0），情况相反。

Consider: N₂O₄(g) ⇌ 2NO₂(g) ΔH = +58 kJ mol⁻¹

考虑：N₂O₄(g) ⇌ 2NO₂(g) ΔH = +58 kJ mol⁻¹

Heating the mixture turns it darker brown because more NO₂ is formed.

加热混合物使其颜色变深，因为生成了更多 NO₂。
Cooling the mixture turns it paler because N₂O₄ is favoured.

冷却混合物使其颜色变浅，因为有利于生成 N₂O₄。

6. Catalysts and Equilibrium | 催化剂与平衡

A catalyst speeds up both the forward and reverse reactions equally. It does not change the position of equilibrium, nor does it change the value of Kc or Kp. A catalyst only helps the system reach equilibrium faster.

催化剂同等程度地加快正反应和逆反应，它不改变平衡位置，也不改变 Kc 或 Kp 的数值，只是帮助体系更快地达到平衡。

Catalyst: no shift in equilibrium, only a shorter time to reach equilibrium.

催化剂：不改变平衡位置，只缩短达到平衡所需的时间。

7. Effect of Inert Gases | 惰性气体的影响

Adding an inert gas (e.g., He or Ar) at constant volume does not change the partial pressures of the reacting gases, so the equilibrium position is unaffected. However, adding an inert gas at constant total pressure increases the total volume, which reduces the partial pressures of the reacting gases and can shift equilibrium in the direction that produces more gas molecules.

在恒容条件下加入惰性气体（如 He 或 Ar），不会改变反应气体的分压，因此平衡位置不受影响。但在恒总压条件下加入惰性气体会增大总体积，从而降低反应气体的分压，可能使平衡向气体分子数增多的方向移动。

8. 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 Kc or Kp, but with concentrations or pressures at any instant, not necessarily at equilibrium. Comparing Q with K predicts the direction of shift:

反应商 Q 使用与 Kc 或 Kp 相同的表达式计算，但代入的是任意时刻的浓度或压力，而不一定是平衡时刻。比较 Q 与 K 可以预测移动方向：

If Q < K, the reaction proceeds forward to reach equilibrium.

若 Q < K，反应正向进行以达到平衡。
If Q > K, the reaction proceeds in reverse to reach equilibrium.

若 Q > K，反应逆向进行以达到平衡。
If Q = K, the system is already at equilibrium.

若 Q = K，体系已经处于平衡。

9. Industrial Applications: Haber and Contact Processes | 工业应用：哈伯法与接触法

Le Chatelier’s principle guides the optimisation of industrial chemical processes.

勒夏特列原理指导工业化学过程的优化。

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

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

High pressure (200 atm) favours the forward reaction because the product side has fewer gas moles.

高压（200 atm）有利于正向反应，因为产物侧气体分子数较少。
Low temperature favours the forward reaction (exothermic), but a compromise temperature of about 450 °C is used to maintain a fast rate.

低温有利于正向反应（放热），但工业上采用约 450 °C 的折中温度以保持较快的反应速率。
Iron catalyst is used to increase the rate without changing the equilibrium position.

使用铁催化剂加快速率，但不改变平衡位置。

Contact process: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −196 kJ mol⁻¹

接触法：2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −196 kJ mol⁻¹

Excess oxygen is used to drive the equilibrium toward SO₃.

使用过量氧气使平衡向 SO₃ 方向移动。
A moderate temperature of 450 °C and V₂O₅ catalyst provide a practical compromise.

450 °C 的适中温度和 V₂O₅ 催化剂提供了实用折中方案。

10. Worked Example | 例题精讲

Example: For the reaction 2A(g) + B(g) ⇌ 2C(g), ΔH is negative. Predict the effect of (a) increasing pressure, (b) increasing temperature, (c) adding a catalyst.

例题：对于反应 2A(g) + B(g) ⇌ 2C(g)，ΔH 为负。预测：(a) 增大压力；(b) 升高温度；(c) 加入催化剂的影响。

Answer:

答案：

(a) Left side has 3 mol gas, right side has 2 mol gas. Increasing pressure shifts equilibrium to the right.

(a) 左侧有 3 mol 气体，右侧有 2 mol 气体。增大压力使平衡向右移动。
(b) Since forward reaction is exothermic, increasing temperature shifts equilibrium to the left.

(b) 因为正向反应放热，升高温度使平衡向左移动。
(c) Adding a catalyst has no effect on the equilibrium position.

(c) 加入催化剂对平衡位置没有影响。

11. Common Misconceptions | 常见误区

Misconception: A catalyst changes the equilibrium position. Truth: A catalyst only speeds up both directions equally.

误区：催化剂会改变平衡位置。事实：催化剂只同等程度地加快正逆反应。
Misconception: Increasing pressure always shifts equilibrium. Truth: Only if the total moles of gas change between reactants and products.

误区：增大压力总是使平衡移动。事实：只有反应物与产物的气体总物质的量不同时才有效。
Misconception: Equilibrium means the reaction stops. Truth: Forward and reverse reactions continue at equal rates.

误区：平衡意味着反应停止。事实：正逆反应仍在进行，只是速率相等。

12. Summary | 总结

Le Chatelier’s principle is essential for predicting how an equilibrium system responds to changes in concentration, pressure, and temperature. It also explains the design of industrial processes such as the Haber and Contact processes. Always compare Q with K for quantitative predictions, and remember that a catalyst does not affect the equilibrium position.

勒夏特列原理是预测平衡体系如何响应浓度、压力和温度变化的关键，也解释了哈伯法、接触法等工业过程的设计原理。进行定量预测时，务必比较 Q 与 K；同时记住催化剂不影响平衡位置。

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