Changing the Position of Equilibrium | 改变平衡位置

📚 Changing the Position of Equilibrium | 改变平衡位置

In a reversible reaction, the forward and reverse reactions never stop. At equilibrium, they proceed at exactly the same rate, so the macroscopic concentrations of reactants and products remain constant. However, equilibrium is dynamic and can be disturbed by changing concentration, pressure or temperature. The position of equilibrium then shifts to a new set of equilibrium concentrations. This article explains how to predict the direction of that shift using Le Chatelier’s principle and how equilibrium constants can help you decide whether a shift has occurred.

在可逆反应中，正反应和逆反应从未停止。达到平衡时，两者速率完全相等，因此反应物和产物的宏观浓度保持不变。但平衡是动态的，改变浓度、压强或温度都会破坏平衡。平衡位置随之移动，形成新的一组平衡浓度。本文将解释如何利用勒夏特列原理预测移动方向，以及如何利用平衡常数判断平衡位置是否发生移动。

1. What Is the Position of Equilibrium? | 什么是平衡位置？

The position of equilibrium describes the relative amounts of reactants and products in an equilibrium mixture. If a mixture contains a high proportion of products, the position lies to the right. If it contains mostly reactants, the position lies to the left. It is important to distinguish this from the rate of reaction: a fast reaction can still have a left-lying equilibrium, and a slow reaction can have a right-lying equilibrium. At A-level, “changing the position of equilibrium” means shifting the balance between reactants and products, not necessarily making the reaction faster.

平衡位置描述平衡混合物中反应物与产物的相对含量。若混合物中产物比例很高，称平衡位置偏右；若主要含反应物，则偏左。必须将其与反应速率区分开：快速反应也可能平衡偏左，慢速反应也可能平衡偏右。在 A-level 中，“改变平衡位置”指的是改变反应物与生成物之间的比例，而不一定加快反应。

2. Le Chatelier’s Principle | 勒夏特列原理

Le Chatelier’s principle states that if a system at equilibrium is subjected to a change in concentration, pressure or temperature, the position of equilibrium will shift in the direction that tends to oppose the change.

勒夏特列原理指出：如果一个处于平衡状态的体系受到浓度、压强或温度的改变，平衡位置将向削弱这种改变的方向移动。

This is a rule of thumb rather than a fundamental proof. It works because the forward and reverse rates are affected differently by the disturbance. For example, adding a reactant momentarily increases the forward rate, so the system consumes the added reactant and shifts to the right.

这是一条经验规律，而不是根本性的证明。其原理在于正、逆反应速率受扰动的程度不同。例如加入反应物会暂时加快正反应，体系因此消耗掉加入的反应物，平衡向右移动。

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

Suppose the equilibrium is A + B ⇌ C + D. If you add more A, the system opposes the increase in A by converting some A and B into C and D; the position shifts right. If you remove C as it forms, the system opposes the loss by making more C; again the position shifts right. Adding C or D shifts the position left.

假设平衡为 A + B ⇌ C + D。若增加 A，体系通过将部分 A 和 B 转化为 C 和 D 来减弱 A 的增加，平衡位置向右移动。若随着生成不断移除 C，体系通过生成更多 C 来抵消损失，平衡位置同样向右移动。加入 C 或 D 则使平衡位置向左移动。

In terms of Kc, concentration changes do not alter the equilibrium constant. The system adjusts concentrations until the ratio [products]/[reactants] returns to the same Kc value at constant temperature.

就 Kc 而言，浓度变化不会改变平衡常数。体系会调整各物质浓度，使 [产物]/[反应物] 的比值在温度不变时回到相同的 Kc 值。

4. Effect of Pressure Changes | 压强变化的影响

Changing pressure only affects equilibria involving gases, and only when there is a change in the total number of gas moles between reactants and products. Increasing pressure shifts the position toward the side with fewer gas moles to reduce the pressure. Decreasing pressure shifts toward the side with more gas moles. If the number of gas moles is the same on both sides, pressure has no effect on the position.

压强变化仅影响有气体参与的平衡，且只有反应物与产物气体总物质的量不同时才会移动。增大压强使平衡向气体物质的量较少的一侧移动以减小压强。减小压强则向气体物质的量较多的一侧移动。若两侧气体物质的量相等，压强对平衡位置无影响。

For example, in 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) the left side has 3 mol of gas and the right side has 2 mol. Increasing pressure therefore shifts the position to the right, favouring SO₃.

例如，在 2SO₂(g) + O₂(g) ⇌ 2SO₃(g) 中，左侧有 3 mol 气体，右侧有 2 mol。增大压强因此使平衡位置向右移动，有利于生成 SO₃。

Kp is unaffected by pressure changes. The partial pressures change initially, but the system adjusts until the ratio of partial pressures returns to the same Kp value at constant temperature.

Kp 不受压强变化影响。分压起初会改变，但体系会不断调整，直到分压比值回到恒温下相同的 Kp 值。

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

Temperature is the only one of the three common changes that alters the value of the equilibrium constant. Increasing temperature shifts the position in the endothermic direction because that helps absorb the added heat. Decreasing temperature shifts the position in the exothermic direction. For the exothermic forward reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹, raising the temperature shifts the position left and reduces the equilibrium yield of ammonia; lowering the temperature favours ammonia formation.

温度是三种常见改变中唯一会改变平衡常数数值的因素。升高温度使平衡向吸热方向移动，因为这样可以吸收加入的热量。降低温度使平衡向放热方向移动。对于正反应放热的 N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，ΔH = −92 kJ mol⁻¹，升高温度平衡位置左移，氨的平衡产率下降；降低温度则有利于生成氨。

If the forward reaction is exothermic, Kc decreases as temperature rises. If the forward reaction is endothermic, Kc increases as temperature rises.

如果正反应放热，Kc 随温度升高而减小；如果正反应吸热，Kc 随温度升高而增大。

6. Effect of Catalysts | 催化剂的影响

A catalyst provides an alternative reaction pathway with a lower activation energy for both the forward and reverse reactions. It increases the rates of the forward and reverse reactions by the same factor, so it does not change the position of equilibrium and does not change the equilibrium constant. A catalyst simply allows equilibrium to be reached faster. This is often confused with increasing yield: a catalyst does not increase the equilibrium yield.

催化剂通过提供活化能更低的替代反应路径，同等加快正反应和逆反应。由于正、逆反应速率按相同倍数增加，它不会改变平衡位置，也不会改变平衡常数。催化剂只是让体系更快达到平衡。这一点常与产率混淆：催化剂不会提高平衡产率。

7. Equilibrium Constants and Position | 平衡常数与平衡位置

For a general homogeneous reaction aA + bB ⇌ cC + dD, the equilibrium constant Kc is expressed as:

对于一般均相反应 aA + bB ⇌ cC + dD，平衡常数 Kc 表示为:

Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ

The position of equilibrium tells you the actual concentrations; the constant tells you their ratio. If Kc is large, the position lies well to the right. If Kc is small, it lies to the left. Changing concentration or pressure changes the position but not Kc. Changing temperature changes both the position and Kc.

平衡位置反映实际浓度；平衡常数反映它们的比值。Kc 很大时平衡位置明显偏右；Kc 很小时偏左。改变浓度或压强会改变平衡位置但不改变 Kc。改变温度则同时改变平衡位置和 Kc。

8. Using the Reaction Quotient Q to Predict Shifts | 用反应商 Q 预测移动方向

The reaction quotient Q has exactly the same form as Kc but uses the current, non-equilibrium concentrations. When Q < Kc, the forward reaction is favoured and the position shifts right until Q = Kc. When Q > Kc, the reverse reaction is favoured and the position shifts left. When Q = Kc, the system is at equilibrium. This provides a more quantitative explanation than Le Chatelier’s principle and is especially useful for concentration changes.

反应商 Q 的表达式与 Kc 完全相同，但代入的是当前非平衡浓度。当 Q < Kc 时，正反应占优势，平衡位置向右移动，直到 Q = Kc。当 Q > Kc 时，逆反应占优势，平衡位置向左移动。当 Q = Kc 时体系处于平衡。这一方法比勒夏特列原理更定量，尤其适用于浓度变化。

For example, adding a reactant makes Q smaller than Kc, so the system consumes reactants and forms more products until Q rises back to Kc.

例如，加入反应物会使 Q 小于 Kc，于是体系消耗反应物并生成更多产物，直到 Q 回升至 Kc。

9. Industrial Applications: Haber Process and Contact Process | 工业应用：哈伯法和接触法

In the Haber process for ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹. The forward reaction is exothermic and reduces gas moles from 4 to 2. Le Chatelier’s principle predicts high pressure and low temperature give the highest equilibrium yield. In practice, conditions are about 200 atm and 400–450 °C with an iron catalyst. Low temperature would give a better yield but the reaction would be too slow; the catalyst allows a moderate temperature to be used, giving a reasonable yield and rate.

哈伯法合成氨：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，ΔH = −92 kJ mol⁻¹。正反应放热，且气体物质的量由 4 减少到 2。勒夏特列原理预测高压和低温可获得最高平衡产率。实际条件约为 200 atm 和 400–450 °C，并使用铁催化剂。低温虽能提高产率但反应太慢；催化剂允许使用中等温度，从而兼顾产率和速率。

In the Contact process: 2

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Changing the Position of Equilibrium | 改变平衡位置