Chemical Equilibria, Le Chatelier's Principle and Kc | 化学平衡、勒夏特列原理与 Kc

📚 Chemical Equilibria, Le Chatelier's Principle and Kc | 化学平衡、勒夏特列原理与 Kc

In AQA A-Level Chemistry, Topic 1.7 develops the core idea that many reactions do not go to completion. Instead, a reversible reaction can reach a state of dynamic equilibrium in a closed system, where the forward and reverse reactions continue at equal rates. Understanding how equilibrium responds to changes in concentration, pressure and temperature is central to Le Chatelier's principle, while the equilibrium constant Kc gives a quantitative measure of the position of equilibrium.

在 AQA A-Level 化学中，主题 1.7 深入探讨一个核心概念：许多反应并不会进行到底。可逆反应在封闭体系中可以达到动态平衡，此时正反应和逆反应仍以相同速率持续进行。理解平衡如何响应浓度、压力和温度的变化是勒夏特列原理的核心，而平衡常数 Kc 则定量描述平衡的位置。

1. Reversible Reactions and Dynamic Equilibrium | 可逆反应与动态平衡

A reversible reaction is represented by the ⇌ symbol, e.g. N₂(g) + 3H₂(g) ⇌ 2NH₃(g). In a closed system, the forward reaction initially dominates, but as products build up, the reverse reaction speeds up.

可逆反应用 ⇌ 符号表示，例如 N₂(g) + 3H₂(g) ⇌ 2NH₃(g)。在封闭体系中，正反应起初占主导，但随着生成物积累，逆反应加快。

At dynamic equilibrium, the rate of the forward reaction equals the rate of the reverse reaction. The concentrations of reactants and products remain constant, but the reactions have not stopped.

达到动态平衡时，正反应速率等于逆反应速率。反应物和生成物的浓度保持恒定，但反应并未停止。

A closed system is essential because no matter or energy can escape; otherwise equilibrium cannot be established.

封闭体系是必要条件，因为物质或能量不能逸出，否则无法建立平衡。

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2. Le Chatelier's Principle: The Core Statement | 勒夏特列原理：核心表述

Le Chatelier's principle states that if a system at equilibrium is disturbed by changing the concentration, pressure or temperature, the position of equilibrium shifts to oppose that disturbance.

勒夏特列原理指出，如果平衡体系受到浓度、压力或温度的扰动，平衡位置会向削弱该扰动的方向移动。

This principle is qualitative. It predicts the direction of shift but not the rate or the new equilibrium position quantitatively.

该原理是定性的，它预测移动方向，但无法定量描述速率或新的平衡位置。

For example, adding a reactant shifts the equilibrium to the right to consume the added substance.

例如，加入反应物会使平衡向右移动，以消耗加入的物质。

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3. 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, favouring the forward reaction and producing more product.

如果增加反应物浓度，平衡向右移动，有利于正反应并生成更多产物。

If the concentration of a product is increased, the equilibrium shifts to the left, favouring the reverse reaction.

如果增加生成物浓度，平衡向左移动，有利于逆反应。

Removing a product as it forms shifts the equilibrium to the right, which is a common strategy in industrial processes to improve yield.

在生成物形成时不断移走它，会使平衡向右移动，这是工业过程中提高产率的常用策略。

Changing concentration does not change the value of Kc; only temperature can do that.

改变浓度不会改变 Kc 的值，只有温度才能改变 Kc。

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4. Effect of Pressure Changes | 压力变化的影响

Pressure changes only affect gaseous equilibria. Increasing the pressure shifts the equilibrium towards the side with fewer moles of gas.

压力变化只影响气体平衡。增大压力会使平衡向气体摩尔数较少的一侧移动。

For example, in N₂(g) + 3H₂(g) ⇌ 2NH₃(g), the right side has 2 moles of gas versus 4 moles on the left. Increasing pressure shifts the equilibrium to the right, increasing ammonia yield.

例如，N₂(g) + 3H₂(g) ⇌ 2NH₃(g) 中，右侧有 2 mol 气体而左侧有 4 mol。增大压力使平衡右移，提高氨的产率。

If both sides have the same number of gas moles, changing pressure has no effect on the position of equilibrium.

如果两侧气体摩尔数相同，改变压力对平衡位置没有影响。

Pressure changes do not alter Kc; the system re-establishes equilibrium with the same Kc at the same temperature.

压力变化不会改变 Kc；在相同温度下，体系重新建立平衡时 Kc 保持不变。

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

Increasing the temperature shifts the equilibrium in the endothermic direction, because that direction absorbs energy and opposes the rise in temperature.

升高温度使平衡向吸热方向移动，因为该方向吸收能量，从而抵消温度的升高。

Decreasing the temperature shifts the equilibrium in the exothermic direction, releasing energy to oppose the fall in temperature.

降低温度使平衡向放热方向移动，释放能量以抵消温度的下降。

Unlike concentration and pressure changes, temperature changes do alter the value of Kc.

与浓度和压力变化不同，温度变化会改变 Kc 的值。

For an exothermic forward reaction, increasing the temperature reduces Kc, so the equilibrium

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