Dynamic Equilibrium | 动态平衡

📚 Dynamic Equilibrium | 动态平衡

In the study of IGCSE Science, dynamic equilibrium is a key concept that appears in chemistry, physics and biology. It describes a state of balance where forward and reverse processes occur at the same rate, so the observable properties of the system stay constant.

在IGCSE科学的学习中，动态平衡是一个关键概念，出现在化学、物理和生物中。它描述了一种平衡状态，其中正向和反向过程以相同速率进行，因此系统的可观察性质保持不变。

1. What is Dynamic Equilibrium? | 什么是动态平衡？

Dynamic equilibrium occurs in a closed system when two opposite reactions and/or physical processes take place at equal rates. The amounts of substances involved do not change overall, but at the molecular level the processes are still actively happening.

动态平衡发生在封闭系统中，当两个相反的反应和/或物理过程以相等速率进行时。所涉及物质的总体数量不变，但在分子层面上这些过程仍活跃进行。

A common misconception is that equilibrium means the reactants and products are present in equal amounts. In fact, they simply remain constant in concentration.

一个常见误解是平衡意味着反应物与生成物的量相等。实际上，它们只是浓度保持不变。

2. Reversible Reactions | 可逆反应

Dynamic equilibrium is only possible in reversible reactions, which can proceed in both the forward and backward directions. These are shown using the symbol ⇌.

动态平衡只可能在可逆反应中出现，这类反应能同时向正方向与反方向进行，用符号⇌表示。

For example, the decomposition of hydrated copper(II) sulphate is reversible and reaches equilibrium under certain conditions.

例如，水合硫酸铜的分解反应是可逆的，在特定条件下会达到平衡。

CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)

This equation shows a typical reversible reaction where heating drives it one way and adding water drives it back.

这个方程展示了一个典型的可逆反应，加热使反应向一个方向进行，加水则使其返回。

3. Dynamic vs Static Equilibrium | 动态平衡与静态平衡

Static equilibrium is a state where no change occurs because there is no ongoing activity. A book resting on a table is in static equilibrium.

静态平衡是指没有任何活动发生，因此没有变化的平衡状态。例如放在桌子上的书就处于静态平衡。

Dynamic equilibrium, however, involves continuous microscopic change. Both directions are active, but their effects cancel out.

而动态平衡涉及微观上持续的变化。两个方向都在活动，但效果相互抵消。

In a closed flask containing water and water vapour, evaporation and condensation happen continually; when their rates become equal, the system is in dynamic equilibrium.

在含有液态水和蒸气的气密烧瓶中，蒸发与凝结持续进行；当速率相等时，系统就处于动态平衡。

4. The Closed System | 封闭系统

A closed system is essential for dynamic equilibrium because matter cannot enter or leave it. Only energy can be exchanged with the surroundings in a closed system.

封闭系统对动态平衡至关重要，因为物质不能进出，只有能量可以与周围环境交换。

If the system is open, products or reactants may escape, and the equilibrium will never be established.

如果系统是敞开的，生成物或反应物可能逸散，平衡将永远无法建立。

In laboratory, a stoppered conical flask can be considered a closed system for many reactions.

在实验室中，带塞的锥形瓶对许多反应而言可视为封闭系统。

5. Le Chatelier’s Principle | 勒夏特列原理

Le Chatelier’s principle states that if a system at equilibrium is disturbed, the position of equilibrium shifts in the direction that reduces the disturbance.

勒夏特列原理指出，如果平衡系统受到扰动，平衡位置会朝减小扰动的方向移动。

This principle helps predict the effect of changes in concentration, pressure, and temperature on a system.

该原理有助于预测浓度、压力和温度变化对系统的影响。

Increasing concentration of reactants shifts equilibrium to the right.
增加反应物浓度会使平衡向右移动。
Increasing pressure shifts equilibrium toward the side with fewer gas molecules.
增加压力会使平衡向气体分子数较少的一侧移动。
Increasing temperature shifts equilibrium in the endothermic direction.
升高温度会使平衡向吸热方向移动。

6. Factors Affecting Equilibrium | 影响平衡的因素

The following table summarises how different disturbances affect the position of equilibrium for a general reaction:

下表总结了不同扰动对一般反应平衡位置的影响：

Change	Effect
Increase [reactant]	Shifts right
Decrease [product]	Shifts right
Increase pressure	Shifts to fewer gas molecules
Increase temperature	Shifts in endothermic direction
Add a catalyst	No shift; equilibrium reached faster

中文翻译：下表总结了不同扰动对一般反应平衡位置的影响：

增加反应物浓度→右移；减少生成物浓度→右移；增加压力→移向气体分子数更少的一侧；升高温度→移向吸热方向；加入催化剂→不移动，但更快达到平衡。

7. The Equilibrium Constant K_c | 平衡常数K_c

For a gaseous or aqueous reaction at equilibrium, the equilibrium constant K_c is defined by the ratio of product concentrations to reactant concentrations, each raised to the power of its stoichiometric coefficient.

对于处于平衡的气态或溶液反应，平衡常数K_c定义为生成物浓度与反应物浓度的比值，每个浓度均按其化学计量系数取幂。

K_c = [C]^c[D]^d / [A]^a[B]^b

Here, aA + bB ⇌ cC + dD. Square brackets indicate concentration in mol dm⁻³.

这里，aA + bB ⇌ cC + dD。方括号表示浓度，单位为 mol dm⁻³。

A large K_c indicates the equilibrium favours products; a small K_c indicates it favours reactants.

K_c值大表明平衡偏向生成物；K_c值小则表明平衡偏向反应物。

The value of K_c is only affected by temperature, not by changes in pressure or concentration.

K_c的值只受温度影响，不受压力或浓度变化的影响。

8. Industrial Applications | 工业应用

Dynamic equilibrium is crucial in industrial processes like the Haber process and the Contact process.

动态平衡在哈伯法和接触法等工业过程中至关重要。

In the Haber process, nitrogen and hydrogen react to form ammonia:

哈伯法中，氮气和氢气反应生成氨：

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

The forward reaction is exothermic. High pressure (200 atm) favours the formation of ammonia because the product side has fewer gas molecules. A moderate temperature of 450°C is used as a compromise between yield and rate.

正反应是放热的。高压（200 atm）有利于氨的生成，因为生成物一侧气体分子数更少。使用450°C的适中温度是产率与速率之间的折衷。

An iron catalyst is used to speed up the attainment of equilibrium, without changing the equilibrium position.

使用铁催化剂以加速达到平衡，而不改变平衡位置。

9. Everyday Examples | 日常实例

Many everyday phenomena involve dynamic equilibrium. For instance, in a sealed bottle of carbonated drink, there is equilibrium between dissolved CO₂ and gaseous CO₂ in the headspace.

许多日常现象涉及动态平衡。例如，在密封的碳酸饮料瓶中，溶解的CO₂与瓶顶空间中的气态CO₂之间存在平衡。

When you open the bottle, the pressure decreases, and equilibrium shifts to produce more gas, which is why it fizzes.

当你打开瓶盖时，压力降低，平衡向产生更多气体的方向移动，这就是为什么会冒泡。

In the human body, oxygen binding to haemoglobin is also a reversible process and reaches dynamic equilibrium under steady conditions.

在人体内，氧与血红蛋白的结合也是可逆过程，在稳定条件下会达到动态平衡。

10. Key Points for IGCSE Exams | IGCSE考试要点

When answering equilibrium questions, always check three conditions for a system to be in dynamic equilibrium: it must be closed, the rates of forward and reverse processes must be equal, and macroscopic concentrations or properties must be constant.

回答平衡问题时，务必检查系统达到动态平衡的三个条件：必须是封闭系统、正逆过程的速率相等、宏观浓度或性质恒定。

Remember that catalysts do not affect the position of equilibrium; they only reduce the time needed to reach equilibrium.

记住催化剂不影响平衡位置，只缩短达到平衡所需的时间。

Use Le Chatelier’s principle to explain shifts, and be careful with the sign of ΔH: treat temperature changes correctly.

运用勒夏特列原理解释移动方向，并小心ΔH的符号：正确看待温度变化的影响。

Practice writing K_c expressions with the correct exponents, and always include states of matter if asked.

练习书写带正确指数的K_c表达式，若题目要求，记得标出状态。

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Dynamic Equilibrium | 动态平衡