📚 Reversible Reactions and Equilibrium | 可逆反应与平衡
In chemistry, many reactions do not proceed to completion but can go in both forward and reverse directions. These are called reversible reactions, and they lead to a state of dynamic equilibrium where the rates of the forward and reverse reactions are equal. Understanding equilibrium is fundamental to predicting the behaviour of chemical systems and optimising industrial processes.
在化学中，许多反应不会进行到底，而是可以同时在正方向和逆方向进行。这类反应称为可逆反应，它们会达到一种动态平衡状态，此时正反应和逆反应的速率相等。理解平衡对于预测化学系统的行为以及优化工业过程至关重要。
1. What are Reversible Reactions? | 什么是可逆反应？
A reversible reaction is one in which the products can react to reform the original reactants under the same conditions. The reaction is represented using a double arrow: ⇌. For example, the thermal decomposition of ammonium chloride: NH₄Cl(s) ⇌ NH₃(g) + HCl(g). The forward reaction is decomposition, and the reverse is recombination.
可逆反应是指产物在相同条件下可以重新反应生成原来的反应物。反应用双箭头 ⇌ 表示。例如，氯化铵的热分解：NH₄Cl(s) ⇌ NH₃(g) + HCl(g)。正反应是分解，逆反应是化合。
Not all reactions are reversible; many go essentially to completion, such as the combustion of hydrocarbons. Reversibility depends on the relative energies of reactants and products and the reaction conditions.
并非所有反应都是可逆的；很多反应基本上会进行到底，比如碳氢化合物的燃烧。可逆性取决于反应物和产物的相对能量以及反应条件。
2. Dynamic Equilibrium | 动态平衡
Dynamic equilibrium occurs in a closed system when the rates of the forward and reverse reactions become equal. At this point, the concentrations of all species remain constant over time, but the reactions are still occurring at the molecular level.
动态平衡发生在密闭系统中，当正反应和逆反应的速率相等时。此时，所有物质的浓度随时间保持不变，但分子水平上的反应仍在进行。
Key features of dynamic equilibrium include:
- It can only be established in a closed system.
- Macroscopic properties (colour, pressure, concentration) remain constant.
- It is dynamic – forward and reverse reactions continue at equal rates.
动态平衡的关键特征包括：
- 只能在密闭系统中建立。
- 宏观性质（颜色、压力、浓度）保持恒定。
- 是动态的——正反应和逆反应以相同速率继续进行。
For example, in the equilibrium: H₂(g) + I₂(g) ⇌ 2HI(g), the purple colour of iodine vapour remains constant once equilibrium is reached because I₂ is consumed and formed at the same rate.
例如，在平衡 H₂(g) + I₂(g) ⇌ 2HI(g) 中，一旦达到平衡，碘蒸气的紫色保持不变，因为 I₂ 的消耗和生成速率相同。
3. Le Chatelier's Principle | 勒夏特列原理
Le Chatelier's principle states: If a system at dynamic equilibrium is subjected to a change in concentration, pressure, or temperature, the position of equilibrium will shift in a direction that tends to counteract the imposed change.
勒夏特列原理指出：如果处于动态平衡的系统受到浓度、压力或温度的改变，平衡位置将朝着削弱这种改变的方向移动。
This principle allows us to predict qualitatively how changing conditions affect the equilibrium composition. It is a guiding rule, not a quantitative law, and it applies to all reversible reactions.
这个原理使我们能够定性地预测条件变化如何影响平衡组成。它是一条指导性规则，不是定量定律，适用于所有可逆反应。
4. Effect of Concentration Changes | 浓度变化的影响
If the concentration of a reactant is increased, the equilibrium shifts to the product side to use up the added reactant. Conversely, if a product is removed, the equilibrium shifts to produce more product. Adding more product or removing reactant shifts equilibrium to the left.
如果增加反应物的浓度，平衡就会向产物方向移动以消耗添加的反应物。相反，如果移走产物，平衡就会移动以产生更多产物。增加产物或移走反应物会使平衡向左移动。
For the equilibrium: Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) (blood-red), adding KSCN increases [SCN⁻], deepening the red colour as
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