📚 Chemical Equilibrium and Le Chatelier’s Principle | 化学平衡与勒夏特列原理
Chemical equilibrium is a core concept in A-Level Chemistry. It describes the state of a reversible reaction where the forward and reverse reactions proceed at the same rate. Understanding equilibrium helps chemists predict how a system responds to changes in concentration, pressure and temperature. This article covers the key principles, equilibrium constant calculations and industrial applications, especially the Haber process.
化学平衡是 A-Level 化学的核心概念。它描述可逆反应中正反应和逆反应速率相等的状态。理解平衡有助于化学家预测系统对浓度、压强和温度变化的响应。本文涵盖关键原理、平衡常数计算以及工业应用，尤其是哈伯法。
1. Dynamic Equilibrium | 动态平衡
A reversible reaction is one that can proceed in both the forward and reverse directions. When the rate of the forward reaction equals the rate of the reverse reaction, the system has reached dynamic equilibrium.
可逆反应是指可以同时向正方向和逆方向进行的反应。当正反应速率等于逆反应速率时，系统达到动态平衡。
At equilibrium, the concentrations of reactants and products remain constant, but the reactions have not stopped. Both forward and reverse reactions continue at the molecular level, which is why we call it dynamic.
在平衡状态下，反应物和产物的浓度保持不变，但反应并未停止。正反应和逆反应在分子水平上仍在继续，因此我们称之为动态平衡。
Dynamic equilibrium can only be established in a closed system where no substances escape. If a product gas is allowed to leave, the forward reaction will continue until one reactant is used up.
动态平衡只能在封闭系统中建立，不能有任何物质逸出。如果允许产物气体逸出，正反应将继续进行，直到某种反应物耗尽。
2. The Equilibrium Constant Kc | 平衡常数 Kc
For a general homogeneous reaction aA + bB ⇌ cC + dD, the equilibrium constant Kc is expressed as Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ, where [ ] represents the equilibrium concentration in mol dm⁻³.
对于一般均相反应 aA + bB ⇌ cC + dD，平衡常数 Kc 表达为 Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ，其中 [ ] 表示以 mol dm⁻³ 为单位的平衡浓度。
Kc = [C]ᶜ[D]ᵈ / [A]ᵃ[B]ᵇ
Solids and pure liquids are omitted from the Kc expression because their concentrations are effectively constant. Only aqueous solutions and gases appear in the quotient.
固体和纯液体从 Kc 表达式中省略，因为它们的浓度实际上是常数。只有水溶液和气体出现在分式中。
A larger Kc value means the equilibrium position lies further to the right, favouring products. A smaller Kc means the equilibrium favours reactants.
Kc 值越大，说明平衡位置越偏右，有利于产物。Kc 值越小，说明平衡有利于反应物。
3. 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 equilibrium position shifts to oppose the change.
勒夏特列原理指出，如果处于平衡状态的系统受到浓度、压强或温度的变化，平衡位置会移动以对抗这种变化。
This principle is a powerful qualitative tool. It does not explain why the shift occurs at the molecular level, but it accurately predicts the direction of change.
该原理是一个强大的定性工具。它并不解释分子水平上发生移动的原因，但能准确预测变化的方向。
Catalysts do not shift the equilibrium position. They increase the rates of both forward and reverse reactions equally, so equilibrium is reached faster.
催化剂不会使平衡位置移动。它们同等程度地加快正反应和逆反应的速率，因此平衡能更快达到。
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 right to consume the added reactant. This reduces the concentration of reactants and increases the products.
如果增加反应物的浓度，平衡向右移动以消耗加入的反应物。这降低了反应物浓度并增加了产物。
If a product is removed, the equilibrium also shifts to the right to replace the removed product. Removing a product is a common way to drive a reversible reaction towards completion.
如果移走产物，平衡也会向右移动以补充被移走的产物。移除产物是推动可逆反应趋于完成的一种常用方法。
Changing concentration does not change the value of Kc. The system adjusts the concentrations until the same Kc value is restored.
改变浓度不会改变 Kc 的值。系统会调整各物质的浓度，直到恢复相同的 Kc 值。
5. Effect of Pressure Changes | 压强变化的影响
Pressure changes only affect equilibria involving gases. If the pressure is increased, the equilibrium shifts to the side with fewer gas molecules, reducing the total pressure.
压强变化只影响涉及气体的平衡。如果增大压强，平衡会向气体分子数较少的一侧移动，从而降低总压强。
For example, in the reaction N₂(g) + 3H₂(g) ⇌ 2NH₃(g), there are 4 moles of gas on the left and 2 moles on the right. Increasing pressure shifts the equilibrium to the right.
例如，在反应 N₂(g) + 3H₂(g) ⇌ 2NH₃(g) 中，左侧有 4 mol 气体，右侧有 2 mol。增大压强会使平衡向右移动。
If both sides have the same number of gas molecules, changing pressure has no effect on the equilibrium position. Pressure changes also do not alter Kc, as long as temperature remains constant.
如果两侧气体分子数相同，改变压强对平衡位置没有影响。只要温度保持不变，压强变化也不会改变 Kc。
6. Effect of Temperature Changes | 温度变化的影响
Temperature is the only factor that changes the value of Kc. If the forward reaction is exothermic, increasing temperature shifts the equilibrium to the left, favouring the endothermic reverse reaction.
温度是唯一能改变 Kc 值的因素。如果正反应是放热反应，升高温度会使平衡向左移动，有利于吸热的逆反应。
If the forward reaction is endothermic, increasing temperature shifts the equilibrium to the right. Decreasing temperature has the opposite effect in each case.
如果正反应是吸热反应，升高温度会使平衡向右移动。在每种情况下，降低温度会产生相反的效果。
For an exothermic forward reaction, Kc decreases as temperature rises. For an endothermic forward reaction, Kc increases with rising temperature.
对于放热正反应，Kc 随温度升高而减小。对于吸热正反应，Kc 随温度升高而增大。
7. Effect of Catalysts | 催化剂的影响
A catalyst provides an alternative reaction pathway with a lower activation energy. It speeds up both the forward and reverse reactions by the same factor.
催化剂提供了一条活化能较低的反应路径。它以相同的倍数加快正反应和逆反应的速率。
Because the rates of both directions increase equally, the equilibrium position does not change. The catalyst only reduces the time needed to reach equilibrium.
由于两个方向的速率同等增加，平衡位置不会改变。催化剂只缩短达到平衡所需的时间。
In industry, catalysts are essential for making reactions economically viable. They allow lower temperatures to be used while still achieving a reasonable rate.
在工业中，催化剂对于使反应在经济上可行至关重要。它们允许在较低温度下使用，同时仍能获得合理的反应速率。
8. Homogeneous vs Heterogeneous Equilibria | 均相与多相平衡
A homogeneous equilibrium involves reactants and products all in the same phase, such as all gases or all aqueous ions. Kc expressions include all species in that phase.
均相平衡涉及所有反应物和产物都处于同一相，例如全部为气体或全部为水合离子。Kc 表达式包括该相中的所有物种。
A heterogeneous equilibrium involves substances in different phases, such as solids with gases. Solids and pure liquids are excluded from the Kc expression.
多相平衡涉及不同相的物质，例如固体与气体。固体和纯液体不包含在 Kc 表达式中。
For example, in CaCO₃(s) ⇌ CaO(s) + CO₂(g), the equilibrium constant is simply Kp = P(CO₂) if expressed in pressure terms, or Kc = [CO₂]. The solid activities are constant.
例如，在 CaCO₃(s) ⇌ CaO(s) + CO₂(g) 中，平衡常数如果用分压表示就是 Kp = P(CO₂)，或者 Kc = [CO₂]。固体的活度是常数。
9. Industrial Applications: The Haber Process | 工业应用：哈伯法
The Haber process combines nitrogen and hydrogen to produce ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g), where the forward reaction is exothermic, ΔH = -92 kJ mol⁻¹.
哈伯法将氮气和氢气合成为氨：N₂(g) + 3H₂(g) ⇌ 2NH₃(g)，其中正反应是放热反应，ΔH = -92 kJ mol⁻¹。
The industrial conditions are about 450 °C and 200 atm with an iron catalyst. High pressure favours the product side because there are fewer gas molecules, but very high pressures are expensive and require thick pipes.
工业条件大约是 450 °C 和 200 atm，使用铁催化剂。高压有利于产物一侧，因为气体分子数较少，但过高的压强成本高且需要厚管道。
A moderate temperature is used despite the fact that lower temperatures would give a higher equilibrium yield. The lower temperature would make the reaction too slow, so a compromise temperature is chosen together with a catalyst.
尽管较低温度会带来更高的平衡产率，但仍采用中等温度。较低温度会使反应太慢，因此选择折中温度并配合催化剂。
The ammonia is continuously removed by cooling and liquefaction, shifting the equilibrium to the right and improving the overall yield.
通过冷却和液化不断移走氨，使平衡向右移动，从而提高总产率。
10. Common Misconceptions and Exam Tips | 常见误区与考试技巧
A common misconception is that catalysts shift the equilibrium towards products. In reality, catalysts do not change the equilibrium position or the value of Kc.
一个常见误区是认为催化剂使平衡向产物方向移动。实际上，催化剂不会改变平衡位置或 Kc 值。
Another error is writing Kc expressions that include solids or pure liquids. Always omit them and double-check the stoichiometric powers.
另一个错误是在 Kc 表达式中包含固体或纯液体。务必省略它们，并仔细检查化学计量数作为幂。
When explaining Le Chatelier shifts, always state the direction of shift and the reason in terms of opposing the change. Use ‘shifts to the right’ or ‘shifts to the left’ rather than vague phrases like ‘moves forward’.
在解释勒夏特列移动时，务必说明移动方向，并从对抗变化的角度说明原因。使用 ‘向右移动’ 或 ‘向左移动’，避免 ‘向前移动’ 等模糊表述。
For temperature changes, remember that only temperature affects Kc. Concentration and pressure changes alter the equilibrium position but not the value of Kc.
对于温度变化，请记住只有温度影响 Kc。浓度和压强变化会改变平衡位置，但不会改变 Kc 值。
| Change / 变化 | Equilibrium shift / 平衡移动 | Effect on Kc / 对 Kc 的影响 |
|---|---|---|
更多咨询请联系16621398022（同微信）
CommentsMore posts |
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导