📚 Le Chatelier’s Principle: Core Rules and Problem-Solving Applications | 化学平衡移动原理：核心规律与解题应用
Chemical equilibrium is a dynamic state in which the forward and reverse reactions proceed at equal rates, so the macroscopic concentrations of reactants and products remain constant over time. When a system at equilibrium is disturbed by a change in conditions, the system responds by shifting the position of equilibrium to counteract the disturbance. This qualitative rule, known as Le Chatelier’s Principle, is one of the most powerful tools for predicting the direction of equilibrium shifts and for optimising industrial reactions.
化学平衡是一种动态状态，正反应和逆反应以相同的速率进行，因此反应物和生成物的宏观浓度随时间保持不变。当处于平衡的体系因条件改变而受到干扰时，体系会通过移动平衡位置来抵消这种干扰。这一定性规则称为勒夏特列原理，是预测平衡移动方向和优化工业反应的最有力工具之一。
1. What Is Le Chatelier’s Principle? | 什么是勒夏特列原理？
Le Chatelier’s Principle states: if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change and a new equilibrium is established.
勒夏特列原理指出：如果一个动态平衡因条件改变而受到干扰，平衡位置会向抵消该改变的方向移动，并建立新的平衡。
It applies to changes in concentration, pressure, and temperature, but not to the addition of a catalyst. A catalyst only speeds up the attainment of equilibrium without shifting its position.
该原理适用于浓度、压强和温度的变化，但不适用于催化剂。催化剂只能加速平衡的到达，而不改变平衡位置。
It is a qualitative tool. For quantitative predictions, we use the equilibrium constant Kc and the reaction quotient Q.
这是一个定性工具。若要进行定量预测，我们使用平衡常数 Kc 和反应商 Q。
2. Effect of Concentration Change | 浓度变化的影响
Increasing the concentration of a reactant causes the equilibrium to shift to the right, favouring the forward reaction, in order to consume the added reactant. Conversely, increasing the concentration of a product shifts the equilibrium to the left.
增加反应物浓度会使平衡向右移动，有利于正反应，目的是消耗所加入的反应物；反之，增加生成物浓度会使平衡向左移动。
Decreasing the concentration of a product, for example by continuously removing it, shifts the equilibrium to the right. This is a common industrial strategy to improve the yield of a desired product.
降低生成物浓度，例如通过连续移走生成物，会使平衡向右移动。这是工业上提高目标产物产率的常用策略。
Note that adding a solid or a pure liquid does not change its concentration in the equilibrium expression, so it has no effect on the position of equilibrium.
注意，加入固体或纯液体不会改变其在平衡表达式中的浓度，因此对平衡位置没有影响。
3. Effect of Pressure Change | 压强变化的影响
For reactions involving gases, an increase in total pressure shifts the equilibrium towards the side with fewer moles of gas. A decrease in pressure shifts it towards the side with more moles of gas.
对于有气体参与的反应，增大总压强会使平衡向气体物质的量减少的一侧移动；减小压强则使平衡向气体物质的量增加的一侧移动。
If the number of moles of gas is the same on both sides of the equation, a change in pressure has no effect on the equilibrium position.
如果反应方程两侧气体总物质的量相等，则压强变化对平衡位置没有影响。
Pressure affects equilibrium by changing the partial pressures of the gaseous species, which in turn changes the reaction quotient Q relative to Kc.
压强通过改变气体组分的分压来影响平衡，分压的改变进而使反应商 Q 相对于 Kc 发生变化。
4. Effect of Temperature Change | 温度变化的影响
An increase in temperature shifts the equilibrium in the endothermic direction, which absorbs heat, while a decrease in temperature shifts it in the exothermic direction, which releases heat.
升高温度，平衡会向吸热方向移动以吸收热量；降低温度，平衡会向放热方向移动以释放热量。
Temperature is the only factor that changes the numerical value of the equilibrium constant Kc. For an exothermic forward reaction, raising the temperature decreases Kc; for an endothermic forward reaction, raising the temperature increases Kc.
温度是唯一能改变平衡常数 Kc 数值的因素。对于正向放热反应，升高温度会使 Kc 减小；对于正向吸热反应，升高温度会使 Kc 增大。
In exam questions, use the sign of ΔH to decide how Kc varies with temperature, and state the direction of shift separately using Le Chatelier’s Principle.
在考试中，应利用 ΔH 的正负号判断 Kc 随温度的变化，并单独使用勒夏特列原理说明平衡移动的方向。
5. Catalysts and Inert Gases | 催化剂与惰性气体
A catalyst lowers the activation energy for both the forward and reverse reactions by the same amount. Therefore, it speeds up the rate at which equilibrium is reached but does not change the equilibrium position.
催化剂同等程度地降低正反应和逆反应的活化能，因此它会加快平衡到达的速率，但不会改变平衡位置。
Adding an inert gas at constant volume does not change the partial pressures of the reacting gases, so the position of equilibrium is unaffected.
在恒容条件下加入惰性气体，不会改变反应气体的分压，因此平衡位置不受影响。
Adding an inert gas at constant pressure increases the total volume, which decreases the partial pressures of all gaseous species. If the reaction has unequal numbers of gas moles on both sides, the equilibrium will shift towards the side with more gas moles.
在恒压条件下加入惰性气体，会使总体积增大，从而降低所有气体组分的分压。若反应前后气体物质的量不等，平衡将向气体物质的量较多的一侧移动。
6. Equilibrium Constant Kc and Reaction Quotient Q | 平衡常数 Kc 与反应商 Q
For a general reaction aA + bB ⇌ cC + dD, the equilibrium constant is written as:
对于一般反应 aA + bB ⇌ cC + dD，平衡常数可写为：
Kc = [C]ᶜ[D]ᵈ / ([A]ᵃ[B]ᵇ)
where the concentrations are those measured at equilibrium.
其中浓度为平衡时的浓度。
The reaction quotient Q has the same algebraic expression as Kc, but it uses the concentrations measured at any moment, not necessarily at equilibrium.
反应商 Q 的代数表达式与 Kc 相同，但使用的是任意时刻测得的浓度，不一定是平衡浓度。
Q is a “snapshot” of the system. By comparing Q with Kc, we can predict the direction of the shift: Q < Kc means the reaction proceeds forward; Q > Kc means it proceeds in reverse; Q = Kc means the system is at equilibrium.
Q 是体系的“快照”。通过比较 Q 与 Kc，我们可以预测移动方向：Q < Kc 表示反应正向进行；Q > Kc 表示反应逆向进行；Q = Kc 表示体系处于平衡状态。
7. Using Q vs Kc to Determine Shift Direction | 用 Q 与 Kc 判断移动方向
In problem solving, always write the expression for Q using the current concentrations or partial pressures, then compare it with the tabulated Kc value at the same temperature.
解题时，先根据当前的浓度或分压写出 Q 的表达式，再与同温度下的 Kc 数值进行比较。
If Q < Kc, the product concentrations are too small and the reactant concentrations are too large, so the reaction shifts to the right until Q equals Kc.
若 Q < Kc，说明生成物浓度过小、反应物浓度过大，反应会向右移动，直到 Q 等于 Kc。
If Q > Kc, the products are in excess and the reaction shifts to the left.
若 Q > Kc，则生成物过量，反应会向左移动。
This method is more precise than applying Le Chatelier’s Principle alone, especially when multiple disturbances occur simultaneously.
这种方法比单独应用勒夏特列原理更精确，尤其当多个干扰同时发生时。
8. Pressure and Volume: Same Direction but Different Mechanism | 压强与体积：方向相同但机理不同
Changing the volume of a gas
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