📚 R2.3 How far? The extent of chemical change | R2.3 反应进行多远？化学变化的程度
In IB Chemistry, understanding how far a chemical reaction proceeds is fundamental. This topic delves into dynamic equilibrium, the equilibrium constant Kc, and the factors that affect the position of equilibrium. Grasping these concepts enables you to predict the extent of a reaction and apply Le Chatelier's principle to real-world situations.
在IB化学中，理解化学反应进行的程度至关重要。本专题深入探讨动态平衡、平衡常数 Kc 以及影响平衡位置的因素。掌握这些概念能让你预测反应的程度，并将勒夏特列原理应用于实际情境中。
1. Introduction to Chemical Equilibrium and Extent of Reaction | 化学平衡与反应程度导论
Many chemical reactions are reversible; they can proceed in both the forward and backward directions. When the rates of the forward and reverse reactions become equal, the system reaches a state of dynamic equilibrium. At this point, the concentrations of reactants and products remain constant, but the reaction has not stopped—it continues in both directions at equal rates. The extent of reaction refers to how far the reaction goes towards products before equilibrium is established.
许多化学反应是可逆的，它们可以正向和反向同时进行。当正反应和逆反应的速率相等时，系统达到动态平衡状态。此时，反应物和生成物的浓度保持不变，但反应并未停止——正逆反应仍在以相同速率进行。反应程度是指在达到平衡之前，反应向生成物方向进行了多少。
Not all reactions reach the same extent. Some proceed almost to completion, leaving very little reactant, while others barely proceed, with mostly reactants present at equilibrium. The equilibrium constant, Kc, provides a quantitative measure of this extent.
并非所有反应都达到相同的程度。有些反应几乎进行到底，只剩极少量反应物；而另一些反应几乎不进行，平衡时主要为反应物。平衡常数 Kc 为这一程度提供了定量的度量。
2. The Equilibrium Constant, Kc | 平衡常数 Kc
For a general reversible reaction at a given temperature:
对于给定温度下的通式可逆反应：
aA + bB ⇌ cC + dD
the equilibrium constant in terms of concentration, Kc, is expressed as:
以浓度表示的平衡常数 Kc 表达为：
Kc = [C]ᶜ [D]ᵈ / [A]ᵃ [B]ᵇ
where [X] denotes the equilibrium concentration of species X in mol dm⁻³. The exponents a, b, c, d are the stoichiometric coefficients from the balanced equation. Kc is dimensionless only when the sum of the stoichiometric coefficients of products equals that of reactants; otherwise, it has units of (mol dm⁻³)Δn, where Δn = (c+d) – (a+b).
其中 [X] 表示物质 X 在平衡时的浓度，单位为 mol dm⁻³。指数 a、b、c、d 为配平方程式中的化学计量系数。仅当生成物化学计量系数之和等于反应物之和时，Kc 无量纲；否则，Kc 具有单位 (mol dm⁻³)Δn，其中 Δn = (c+d) – (a+b)。
It is essential to note that Kc is temperature-dependent. Changing the temperature alters the value of Kc, whereas changes in concentration, pressure, or the addition of a catalyst do not affect Kc.
务必注意，Kc 与温度相关。改变
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