IB Chemistry: Writing Equilibrium Constant Expressions | IB化学：平衡常数表达式书写

📚 IB Chemistry: Writing Equilibrium Constant Expressions | IB化学：平衡常数表达式书写

In a chemical equilibrium, the equilibrium constant, K, is a fundamental quantity that links the concentrations or partial pressures of reactants and products at equilibrium. Mastering how to write equilibrium constant expressions is an essential skill in IB Chemistry, appearing in both SL and HL papers. This article will guide you through the rules, notation, and common pitfalls.

在化学平衡中，平衡常数 K 是在平衡状态下将反应物与产物的浓度或分压联系起来的核心物理量。掌握如何书写平衡常数表达式是 IB 化学中的一项必备技能，在 SL 和 HL 考试中都会出现。本文将引导你学习相关规则、符号和常见易错点。

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1. What is the Equilibrium Constant? | 1. 什么是平衡常数？

Dynamic equilibrium occurs when the forward and reverse reaction rates are equal, so the macroscopic concentrations of all species remain constant over time. For a reaction at equilibrium, the ratio of product activities (or concentrations) to reactant activities, each raised to the power of its stoichiometric coefficient, is called the equilibrium constant.

动态平衡是指正逆反应速率相等时的状态，此时各物种的宏观浓度随时间不再改变。对于处于平衡的反应，产物活度（或浓度）与反应物活度之比（各自以其化学计量数为指数）称为平衡常数。

At a given temperature, K is a fixed value. It does not depend on the initial concentrations of reactants or products, nor on the total pressure or the presence of a catalyst. Changing the temperature is the only way to change the value of K for a given reaction.

在给定温度下，K 是一个固定值。它与反应物或产物的初始浓度无关，也与总压力无关，并且不受催化剂影响。对于给定反应，只有改变温度才能改变 K 的数值。

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2. The General Form of Kc | 2. Kc 的一般形式

For a general homogeneous reaction:

对于一个一般的均相反应：

aA + bB ⇌ cC + dD

The equilibrium constant in terms of concentration, Kc, is written as:

基于浓度的平衡常数 Kc 可写成：

Kc = [C]^c[D]^d / ([A]^a[B]^b)

Square brackets denote concentration in mol dm⁻³. The exponents are the stoichiometric coefficients from the balanced equation. Products are placed in the numerator and reactants in the denominator. If the reaction is written in the reverse direction, the new equilibrium constant is the reciprocal of the original Kc.

方括号表示浓度，单位为 mol dm⁻³。指数来自配平方程式中的化学计量数。产物写在分子中，反应物写在分母中。如果反应按相反方向书写，新的平衡常数是原 Kc 的倒数。

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3. Writing Kc for Homogeneous Equilibria | 3. 均相平衡的 Kc 表达式

In a homogeneous equilibrium, all reactants and products are in the same physical phase. Common examples include all gases or all aqueous solutions.

在均相平衡中，所有反应物和产物均处于相同的物理状态。常见的例子包括全部为气体或全部为水溶液的情况。

For the Haber process:

以哈伯法为例：

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The Kc expression is:

其 Kc 表达式为：

Kc = [NH₃]² / ([N₂][H₂]³)

Notice that the coefficient 2 for ammonia becomes the exponent 2, and the coefficient 3 for hydrogen becomes the exponent 3. The coefficient 1 for nitrogen is understood and not written.

注意氨的系数 2 变为指数 2，氢的系数 3 变为指数 3。氮的系数 1 默认省略不写。

For hydrogen iodide formation:

对于碘化氢的生成反应：

H₂(g) + I₂(g) ⇌ 2HI(g)

Kc = [HI]² / ([H₂][I₂])

For an aqueous equilibrium such as the formation of the iron(III) thiocyanate complex:

对于水溶液中的平衡，例如铁(III)硫氰酸根配合物的生成：

Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq)

Kc = [FeSCN²⁺] / ([Fe³⁺][SCN⁻])

Here, all species are aqueous, so each concentration appears in the expression with an implied exponent of 1.

这里所有物种均为水溶液，因此每个浓度项都以其隐含的指数 1 出现在表达式中。

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4. Writing Kc for Heterogeneous Equilibria | 4. 非均相平衡的 Kc 表达式

In heterogeneous equilibria, species exist in more than one phase. Pure solids and pure liquids have constant concentration (or activity equal to 1) during the reaction, so they are omitted from the equilibrium expression.

在非均相平衡中，物种存在于多个相中。纯固体和纯液体在反应过程中浓度恒定（活度等于 1），因此不出现在平衡表达式中。

Consider the thermal decomposition of calcium carbonate:

以碳酸钙的热分解为例：

CaCO₃(s) ⇌ CaO(s) + CO₂(g)

The Kc expression is simply:

其 Kc 表达式简化为：

Kc = [CO₂]

Both CaCO₃ and CaO are solids, so they do not appear. Only gaseous CO₂ appears in the expression. Similarly, for the dissolution of silver chloride:

CaCO₃ 和 CaO 都是固体，因此不出现。只有气态 CO₂ 出现在表达式中。类似地，对于氯化银的溶解：

AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq)

Kc = [Ag⁺][Cl⁻]

This particular equilibrium constant is also known as the solubility product, Ksp. Remember: in aqueous solutions, pure water is also excluded from the expression when it acts as the solvent or appears as a pure liquid.

这一特殊平衡常数也称为溶度积 Ksp。请记住：在水溶液中，当纯水作为溶剂或以纯液体出现时，也应在表达式中省略。

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5. Kp Expressions for Gases | 5. 气体的 Kp 表达式

For reactions involving gases, the equilibrium constant can also be expressed in terms of partial pressures. This constant is denoted Kp. Only gaseous species have partial pressures; solids and pure liquids are omitted just as in Kc.

对于涉及气体的反应，平衡常数也可以用分压来表示，记为 Kp。只有气态物种具有分压；固体和纯液体如同在 Kc 中一样被省略。

For the oxidation of sulfur dioxide:

以二氧化硫的氧化为例：

2SO₂(g) + O₂(g) ⇌ 2SO₃(g)

The Kp expression is:

其 Kp 表达式为：

Kp = p(SO₃)² / (p(SO₂)² × p(O₂))

Here p(X) represents the partial pressure of species X at equilibrium. Partial pressures are usually measured in Pa, kPa, bar, or atm, so the units of Kp depend on the pressure unit used.

这里 p(X) 表示物种 X 在平衡时的分压。分压通常以 Pa、kPa、bar 或 atm 为单位，因此 Kp 的单位取决于所使用的压力单位。

The two constants are related by the equation Kp = Kc(RT)^Δn, where Δn is the change in the number of moles of gas between products and reactants. For example, for 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), Δn = 2 − 3 = −1, so Kp = Kc(RT)⁻¹.

Kp 与 Kc 的关系为 Kp = Kc(RT)^Δn，其中 Δn 是产物气体物质的量与

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