Equilibrium and Solubility | A-Level 化学：平衡与溶解度

📚 Equilibrium and Solubility | A-Level 化学：平衡与溶解度

In A-Level Chemistry, solubility equilibria connect the idea of dynamic equilibrium to sparingly soluble salts. The solubility product, Ksp, lets us calculate how much solid dissolves and predict whether a precipitate forms when solutions are mixed.

在 A-Level 化学中，溶解平衡将动态平衡概念与难溶盐联系起来。溶度积 Ksp 使我们能够计算固体溶解多少，并预测混合溶液时是否会生成沉淀。

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1. Dynamic Equilibrium in Saturated Solutions | 饱和溶液中的动态平衡

A saturated solution contains undissolved solid in contact with its aqueous ions. At equilibrium, the rate of dissolving equals the rate of precipitation, so the ion concentrations remain constant.

饱和溶液含有未溶解的固体与其水合离子相接触。达到平衡时，溶解速率等于沉淀速率，因此离子浓度保持恒定。

For a general sparingly soluble salt MₘAₙ, the heterogeneous equilibrium is written as MₘAₙ(s) ⇌ mMⁿ⁺(aq) + nAᵐ⁻(aq).

对于通式难溶盐 MₘAₙ，多相平衡可写作 MₘAₙ(s) ⇌ mMⁿ⁺(aq) + nAᵐ⁻(aq)。

The solid is omitted from the equilibrium constant expression because its concentration is effectively constant. This is a key difference from homogeneous equilibria.

固体在平衡常数表达式中被省略，因为其浓度基本恒定。这是与均相平衡的一个关键区别。

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2. Defining Solubility Product Ksp | 定义溶度积 Ksp

Ksp is the equilibrium constant for the dissolution of a sparingly soluble salt. It relates the concentrations of the aqueous ions in a saturated solution at a given temperature.

Ksp 是难溶盐溶解过程的平衡常数。它表示在给定温度下饱和溶液中各水合离子的浓度关系。

For the equilibrium MₘAₙ(s) ⇌ mMⁿ⁺(aq) + nAᵐ⁻(aq), the Ksp expression is:

对于平衡 MₘAₙ(s) ⇌ mMⁿ⁺(aq) + nAᵐ⁻(aq)，Ksp 表达式为：

Ksp = [Mⁿ⁺]ᵐ [Aᵐ⁻]ⁿ

Ksp is affected only by temperature. It does not change when more solid is added or when the solution volume changes at constant temperature.

Ksp 只受温度影响。加入更多固体或在恒温下改变溶液体积都不会改变 Ksp。

Although the true equilibrium constant has no units, A-Level questions often quote Ksp with units such as mol² dm⁻⁶ or mol³ dm⁻⁹. Always calculate units from the expression.

虽然真正的平衡常数没有单位，但 A-Level 题目中常给出 Ksp 的单位，例如 mol² dm⁻⁶ 或 mol³ dm⁻⁹。应始终根据表达式计算单位。

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3. Common Ksp Expressions | 常见 Ksp 表达式

The stoichiometric coefficients in the dissolution equation become powers in the Ksp expression. The following examples show this pattern.

溶解方程式中的化学计量数在 Ksp 表达式中成为指数。以下示例展示了这一规律。

AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq) gives Ksp = [Ag⁺][Cl⁻].

AgCl(s) ⇌ Ag⁺(aq) + Cl⁻(aq) 得到 Ksp = [Ag⁺][Cl⁻]。

Ag₂CrO₄(s) ⇌ 2Ag⁺(aq) + CrO₄²⁻(aq) gives Ksp = [Ag⁺]²[CrO₄²⁻].

Ag₂CrO₄(s) ⇌ 2Ag⁺(aq) + CrO₄²⁻(aq) 得到 Ksp = [Ag⁺]²[CrO₄²⁻]。

PbCl₂(s) ⇌ Pb²⁺(aq) + 2Cl⁻(aq) gives Ksp = [Pb²⁺][Cl⁻]².

PbCl₂(s) ⇌ Pb²⁺(aq) + 2Cl⁻(aq) 得到 Ksp = [Pb²⁺][Cl⁻]²。

Fe(OH)₃(s) ⇌ Fe³⁺(aq) + 3OH⁻(aq) gives Ksp = [Fe³⁺][OH⁻]³.

Fe(OH)₃(s) ⇌ Fe³⁺(aq) + 3OH⁻(aq) 得到 Ksp = [Fe³⁺][OH⁻]³。

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