📚 Enthalpy Changes in Solution | 溶解焓变
When an ionic solid dissolves in water, its lattice breaks apart and the released ions become surrounded by water molecules. The overall enthalpy change for this process is called the enthalpy change of solution, ΔH°sol. A complete understanding of this topic requires combining lattice enthalpy with hydration enthalpies in an energy cycle based on Hess’s law.
当离子固体溶于水时，其晶格会拆散，释放出的离子被水分子包围。这一过程的总焓变称为溶解焓变，记作 ΔH°sol。要全面理解这一主题，需要基于盖斯定律将晶格焓与水合焓结合在一个能量循环中。
1. Defining Enthalpy Change of Solution | 溶解焓变的定义
The standard enthalpy change of solution, ΔH°sol, is the enthalpy change when one mole of an ionic substance dissolves in enough water to form an infinitely dilute solution under standard conditions of 298 K and 100 kPa.
标准溶解焓变 ΔH°sol 是指在 298 K、100 kPa 的标准条件下，一摩尔离子物质溶于足量水中形成无限稀释溶液时的焓变。
‘Infinitely dilute’ means the ions are so well separated by water molecules that further dilution produces no measurable heat change. For sodium chloride, the process can be written as NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq).
‘无限稀释’ 表示离子被水分子充分隔开，进一步稀释不会产生可测量的热量变化。对于氯化钠，该过程可写作 NaCl(s) + aq → Na⁺(aq) + Cl⁻(aq)。
The (aq) state does not simply mean ‘in water’; it means each ion is hydrated, with polar water molecules attracted to the charged ion. This hydration is an essential part of the dissolving process.
(aq) 状态并不仅仅表示 ‘在水中’；它表示每个离子都发生了水合，即极性水分子被带电离子吸引。水合是溶解过程中必不可少的部分。
2. Standard Conditions and Key Symbols | 标准条件与关键符号
Standard enthalpy changes are measured at 298 K and 100 kPa. The degree sign in ΔH° shows that the value refers to standard conditions. The subscript indicates the type of enthalpy change being described.
标准焓变在 298 K 和 100 kPa 下测定。ΔH° 中的 ° 表示该数值对应标准条件，下标则说明所描述的焓变类型。
The three symbols you must know are ΔH°sol for solution, ΔH°hyd for hydration, and ΔH°latt for lattice formation. Always check whether a lattice enthalpy value is quoted as formation or dissociation because the sign is opposite in the two cases.
你必须掌握的三个符号是：ΔH°sol 表示溶解焓变，ΔH°hyd 表示水合焓变，ΔH°latt 表示晶格形成焓。始终要检查晶格焓数值是形成焓还是解离焓，因为两者的符号相反。
In solution chemistry, standard states for the species involved are usually the solid lattice for the ionic compound and 1 mol dm⁻³ solutions for aqueous ions, but the solution is assumed to be dilute enough for ideal behaviour.
在溶液化学中，涉及物质的标准状态通常对离子化合物为固体晶格，对水合离子为 1 mol dm⁻³ 溶液，但一般假设溶液足够稀，表现出理想行为。
3. Enthalpy Change of Hydration | 水合焓变
The standard enthalpy change of hydration, ΔH°hyd, is the enthalpy change when one mole of gaseous ions dissolves in water to form one mole of aqueous ions under standard conditions.
标准水合焓变 ΔH°hyd 是指在标准条件下，一摩尔气态离子溶于水形成一摩尔水合离子时的焓变。
Hydration is always exothermic because the attraction between the charged ion and the polar water molecules releases energy. For example, Mg²⁺(g) + aq → Mg²⁺(aq) has a large negative ΔH°hyd.
水合过程总是放热的，因为带电离子与极性水分子之间的吸引力会释放能量。例如，Mg²⁺(g) + aq → Mg²⁺(aq) 的 ΔH°hyd 为较大的负值。
Smaller ions and ions with higher charges have more exothermic hydration enthalpies. This is because their charge density is greater, so they attract the negative or positive ends of water dipoles more strongly.
半径较小、电荷较高的离子具有更负的水合焓。这是因为它们的电荷密度更大，因此更强烈地吸引水分子的负端或正端。
You should be able to compare hydration enthalpies: Na⁺ has a less negative ΔH°hyd than Mg²⁺, and Cl⁻ has a less negative ΔH°hyd than F⁻ because F⁻ is smaller.
你应该能够比较水合焓：Na⁺ 的 ΔH°hyd 不如 Mg²⁺ 负，Cl⁻ 的 ΔH°hyd 不如 F⁻ 负，因为 F⁻ 半径更小。
4. Lattice Enthalpy and Its Role | 晶格焓及其作用
Lattice formation enthalpy, ΔH°latt(formation), is the enthalpy change when one mole of a solid ionic lattice is formed from its gaseous ions. It is always exothermic and therefore always has a negative sign.
晶格形成焓 ΔH°latt(形成) 是由气态离子形成一摩尔固体离子晶格时的焓变。它总是放热的，因此总是带负号。
The reverse process, lattice dissociation, is endothermic because energy must be supplied to overcome the strong electrostatic attractions between the oppositely charged ions in the lattice.
其逆过程，即晶格解离，是吸热的，因为必须提供能量来克服晶格中带相反电荷离子之间的强静电吸引力。
In a solution energy cycle, the solid lattice is first broken into gaseous ions, and then those gaseous ions are hydrated. The sum of these two steps gives the enthalpy change of solution.
在溶解能量循环中，固体晶格先被拆分为气态离子，然后这些气态离子再发生水合。这两步的加和就是溶解焓变。
Lattice enthalpy itself cannot be measured directly; it is usually calculated from a Born-Haber cycle. The solution enthalpy calculation simply uses this experimentally derived value or a data-book value.
晶格焓本身不能直接测量；它通常通过玻恩-哈伯循环计算得到。溶解焓的计算只是使用这一实验导出值或数据手册中的数值。
5. The Solution Energy Cycle | 溶解能量循环
For an ionic solid MX, the solution energy cycle can be represented as:
对于离子固体 MX，溶解能量循环可表示为：
MX(s) → M⁺(g) + X⁻(g) → M⁺(aq
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