📚 Enthalpy Change of Solution | 溶解过程的焓变
When an ionic compound dissolves in water, energy is always absorbed to break the crystal lattice, but energy is also released as water molecules surround the separated ions. The balance between these two opposing energy terms determines whether dissolution feels hot or cold to the touch. In this article, we explore the enthalpy change of solution, the energy landscape of dissolving ionic solids, and how to calculate and predict these enthalpy changes using the ionic model.
当离子化合物溶于水时，破坏晶格总是需要吸收能量，而水分子包围分离后的离子时又会释放能量。这两项能量之间的平衡决定了溶解过程是放热还是吸热。在本文中，我们将探讨溶解过程的焓变、离子固体溶解的能量图景，以及如何利用离子模型计算和预测这些焓变。
1. Enthalpy Change of Solution Defined | 溶解焓变的定义
The standard enthalpy change of solution, ΔHsol, is defined as the enthalpy change that occurs when one mole of a solute is dissolved in sufficient solvent to form an infinitely dilute solution, with all substances in their standard states. For an ionic solid MX, the process can be written as:
标准溶解焓变 ΔHsol 的定义是：当一摩尔溶质溶于足量溶剂形成无限稀释溶液时发生的焓变，所有物质均处于标准状态。对于离子固体 MX，该过程可表示为：
MX(s) + aq → M⁺(aq) + X⁻(aq) ΔH = ΔHsol
"Infinitely dilute" means that further addition of solvent produces no additional enthalpy change — in practice, a large excess of water is used so that each ion is fully surrounded by water molecules and ion-ion interactions
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