Calculating the Enthalpy Change of Hydration of an Anhydrous Salt | 计算无水盐的水合焓变

📚 Calculating the Enthalpy Change of Hydration of an Anhydrous Salt | 计算无水盐的水合焓变

When an anhydrous salt is added to water, it may dissolve directly to give aqueous ions, or it may first combine with water of crystallisation to produce a hydrated salt. The enthalpy change of hydration of an anhydrous salt is the enthalpy change when one mole of the anhydrous salt reacts with water to form one mole of the hydrated salt. Since this process is often difficult to measure directly in a simple calorimeter, we usually calculate it indirectly by measuring the enthalpy changes of solution of the anhydrous salt and the hydrated salt, then applying Hess’s law.

当无水盐加入水中时，它可能直接溶解给出水合离子，也可能先与结晶水结合生成水合盐。无水盐的水合焓变是指 1 mol 无水盐与水反应生成 1 mol 水合盐的焓变。由于该过程通常难以用简单量热计直接测量，我们一般通过分别测量无水盐和水合盐的溶解焓变，再利用赫斯定律间接计算。

1. The Chemical Context: Anhydrous vs Hydrated Salts | 化学背景：无水盐与水合盐

An anhydrous salt contains no water of crystallisation in its solid lattice. Many salts can exist in both anhydrous and hydrated forms. For example, anhydrous copper(II) sulfate is white CuSO₄(s), while hydrated copper(II) sulfate is blue CuSO₄·5H₂O(s). The dot formula indicates that five moles of water molecules are incorporated per mole of copper(II) sulfate in the crystal lattice.

无水盐的晶格中不含结晶水。许多盐可以同时存在无水形式和水合形式。例如，无水硫酸铜(II) 是白色的 CuSO₄(s)，而五水合硫酸铜(II) 是蓝色的 CuSO₄·5H₂O(s)。点式表示每摩尔硫酸铜(II) 晶格中含有 5 mol 水分子。

Other common examples include Na₂CO₃ and Na₂CO₃·10H₂O, MgSO₄ and MgSO₄·7H₂O, and CaCl₂ and CaCl₂·6H₂O. The water molecules in a hydrated salt are not just attached to the surface; they occupy fixed positions in the ionic lattice and form ion-dipole interactions with the cations and anions.

其他常见例子包括 Na₂CO₃ 和 Na₂CO₃·10H₂O、MgSO₄ 和 MgSO₄·7H₂O、CaCl₂ 和 CaCl₂·6H₂O。水合盐中的水分子并不只是附着在表面，它们在离子晶格中占据固定位置，并与阳离子和阴离子形成离子-偶极相互作用。

Do not confuse this solid-state hydration process with the standard enthalpy change of hydration of an ion. The latter refers to one mole of gaseous ions dissolving in water to form aqueous ions, e.g. M⁺(g) + aq → M⁺(aq). Here we are concerned with a solid anhydrous salt combining with water of crystallisation to form a solid hydrated salt.

不要将该固态水合过程与离子的标准水合焓变混淆。后者是指 1 mol 气态离子溶于水形成水合离子，例如 M⁺(g) + aq → M⁺(aq)。本主题关注的是固态无水盐与结晶水结合生成固态水合盐的过程。

2. Standard En

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