What are hydrated and anhydrous substances

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Enthalpy of hydration and lattice energy

The energetic conditions are of particular importance for understanding the proposed solution. The hydration enthalpy is released during hydration.

This energy results from the electrostatic attraction of the ion dipole and depends accordingly on the charge and radius of the ion in question. Small ions with a high charge therefore have particularly high amounts of hydration enthalpy:

Tab. 1
Enthalpies of hydration of various ions
ionEnthalpy of hydration []ionEnthalpy of hydration []ionEnthalpy of hydration []

The smaller ions thus form more strongly developed hydrate shells than the larger ones. This means that in solutions of ions, the hydration changes the size ratios compared to the crystalline state, because the ions, when dissolved in water, build up hydration shells of different sizes. Thus, among the alkali ions, the hydrated ion is the largest. Many ions of the subgroup metals are particularly strongly hydrated, because here stable aqua complexes can form due to the interactions of the free electron pairs of the water molecules with the incompletely occupied d orbitals of the metal ions.

As already mentioned, the enthalpy of hydration is released when an ion crystal dissolves. On the other hand, however, energy is also used to dissolve the crystal lattice. The energy to be used corresponds to the amount of the lattice energy of the salt. Lattice energy and hydration enthalpy determine the enthalpy of solution, so that qualitatively whether the dissolution process is exothermic or endothermic. If the amount of the lattice energy is greater than that of the hydration enthalpy, the dissolution process consumes energy. This is shown in the experiment by cooling the salt solution. Conversely, if the enthalpy of hydration is greater than the lattice energy, the dissolution process is exothermic and the solution heats up.

Dissolution of CaCl2
  • The dissolution of anhydrous calcium chloride in water provides energy, which means that the solution heats up. Obviously the amount of energy that is released during the hydration of the and ions is greater than the lattice energy of the crystal.
  • The dissolution of the hexahydrate, the crystal lattice of which is formed by -, ions and molecules, on the other hand, is endothermic. Here the lattice energy of the crystal is greater than the hydration enthalpy of the ions. The lower enthalpy of hydration results from the fact that the ions of the crystal are already partially hydrated by the water of crystallization, which means that fewer water molecules can be added than in the case of anhydrous salt.