Question

When anhydrous salts, which can exist in hydrated form dissolves, its enthalpy of solution is usually negative whereas when the hydrated form of the same salt dissolves, its enthalpy of solution is always positive. Explain.

Answer 1

Answer:

Hydration energy (also hydration enthalpy) is the amount of energy released when one mole of ions undergo hydration which is a special case of solvation. It is a special case of dissolution of energy, with the solvent being water.

For example, upon dissolving a salt in water, the outermost ions (those at the edge of the lattice) move away from the lattice and become covered with the neighbouring water molecules. If the hydration energy is equal to or greater than the lattice energy, then the salt is water-soluble. In salts for which the hydration energy is higher than the lattice energy, solvation occurs with a release of energy in the form of heat. For instance, CaCl2 (anhydrous calcium chloride) heats the water when dissolving. However, the hexahydrate, CaCl2·6H2O cools the water upon dissolution. The latter happens because the hydration energy does not completely overcome the lattice energy, and the remainder has to be taken from the water in order to compensate the energy loss.

What is the energy of hydration?

Enthalpy of hydration, Hhyd, of an ion is the amount of energy released when a mole of the ion dissolves in a large amount of water forming an infinitely dilute solution in the process,

Mz+(g) + mH2O ® Mz+(aq)

where Mz+(aq) represents ions surrounded by water molecules and dispersed in the solution. The approximate hydration energies of some typical ions are listed here. The table illustrates the point that as the atomic numbers increases, so do the ionic size, leading to a decrease in absolute values of enthalpy of hydration.

Answer 2

Answer

Hydration energy (also called hydration enthalpy) is the amount of energy released when one mole of ions hydrates, and it is a special case of solvation. A special case of energy dissolution where the solvent is water.

Explanation:

Concept: What is Hydration Energy?

The hydration enthalpy of an ion is the amount of energy released when one mole of an ion dissolves in a large volume of water, forming an infinitely diluted solution.

$Mz+(g) + mH_2O Mz+(aq)$

where Mz+(aq) represents an ion dispersed in a solution surrounded by water molecules. Approximate hydration energies for some typical ions are listed here. The table shows that as the atomic number increases so does the ion size and the absolute value of the enthalpy of hydration decreases.

Solution

when a salt dissolves in water, the outermost ions (the ions at the edges of the lattice) move away from the lattice and are covered by neighboring water molecules. Salt is water soluble if its hydration energy is greater than or equal to its lattice energy. For salts with higher hydration energy than the lattice energy, solvation occurs with the release of energy in the form of heat. For example, $CaCl_2$ (anhydrous calcium chloride) heats water as it dissolves. However, the hexahydrate, $CaCl_2 6H_2O$, cools water as it dissolves.

The latter occurs because the hydration energy does not completely overcome the lattice energy and the rest must be extracted from the water to compensate for the energy loss.

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