Sodium chloride has a lattice crystalline structure which corresponds to good solubility in water and its brittleness. Lattice structures are readily soluble in water when the hydration energy released as a result of dissolution equals or becomes greater than the lattice energy which keeps the ions together (undissociated).
The hydration energy for NaCl is almost equal to its lattice energy. So, the undissociated form becomes less stable than the dissolved or dissociated form of NaCl. This becomes the thermodynamic parameter of why is NaCl soluble in water.
Lattice energy of NaCl crystals
Lattice energy is an estimation of bond energy in a compound which can be explained as the amount of energy released when gaseous opposite ions combine to make a lattice crystal. The same happens in the case of NaCl lattice.
The sodium and chloride ions in gaseous forms combine with each other to form a NaCl crystal lattice structure.
Na+(aq) + Cl–(aq) → NaCl(s)
ΔHlatt = -787.3 KJ/mol
Hydration energy of NaCl crystals
The concept of hydration energy rests entirely on charge density. The charge density is the amount of charge per unit volume of an ion. The smaller the size of ions, the more will be the charge density and hence, more will be the hydration energy. The opposite goes for larger ions making the hydration energy of sodium be -406 KJ/mol and chloride to be -378 KJ/mol making the whole hydration energy of NaCl, -784 KJ/mol.
NaCl(s) → Na+(aq) + Cl–(aq)
ΔHhyd = -784 KJ/mol
The hydration energy of NaCl is almost equal to the lattice energy, making NaCl more stable in hydrolyzed form. As the principle, ‘Entropy increase makes a system stable‘ suggests, the dissociated form of NaCl is thermodynamically more stable than solid form, making NaCl soluble in water.
Other parameters of dissolution
Another parameter of dissolution of NaCl in water is the solvation or spectatoring by water molecules. Na+ and Cl– ions are surrounded by water molecules having their opposite charged sides, facing the ions. These water molecules are called spectator ions. The number of water molecules surrounding each sodium or chloride ion is 6 in the case of NaCl, which makes these ions stable in dissociated form. It is these spectator ions that do not let the sodium and chloride ions get back together unless all of the solvent is evaporated.
Similar to NaCl, NaBr and other lattice structures also dissolves in water,
