Question

Anhydrous \[AlC{l_3}\;\] is covalent however when it is dissolved in water hydrated ionic species are formed. This transformation is owing to:
A. The trivalent state of Al
B. The large hydration energy of \[A{l^{ + 3}}\]
C. Low hydration energy of \[A{l^{ + 3}}\]
D. The polar nature of water

Answer 1

Hint: Chemical compounds are generally grouped into one of two categories: covalent compounds and ionic compounds. Ions of opposite charges form ionic compounds, usually as a result of a metal reacting with a nonmetal. In covalent compounds, the elements form a compound by sharing electrons, resulting in an electrically neutral molecule.

Step by step answer: There is no clear cut distinction between covalent and ionic compounds. There are many compounds that fall somewhere in the middle. In the solid state it adopts an 'ionic lattice' structure with octahedral coordination for the \[A{l^{ + 3}}\] ions but in the liquid and gas phases it exists as a covalent compound, either as AlCl3or as a dimer \[\] \[A{l_2}C{l_6}\]
In solution state, When \[AlC{l_3}\;\]​ dissolves in water, the dissociation occurs and \[AlC{l_3}\;\] is dissociated into Al+3 and \[C{l^ - }\] Hydration of \[A{l^{ + 3}}\]and\[\] \[C{l^ - }\]. Ions occur and due to large hydration energy of ions, \[AlC{l_3}\;\] dissolves in water.
Anhydrous \[AlC{l_3}\;\] is covalent however when it is dissolved in water hydrated ionic species are formed. This transformation is owing to the large hydration energy of \[A{l^{ + 3}}\]

Hence, Option (B) is correct.

Additional Information: Hydration energy (also hydration enthalpy) is the amount of energy released when one mole of ions undergoes hydration. Hydration energy is the energy liberated when something dissolves in water. If the hydration energy is greater than lattice energy, the substance will be soluble. The hydration energies of the gaseous Li+, Na+, and Cs+ are respectively 520, 405, and 265 kJ/mole respectively.

Note: Students may be confused in deciding the ionic and covalent nature of aluminum chloride and aluminum fluoride. \[AlC{l_3}\;\] Is covalent while \[AI{F_{\mathbf{3}}}\] is ionic this can be justified on the basis of Fagan’s Rule. Larger the size of the cation and smaller the size of the anion, more ionic is the molecule.