Q. Illustrate Born-Haber cycle?

Boron is always covalent and does not   form,B³⁺ ions, because' the energy required to remove three electrons is very high.  Many simple compounds of A1 and Ga like AlCl₃ and GaCl₃ are also covalent when anhydrous. However, in solution, the large amount of hydration energy evolved compensates the high ionisation energy and all the' metal ions exist in a hydrated state.

This can be explained with the help of Born-Haber cycle as given below. However, this is a simplified approach given only on the bas; of enthalpy changes, whereas the direction and extent of any reaction depend on the free energy changes which take into account changes both in enthalpy and entropy.

Thus, despite the large ionisation energies of aluminium, the enthalpy of' solution of  AlCl₃ has a large negative value. Therefore, in aqueous solution, AIC₃ exists as AI³⁺(aq) and Cl^-(aq). It might be thought that the same argument would apply to BCI₃; But to make the enthalpy of solution of BCl₃ nethalpy the enthalpy of hydration of B³⁺ should be - 6009 KJ which is unlikely for the small B³⁺cation. Therefore, BC13 hydrolyses in aqueous solution to give boric acid and not B³⁺ (aq).

BCl₃ + 3H₂0 -----------------------------------> H₃B0₃ + 3HCI