Illustrate born-haber cycle, Chemistry

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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.

365_Illustrate Born-Haber cycle.png

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


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