Question

AlCl3 and BCl3 are strong Lewis Acids. But, CaCl2 is not a Lewis acid. Explain why. (Hint: Write the reactions of these substances with H2O and explain).

Answer 1

Solution:

In BCl3 and AlCl3, both B and Al are electron deficient atom, hence are able to accept lone pair of electrons. In the case of CaCl2, Ca is not an electron deficient atom hence not able to accept lone pairs and therefore not considered as lewis acid. AlCl3 and BCl3 are covalent compounds hence reacts with water and forms Al(OH)3 and B(OH)3, while CaCl2 is an ionic compound, thus it dissociated in water as Ca2+ and Cl- ions.

The reactions of AlCl3, BCl3 and CaCl2 with water can be written as,

AlCl3 + 3H2O = Al(OH)3 + 3HCl

BCl3 + 3H2O = B(OH)3 + 3HCl

CaCl2 + H2O = Ca2+(aq) + 2Cl- (aq)