Question

Account for the following:
a) La(OH)₃ is more basic than Lu(OH)_{3.
b) Zn2+ salts are white.
c) cu compounds are unstable in aqueous solution and undergo disproportination.}

Answer 1

a) The size of Lu^​3+ ion is small than La³⁺ ion due to lanthanoid contraction. As a result of small size, covalent character between Lu^3+​​ ion and OH^- increases and therefore its basic strength decreases. Hence, La(OH)₃ is more basic than Lu(OH)_3.

b)Zn²⁺ is white in colour. Its electronic configuration is 3d¹⁰. Since there is no unpaired electron present in Zn²⁺ ion there is no d-d transition taking place and therefore Zn²⁺ is white in colour.

c) The disproportionation reactions are those in which the same substance gets oxidised as well as reduced. In case of Cu compounds +2 oxidation state is more stable than +1 oxidation state in aqueous medium . This can be explained on the basis of much larger negative hydration enthalapy of Cu²⁺ (aq) than Cu⁺. Therefore, Cu(I) compounds are unstable in aqueous solution and they undergo disproportionation to Cu(II) and Cu(0) as:$2C{u}^{+}\to C{u}^{2+}+Cu$