Chapter 22: Problem 4

Give reasons for: (a) \(\mathrm{La}(\mathrm{OH})_{3}\) is most basic among \(\mathrm{Ln}(\mathrm{OH})_{3}\). (b) Lanthanides have similar properties (c) Most of the lanthanide ions are coloured.

Short Answer

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(a) La(OH)3 is most basic due to the 'double effect' of ionic size and hydration enthalpy. (b) Lanthanides have similar properties because of their similar electronic configurations and the effect of lanthanide contraction. (c) Most of the lanthanide ions are coloured due to the f->f transitions.

Step by step solution

Most Basic

(a) The basic strength of hydroxides of lanthanides increase from La to Lu. This is due to the fact that the size of lanthanide ions decrease from La to Lu (lanthanide contraction). Thus, the smaller the size of the ion, the greater is its polarizing power, causing greater degree of ionization of water molecules and hence increasing the basic strength of hydroxides. However, La(OH)3 is considered to be most basic among Ln(OH)3, despite being the first in the series, due to what's known as the 'double effect' of the ionic size and hydration enthalpy. Since La3+ is largest in size, it is less polarised, leading to less covalent and more ionic character of La(OH)3 and hence it is most basic.

Similar Properties

(b) The properties of lanthanides are similar due to the similar electronic configurations of their atoms. The 'lanthanide contraction', a steady decrease in atomic and ionic size of elements with increasing atomic number, as one proceeds in the period from left to right, causes a shielding of the nuclear charge by 4f electrons so that outer electrons feel similar effective nuclear charges. Thus, their chemical and physical properties are very much similar to each other.

Coloured Ions

(c) Most of the lanthanide ions are coloured due to the f -> f transitions. The ions of lanthanides with partially filled inner 4f levels give colours to their solutions and salts because of the absorption of radiant energy corresponding to the visible region to excite an electron from one 4f orbital to another. The absorption of radiant energy corresponding to the visible region results in the promotion of an electron from a lower to higher energy level.

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Give reasons for: (a) \(\mathrm{La}(\mathrm{OH})_{3}\) is most basic among \(\mathrm{Ln}(\mathrm{OH})_{3}\). (b) Lanthanides have similar properties (c) Most of the lanthanide ions are coloured.

Short Answer

Step by step solution

Most Basic

Similar Properties

Coloured Ions

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