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18 5a.png


The blue silica gel

desiccants commonly found in the laboratory are prepared by

impregnating silica gel with an aqueous solution of hydrated cobalt(II) chloride

(C0Ch•6H20), followed by drying it in an oven to give a distinct blue colour. On

prolonged use for drying purposes in the desiccator or upon exposure to a moist

environment, the colour of the silica gel will eventually tum from the distinct blue

colour to a pale pink colour. Regeneration of the silica gel to its original distinct blue

colour can be achieved by baking the silica gel desiccant in an oven. The blue colour

observed in the anhydrous state is believed to be due to the presence of a tetrahedral

complex.

(a) Give the ground state Russell-Saunders term for cobalt(II). Give also the

spectral terms for cobalt(II) in (i) an octahedal field and (ii) a tetrahedral field that

would correlate with the Russell-Saunders term.(3 marks)

(b) Identify the blue species and also the species that gives rise to the pale pink colour

when exposed to moist environment. Rationalize the observed colour changes.

(4 marks)

(c) Suggest the origin of the electronic transitions that are responsible for the distinct

blue colour and the pale pink colour and give the spectral assignments for these

electronic transitions.

(4 marks)

(d) State the Laporte selection rule for electronic transitions and describe two

mechanisms for the relaxation of this selection rule. Rationalize the observed

differences in both their colour and intensity, and discuss them in the context of

ligand field theory and the selection rules.

(9 marks)

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18 5a.png

The blue silica gel desiccants commonly found in the laboratory are prepared by
impregnating silica gel with an aqueous solution of hydrated cobalt(II) chloride
(CoClg-6H20), followed by drying it in an oven to give a distinct blue colour. On
prolonged use for drying purposes in the desiccator or upon exposure to a moist
environment, the colour of the silica gel will eventually turn flom the distinct blue
colour to a pale pink colour. Regeneration of the silica gel to its original distinct blue
colour can be achieved by baking the silica gel desiccant in an oven. The blue colour
observed in the anhydrous state is believed to be due to the presence of a tetrahedral
complex. (a) Give the ground state Russell-Saunders term for cobalt(II). Give also the
spectral terms for cobalt(II) in (i) an octahedal field and (ii) a tetrahedral field that
would correlate with the Russell-Saunders term.

18 5bcd.png

(b) Identify the blue species and also the species that gives rise to the pale pink colour when exposed to moist environment. Rationalize the observed colour changes.
(4 marks) (c) Suggest the origin of the electronic transitions that are responsible for the distinct
blue colour and the pale pink colour and give the spectral assignments for these electronic transitions.
(4 marks) (d) State the Laporte selection rule for electronic transitions and describe two
mechanisms for the relaxation of this selection rule. Rationalize the observed differences in both their colour and intensity, and discuss them in the context of ligand field theory and the selection rules.

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