Cobalt and its compounds (A-level inorganic chemistry)
Cobalt and its compounds
The electron configuration of cobalt
[Ar]4s3d7
Properties of cobalt as a transition element
- Forms colored compounds; for instance, Co2+ is pink
- Has variable oxidation states; +2, +3,
- Forms complexes, e.g. CoCl42-.
Physical properties of cobalt
Cobalt is a silver-white lustrous metal, slightly harder than iron. It is malleable and ductile.
It is more magnetic than all other metals except iron.
It has a density equal to 8.9. It melts at 14950C and boils at 29000C.
The action of air:
Dry air has no action on cobalt but the metal reacts with O2 to form Co3O4 when red hot.
3Co(s) + 2O2(g) → Co3O4(s)
The reaction of cobalt with water:
Cobalt does not react with cold water. However, it decomposes steam to hydrogen.
Co(s) + H2O(steam) → CoO(s) + H2(g)
The reaction of cobalt with acids
Dilute Hydrochloric acid and sulphuric acid form salts and hydrogen with cobalt.
Co(s) + 2H+(g) → Co2+(aq) + H2(g)
Dilute nitric acid dissolves the metal and renders it passive due to formation of a protecting thin layer of metal oxide on the surface of the metal.
The reaction of cobalt with alkalis:
Even concentrated alkali has no action on the metal. The metal is dissolved in fused caustic potash at 5500C.
The reaction of cobalt of Carbon monoxide:
Finely divided Cobalt combines with carbon monoxide at 1500C and under 30 atmospheres to form orange-red Co2(CO)8
The action of ammonia:
Co4N2 is formed.
4Co(s) + 2NH3(g) → Co4N2(s) + 3H2(g)
The reaction of cobalt with other elements:
Cobalt readily combines with H2, S, P, and B but N2 has no action on it.
Co(s) + H2(g) → CoH2(s)
Co(s) + S(s) → CoS(s)
3Co(s) + 2P(s) → Co3P2(s)
Uses of cobalt and its salts
Cobalt is used for electroplating.
For the preparation of alloys; e.g. cobalt steel for making permanent magnets, stellite (Co, Cr and W (tungsten)) for making surgical instruments, and Co-chrome (Co, Cr, and Fe) for making resistance wires.
Cobalt oxides and silicates are used for coloring glass, pottery, and glazes.
Qualitative analysis of cobalt II salts
Common salt: Cobalt sulphate: Co2+
With dilute sodium hydroxide solution of ammonium solution
Observation
Blue precipitate insoluble in excess turns brown on standing
Confirmatory test of cobalt
Forms deep blue solution with concentrated hydrochloric acid.
Co(H2O)62+ (aq) + 4Cl– (aq) →CoCl42- (aq) + 6H2O (l)
(b) Concentrated ammonia forms a blue precipitate soluble in excess to give a pale yellow solution; the solution turns brown on standing.
[Co(H2O)6]2+ (aq) + 2OH– (aq) → Co(OH)2 (s)
pink blue ppt
Co(OH)2 (s) + 6NH3 (aq) s → [Co(NH3)6]2+(aq)
red-brown
[Co(NH3)6]2+(aq) →Co(NH3)6]3+(aq)
red-brown yellow solution
Trial 23 (2004/1/15)
(a) Write equations for the reactions which take place and in each case, state what would be observed when;(i) Concentrated ammonia is added dropwise to an aqueous solution of cobalt (II) sulphate and when the resultant solution is allowed to stand in air. (4 marks)
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