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Chromium(III)

Copper(II)

Iron(II) and Iron(III)

Zinc(II) and Aluminium(III)

Nickel(II)

Manganese(II)

Cobalt(II)

EDTA

Cobalt(II) chemistry.

Acid-Base Reaction ⇒
Acid-Base Reaction ⇒
Ligand substitution ⇒
Carbonate with M 2+ (aq) ⇒
Ionic equation – precipitation ⇒
Ligand substitution ⇒

 

  • [Co(H2O)6] 2+ (aq) + 2OH ¯ (aq)  → Co(OH)2(H2O)4 (s) + 2H2O (l)
  • [Co(H2O)6] 2+ (aq) + 2NH3 (aq)  ⇌   Co(OH)2(H2O)4 (s) + 2NH4 + (aq)
  • Co(OH)2(H2O)4 (s) + 6NH3 (aq) → [Co(NH3)6] 3+ (aq) + 4H2O (l) + 2OH – (aq)
  • [Co(H2O)6] 2+ (aq) + CO3 2- (aq)  →  CoCO3 (s) + 6H2O (l) 
    Alternatively: Co 2+ (aq) + CO3 2- (aq) → CoCO3 (s)
  • [Co(H2O)6] 2+ (aq) + 4Cl ¯ (aq)   ⇌  [CoCl4] 2- (aq) + 6H2O (l)
1. Tests: sodium hydroxide and ammonia solutions.

CoSO4 (aq)  + 2NaOH  (aq)  →  Co(OH)2 (s)  +  Na2SO4 (aq)

The cobalt(II) ion actually exists as a complex ion in solution:
[Co(H2O)6] 2+. This hexaaquacobalt(II) ion can act as an acid
– giving up H+ ion.

[Co(H2O)6] 2+(aq)   +  2OH – (aq)  →  Co(H2O)4(OH)2 (s) + 2H2O (l)

  • when a few drops of concentrated ammonia solution  solution are added, again, a dark blue precipitate of cobalt(II) hydroxide is formed
  • this precipitate does dissolve in excess ammonia solution,
    forming a brown solution.

[Co(H2O)6] 2+ (aq) + 2NH3 (aq)  ⇌   Co(OH)2(H2O)4 (s) + 2NH4 + (aq)

Co(OH)2(H2O)4 (s) + 6NH3 (aq) → [Co(NH3)6] 3+ (aq) + 4H2O (l) + 2OH – (aq)

2. Oxidation of cobalt(II) by hydrogen peroxide

2[Co(H2O)6] 2+ (aq)  +  3H2O2 (l) + 4e – → Co2O3(s) + 15H2O (l) 
pink                                                             brown

Alternatively

2[Co(H2O)6] 2+ (aq)  +  H2O2 (l) →  2[Co(H2O)6] 3+(aq) + 2H2O (l) + O2 (g) + 2e –

 

3. Adding sodium carbonate to cobalt(II)

[Co(H2O)6] 2+ (aq) + CO3 2- (aq)  →  CoCO3 (s) + 6H2O (l) 
                                                           Lilac precipitate
Alternatively: Co 2+ (aq) + CO3 2- (aq) → CoCO3 (s)

4. Ligand Substitution

[Co(H2O)6] 2+ (aq) + 4Cl ¯ (aq)   ⇌  [CoCl4] 2- (aq) + 6H2O (l)

  • [Co(H2O)6] 2+ (aq) is pink 
  • [CoCl4] 2- (aq)  is blue
  • The reaction can be reversed by adding water
  • Use le Chatelier’s principle to explain why

5. Ligand Substitution: BIDENTATE FOR MONODENTATE

[Co(H2O)6] 2+ (aq) + 3en (aq)   ⇌  [Co(en)3] 2+ (aq) + 6H2O (l)

  • [Co(H2O)6] 2+ (aq) is pink 
  • [Co(en)3] 2+ (aq)  is orange

[Co(H2O)6] 2+ (aq) + ox (aq)   ⇌  Co(H2O)2ox  (s) + 4H2O (l)

  • [Co(H2O)6] 2+  is a pink solution 
  • Co(H2O)2ox is a pale pink solid
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