1. Reactions of cobalt and iron complexes

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3. Cu2+(aq) + 2 OH-(aq) → Cu(OH)2(s) Blue Soln Blue PPT
With a few drops of OH-:
[Cu(H2O)6]2+ (aq) + 2 OH- (aq) → [Cu(OH)2 (H2O)4] (s) H2O (l)
Blue Soln Blue PPT
Cu2+(aq) OH-(aq) → Cu(OH)2(s)
Blue Soln Blue PPT
With excess OH-:
The neutral metal hydroxide M(H2O)4(OH)2 (s) is in effect M(OH)2(s), which is uncharged and insoluble – forms a PPT
Ammonia, which is basic, has the SAME effect as OH- in removing protons
No Change (Blue PPT)

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5. Acid – Base Reaction With a few drops of OH-:
[Co(H2O)6]2+ (aq) + 2 OH- (aq) → [Co(OH)2 (H2O)4] (s) H2O (l)
Pink Soln Blue PPT
Co2+(aq) OH-(aq) → Co(OH)2(s)
Pink Soln Blue PPT
With excess OH-:
Acid – Base Reaction
The neutral metal hydroxide M(H2O)4(OH)2 (s) is in effect M(OH)2(s), which is uncharged and insoluble – forms a PPT
Ammonia, which is basic, has the SAME effect as OH- in removing protons
No Change (Blue PPT)

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7. 2NH3(aq) + 2H+(aq) → 2NH4+(aq)
With a few drops of NH3:
[Co(H2O)6]2+(aq) NH3 (aq) → [Co(H2O)4(OH)2] (s) NH4+(aq)
Pink Soln blue PPT
2NH3(aq) H+(aq) → 2NH4+(aq)
Co2+(aq) OH-(aq) → Co(OH)2
With excess NH3:
[Co(H2O)4(OH)2] (s)+ 6NH3(aq) → [Co(NH3)6]2+(aq) + 4H2O(l) + 2 OH-(aq)
Blue PPT Pale brown solution

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9. [Co(H2O)6]2+ + 6 NH3 → [Co(NH3)6]2+ + 6 H2O Pink Soln Brown soln
Cobalt
Small amount added:
[Co(H2O)6]2+(aq)+2NH3(aq) →[Co(H2O)4(OH)2] (aq) +2NH4+(aq)
Pink Soln Blue PPT
[Co(H2O)6] NH3 → [Co(NH3)6] H2O
Pink Soln Brown soln
air
In 1st step ammonia is acting as a base
If add more of the conc ammonia, both OH- and 4H2O ligands are replaced by ammonia
[Co(NH3)6]3+

10. Why does this look darker than yellow?
© A Jul-12

11. Change in co-ordination number, shape and colour
SUBSTITUTION BY Cl-
Charged
Bigger than H2O
Change in co-ordination number, shape and colour
In general for M2+ complexes:
[M(H2O)6]2+ (aq) + 4Cl- (aq) ↔ [MCl4]2- (aq) + 6H2O
Addition of conc HCl to aqueous TM ions causes substitution reactions
General for M2+
[M(H2O)6]2+ (aq) + 4Cl- (aq) ↔ [MCl4]2- (aq) + 6H2O
Tetrahedral shape – draw the shape p 241 of textbook

12. Substitution by Larger Ligands
Cl- bigger than O of H2O – only four Cl-’s fit around Mn+ H
Cl- O H

13. [Co(H2O)6]2+ + 4 Cl- ↔ [CoCl4]2- + 6 H2O
Pink soln Blue soln
Octahedral Tetrahedral
Demo of this in Fume cupboard in A Level prep room
© A Jul-12

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15. Acid – Base Reaction Aqua ions (2+) + OH- With a few drops of OH-:
[Fe(H2O)6]2+ (aq) + 2 OH- (aq) → [Fe(OH)2 (H2O)4] (s) H2O (l)
Green Soln (goes brown upon standing in air)
Fe2+(aq) OH-(aq) → Fe(OH)2(s)
Green Soln Green PPT
With excess OH-:
Acid – Base Reaction
The neutral metal hydroxide M(H2O)4(OH)2 (s) is in effect M(OH)2(s), which is uncharged and insoluble – forms a PPT
Ammonia, which is basic, has the SAME effect as OH- in removing protons
No Change (Green PPT)

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17. 2NH3(aq) + 2H+(aq) → 2NH4+(aq)
With a few drops of NH3:
[Fe(H2O)6]2+ (aq) + 2 NH3 (aq) → [Fe(H2O)4(OH)2](s) NH4+(aq)
Green soln Green PPT
(goes brown upon standing in air)
2NH3(aq) H+(aq) → 2NH4+(aq)
Fe2+(aq) OH-(aq) → Fe(OH)2
With excess NH3:
No Change (green PPT)

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19. Difficult to tell solutions apart.
With a few drops of OH-:
[Fe(H2O)6] OH- → [Fe(H2O)3(OH)3] H2O
Brown/Yellow Soln Brown PPT
Fe3+(aq) OH-(aq) → Fe(OH)3(s)
Brown/Yellow Soln Brown PPT
With excess OH-:
Fe2+ and Fe3+ exist in aqueous solution as octahedral hexaaqua complexes.
[Fe(H2O)6]2+ = pale green
[Fe(H2O)6]3+ = pale brown
Difficult to tell solutions apart.
Simple test is to add dilute alkali – precipitates the hydroxides – colours different
Fe(H2O)4(OH)2 = green
Fe(H2O)3(OH)3 = brown
No Change (Brown PPT)
Acid – Base Reaction

20. 3NH3(aq) + 3H+(aq) → 3NH4+(aq)
With a few drops of NH3:
[Fe(H2O)6]3+(aq) + 3NH3 (aq) → [Fe(H2O)3(OH)3](s) + 3 NH4+(aq)
Yellow/Brown Soln brown PPT
3NH3(aq) H+(aq) → 3NH4+(aq)
Fe3+(aq) OH-(aq) → Fe(OH)3
With excess NH3:
No Change (Brown PPT)

21. Exam Questions