Reactions of cobalt and iron complexes

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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) + 2 H2O (l) Blue Soln Blue PPT Cu2+(aq) + 2 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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Acid – Base Reaction With a few drops of OH-: [Co(H2O)6]2+ (aq) + 2 OH- (aq) → [Co(OH)2 (H2O)4] (s) + 2 H2O (l) Pink Soln Blue PPT Co2+(aq) + 2 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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2NH3(aq) + 2H+(aq) → 2NH4+(aq) With a few drops of NH3: [Co(H2O)6]2+(aq) + 2 NH3 (aq) → [Co(H2O)4(OH)2] (s) + 2 NH4+(aq) Pink Soln blue PPT 2NH3(aq) + 2H+(aq) → 2NH4+(aq) Co2+(aq) + 2 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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[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]2+ + 6 NH3 → [Co(NH3)6]2+ + 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+

Why does this look darker than yellow? © www.chemsheets.co.uk A2 042 20-Jul-12

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

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

[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 © www.chemsheets.co.uk A2 042 20-Jul-12

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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) + 2 H2O (l) Green Soln (goes brown upon standing in air) Fe2+(aq) + 2 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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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) + 2 NH4+(aq) Green soln Green PPT (goes brown upon standing in air) 2NH3(aq) + 2H+(aq) → 2NH4+(aq) Fe2+(aq) + 2 OH-(aq) → Fe(OH)2 With excess NH3: No Change (green PPT)

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Difficult to tell solutions apart. With a few drops of OH-: [Fe(H2O)6]3+ + 3 OH- → [Fe(H2O)3(OH)3] + 3 H2O Brown/Yellow Soln Brown PPT Fe3+(aq) + 3 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

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) + 3H+(aq) → 3NH4+(aq) Fe3+(aq) + 3 OH-(aq) → Fe(OH)3 With excess NH3: No Change (Brown PPT)

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