* Advanced Inorganic Chemistry Transition metals 10.9 Cobalt Chemistry Doc Brown's

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 Doc Brown's Chemistry  Periodic Table revision notes 10.9

Part 10. Transition Metals 3d-block:  10.9 Cobalt Chemistry

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses)  GCSE Periodic Table * GCSE notes Transition Metals

INORGANIC Part 10 The 3d block TRANSITION METALS sub-index: 10.1-10.2 Introduction 3d-block Transition Metals * 10.3 Scandium * 10.4 Titanium * 10.5 Vanadium * 10.6 Chromium * 10.7 Manganese * 10.8 Iron * 10.9  Cobalt * 10.10 Nickel * 10.11 Copper * 10.12 Zinc * 10.13 Other Transition Metals e.g. Ag and Pt * Appendix 1. Hydrated salts, acidity of hexa-aqua ions * Appendix 2. Complexes & ligands * Appendix 3. Complexes and isomerism * Appendix 4. Electron configuration & colour theory * Appendix 5. Redox equations, feasibility, Eø * Appendix 6. Catalysis * Appendix 7. Redox equations * Appendix 8. Stability Constants and entropy changes * Appendix 9. Colorimetric analysis and complex ion formula * Appendix 10 3d block - extended data * Appendix 11 Some 3d-block compounds, complexes, oxidation states & electrode potentials * Appendix 12 Hydroxide complex precipitate 'pictures', formulae and equations

Advanced Level Inorganic Chemistry Periodic Table Index * Part 1 Periodic Table history * Part 2 Electron configurations, spectroscopy, hydrogen spectrum, ionisation energies * Part 3 Period 1 survey H to He * Part 4 Period 2 survey Li to Ne * Part 5 Period 3 survey Na to Ar * Part 6 Period 4 survey K to Kr and important trends down a group * Part 7 s-block Groups 1/2 Alkali Metals/Alkaline Earth Metals * Part 8  p-block Groups 3/13 to 0/18 * Part 9 Group 7/17 The Halogens * Part 10 3d block elements & Transition Metal Series * Part 11 Group & Series data & periodicity plots * All 11 Parts have their own sub-indexes near the top of the pages


10.9. Chemistry of Cobalt Co, Z=27, 1s22s22p63s23p63d74s2 

  • Co data table 1 summary * extended cobalt data table 2 * Cobalt & electrode potential chart of 3d-block

  • Summary of some complexes-compounds & oxidation states of cobalt compared to other 3d-block elements

  • Cobalt is alloyed with chromium and tungsten to make a metal hard enough, even at red heat, to be used for high speed cutting tools and valves for internal combustion engines.

  • COBALT(II) chemistry

  • In aqueous solution, in the absence of complexing agents,

    • cobalt forms the stable pink hexaaqua cobalt(II) ion, [Co(H2O)6]2+(aq) 

  • The alkalis sodium hydroxide and ammonia, produce the hydrated cobalt(II) hydroxide blue ppt. which turns pink on standing. There is no further reaction with excess of NaOH or Na2CO3, but see further down for excess NH3.

    • Co2+(aq) + 2OH-(aq) ==> Co(OH)2(s)  (can be written as [Co(OH)2(H2O)4])

    •    (a precipitation reaction)

  • Alkaline aqueous sodium carbonate solutions produces a precipitate of pink/blue? cobalt(II) carbonate.

    • Co2+(aq) + CO32-(aq) ==> CoCO3 (s) (maybe basic carbonate? - mix of CoCO3 + Co(OH)2)

  • When excess ammonia is added to a cobalt(II) salt solution, the hexamine complex is formed BUT this is unstable in the presence of dissolved oxygen and is oxidised to the cobalt(III) complex. This change in cobalt's oxidation state from +2 to +3 via an oxidising agent is quite common if a complexing agent is present too.

    • [Co(H2O)6]2+(aq) + 6NH3(aq) ==> [Co(NH3)6]2+(aq) + 6H2O(l) 

    • pink hexaaquacobalt(II) ion == oxygen ==> brown hexaamminecobalt(II) ion.

      • The uncharged ligand molecules ammonia NH3 and water H2O are similar in size and ligand exchange occurs without change in co-ordination number (stays at 6).

      • Oxidation then follows from dissolved oxygen, or you can add hydrogen peroxide for a more efficient job!

    • (i) 4[Co(NH3)6]2+(aq) + O2(g/aq) + 4H+(aq) ==> 4[Co(NH3)6]3+(aq) + 2H2O(l)

    • (ii) 2[Co(NH3)6]2+(aq) + H2O2(g/aq) + 2H+(aq) ==> 2[Co(NH3)6]3+(aq) + 2H2O(l)

    • brown ==> colour? hexaamminecobalt(III) ion.

    • Oxidation state changes: in both (i) & (ii) Co from +2 to +3, (i) O from 0 to -2, (ii) O from -1 to -2.

      • +1.82V for [Co(H2O)6]3+(aq) + e- rev [Co(H2O)6]2+(aq)

      • +0.10V for [Co(NH3)6]3+(aq) + e- rev [ Co(NH3)6]2+(aq)

      • more EØ data & comments?

    • Comparison of the stability of the hexammine complexes irrespective of redox stability

      • [Co(H2O)6]2+(aq) + 6NH3(aq) ==> [Co(NH3)6]2+(aq) + 6H2O(l)

        • Kstab = {[Co(NH3)6]2+(aq)} / {[Co(H2O)6]2+(aq)} [NH3(aq)]6

        • Kstab = 7.7 x 104 mol-6 dm18  [lg(Kstab) = 4.9]

      • [Co(H2O)6]3+(aq) + 6NH3(aq) ==> [Co(NH3)6]3+(aq) + 6H2O(l)

        • Kstab = {[Co(NH3)6]3+(aq)} / {[Co(H2O)6]3+(aq)} [NH3(aq)]6

        • Kstab = 4.5 x 1033 mol-6 dm18  [lg(Kstab) = 33.7]

      • Note that the more highly charged Co3+(aq) ion complexes more strongly than the Co2+(aq) ion i.e. forms a more stable complex

  • VIEW ppts. with OH-, NH3 and CO32-, & complexes, if any, with excess reagent.

  • When hydrogen peroxide is added to an alkaline cobalt(II) solution, oxidation occurs to give cobalt(III) complexes.

    • The air oxidation described above in alkaline ammonia solution can also be effected via hydrogen peroxide giving the hexa-amminecobalt(III) ion - described in detail above.

  • If e.g. sodium chloride or hydrochloric acid is added to cobalt(II) sulphate solution the blue tetrachlorocobaltate(II) complex ion is formed.

    • [Co(H2O)6]2+(aq) + 4Cl-(aq) rev [CoCl4]2-(aq) + 6H2O(l) 

    • This particular ligand substitution/exchange reaction involves several changes (L to R):

      • the larger chloride ion ligand leads to a change in co-ordination number from 6 to 4,

      • the complex ion shape changes from octahedral to tetrahedral

      • the colour of the complex changes from pink to blue,

      • the complex changes from a cationic to an anionic ion.

      • There is no oxidation state change at all.

    • This is quite a good reaction to demonstrate Le Chatelier's equilibrium principles:

      • dilution shifts the equilibrium to the left, more pink,

      • increasing the chloride ion concentration shifts the equilibrium to the right, more blue,

      • increasing the solution temperature shifts the equilibrium to the right, more blue

      • or if prepared at higher temperature, with just enough chloride to turn the solution blue, on cooling it becomes pink,

      • this shows that left to right is endothermic and right to left is exothermic.

  • COBALT(III) chemistry

    • As we have seen above the hexaaquacobalt(III) cation is unstable in aqueous solution but can be stabilised by a suitable ligand.

    • The formation of [Co(NH3)6]3+ is described above and two other stable complex anions are with the ...

      • nitrate(III) ion (nitrite, ion NO2-) it forms the anionic octahedral complex [Co(NO2)6]3-

      • cyanide ion CN- it forms the anionic octahedral complex [Co(CN)6]3-

  • Isomerism in cobalt(III) complexes e.g. with the ligands ammonia + chloride (i)-(iii)

    • (i) crystalline [Co(NH3)6]3+(Cl-)3 is orange-yellow, no isomers possible

    • (ii) crystalline [Co(NH3)5Cl]2+(Cl-)2 is violet, no isomers possible

    • (iii) crystalline [Co(NH3)4Cl2]+Cl- is violet or green - there are two geometrical E/Z isomers (trans/cis)

      • (c) doc b

      • Geometrical isomerism diagrams: The Z and E (cis and trans geometrical isomers) isomeric octahedral complexes of the dichlorotetraamminecobalt(III) complex ion

      • (1) is the cis or Z isomer, (2) is the trans or E isomer

    • (iv) -

  • More examples of the complexes of cobalt(II) and cobalt(III)

    • Both the hexa-aqua ions of cobalt(II) and cobalt(III) readily complex with EDTA

      • [Co(H2O)6]2+(aq) + EDTA4-(aq) ===> [Co(EDTA)]2-(aq) + 6H2O(l)

        • Kstab = {[Co(EDTA)3]2-(aq)} / {[Co(H2O)6]2+(aq)} [EDTA4-(aq)]

        • Kstab = 2.0 x 1016 mol-1 dm3 [lg(Kstab) = 16.3]

      • [Co(H2O)6]3+(aq) + EDTA4-(aq) ===> [Co(EDTA)]-(aq) + 6H2O(l)

        • Kstab = {[Co(EDTA)3]-(aq)} / {[Co(H2O)6]3+(aq)} [EDTA4-(aq)]

        • Kstab = 1.0 x 1036 mol-1 dm3 [lg(Kstab) = 36.0]

      • Note that the more highly charged Co3+(aq) ion complexes more strongly than the Co2+(aq) ion.

    • The cobalt(II) ion complexes with 1,2-diaminoethane, a bidentate ligand

      • [Co(H2O)6]2+(aq) + 3en(aq) ==> [Co(en)3]2+(aq) + 6H2O(l)

      • Kstab = {[Co(en)3]2+(aq)} / {[Co(H2O)6]2+(aq)} [en(aq)]3

      • Kstab = 6.3 x 1013 mol-3 dm9 [lg(Kstab) = 13.8]

    • -


Scandium * Titanium * Vanadium * Chromium * Manganese * Iron * Cobalt * Nickel * Copper * Zinc * Silver & Platinum

(german) Chemie Periodensystem 10,9 Cobalt Chemie *

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