* Advanced Inorganic Chemistry Transition metals 10.12 Zinc Chemistry Doc Brown's

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 Doc Brown's Chemistry  Periodic Table revision notes 10.12

Part 10. Transition Metals 3d-block:   10.12 Zinc 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 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

10.12. Chemistry of Zinc Zn, Z=30, 1s22s22p63s23p63d104s2 

  • Zn data table 1 summary * extended zinc data table 2 * Zinc & electrode potential chart of 3d-block

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

  • Some basic reactions of zinc metal, oxide and carbonate are on the GCSE Reactivity Series of Metals Notes

  • Although a member of the 3d-block, zinc is NOT a true transition metal.

  • The Zn2+ ion has a full sub-shell, 3d10, which does not allow the electronic transitions which account for the colour in transition metal compounds (see Appendix 4. complex ion colour theory).

  • In aqueous solution zinc forms the colourless stable zinc ion, [Zn(H2O)4]2+(aq) and most complexes of the zinc ion have a co-ordination number of 4.

  • The alkalis sodium hydroxide or ammonia, produce the hydrated white gelatinous zinc hydroxide precipitate. There is a further reaction with excess of NaOH or NH3.

    • Zn2+(aq) + 2OH-(aq) ==> Zn(OH)2(s)  (can be written as [Zn(OH)2(H2O)2])

    •    (a precipitation reaction)

  • with excess sodium hydroxide:

    • [Zn(H2O)4]2+(aq)  + 4OH-(aq) rev [Zn(OH)4]2-(aq) + 4H2O(l)  (from original aqueous ion)

    • or Zn(OH)2(s) + 2OH-(aq) rev [Zn(OH)4]2-(aq)  (from hydroxide ppt.)

      • formation of the tetrahydroxozincate ion.

    • In fact zinc oxide is a classic amphoteric oxide e.g. giving a 'zincate' with alkali and a chloride salt with hydrochloric acid.

      • ZnO(s) + 2NaOH(aq) + H2O(l) ==> Na2Zn(OH)4(aq)

      • ZnO(s) + 2HCl(aq) ==> ZnCl2(aq) + H2O(l)

  • with excess ammonia:

    • [Zn(H2O)4]2+(aq) + 4NH3(aq) rev [Zn(NH3)4]2+(aq) + 4H2O(l)  (formation from original aqueous ion)

    • or  Zn(OH)2(s)  + 4NH3(aq) rev [Zn(NH3)4]2+(aq) + 2OH-(aq) (or from hydroxide precipitate)

      • The ammonia ligand displaces the water/hydroxide ion ligands.

  • Aqueous sodium carbonate solutions produces a precipitate of white zinc carbonate, but its a basic carbonate, i.e. the carbonate precipitate is mixed with the hydroxide, Zn(OH)2.

    • Zn2+(aq) + CO32-(aq) ==> ZnCO3(s) 

    • better prepared using NaHCO3: Zn2+(aq) + 2HCO3-(aq) ==> ZnCO3(s) + H2O(l) + CO2(g) 

  • -

 

Advanced Inorganic Chemistry Page Index and Links

 

The Extraction and Purification of Zinc
  • Zinc is extracted from either zinc blende/sphalerite ore (zinc sulphide) or sometimes calamine/Smithsonite ore (zinc carbonate).
  • (1) The zinc sulphide ore is roasted in air to give impure zinc oxide.
    • 2ZnS(s) + 3O2(g) ==> 2ZnO(s) + 2SO2(g)
    • Note: calamine ore can be used directly in a zinc smelter because on heating it also forms zinc oxide.
      • ZnCO3(s)  ==> ZnO(s) + CO2(g) (endothermic thermal decomposition)
  • (2) The impure zinc oxide can be treated in two ways to extract the zinc:
    • (a) It is roasted in a smelting furnace with carbon (coke, reducing agent) and limestone (to remove the acidic impurities).
      • C(s) + O2(g) ==> CO2(g) (very exothermic oxidation, raises temperature considerably)
      • C(s) + CO2(g) ==> 2CO(g) (C oxidised, CO2 reduced)
      • ZnO(s) + CO(g) ==> Zn(l) + CO2(g) (zinc oxide reduced by CO, Zn undergoes O loss)
      • or direct reduction by carbon: ZnO(s) + C(s) ==> Zn(l) + CO(g) (ZnO reduced, C oxidised)
      • The carbon monoxide acts as the reducing agent i.e. it removes the oxygen from the oxide.
      • The impure zinc is  then fractionally distilled from the mixture of slag and other metals like lead and cadmium out of the top of the furnace in an atmosphere rich in carbon monoxide which stops any zinc from being oxidised back to zinc oxide.
      • The slag and lead (with other metals like cadmium) form two layers which can be tapped off at the base of the furnace.
      • The zinc can be further purified by a 2nd fractional distillation or more likely by dissolving it in dilute sulphuric acid and purified electrolytically as described below.
    • (b)Two stages
      • (i) It is dissolved and neutralised with dilute sulphuric acid to form impure zinc sulphate solution.
      • ZnO(s) + H2SO4(aq) ==> ZnSO4(aq) + H2O(l)
      • or using calamine ore/zinc carbonate directly:
        • ZnCO3(s) + H2SO4(aq) ==> ZnSO4(aq) + H2O(l)+ CO2(g)
      • (ii) Quite pure zinc is produced from the solution by electrolysis. It can be deposited on a pure zinc negative electrode (cathode) in the same way copper can be purified. The other electrode, must be inert e.g. for laboratory experiments, carbon (graphite) can be used and oxygen is formed.
        • Zn2+(aq) + 2e- ==> Zn(s)
          • A reduction process, electron gain, as zinc metal is deposited on the (-) electrode.
        • You can't use solid zinc oxide directly because its insoluble and the ions must free to carry the current and migrate to the electrodes in some sort of solution.
        • For more details of the type of electrolysis system used, see purification of copper (just swap Zn for Cu in the method/diagram).
        • PLEASE note: In the industrial production of zinc by electrolysis (called electro-winning) the negative (-) cathode is made of aluminium (Al, where zinc deposits) and the positive (+) electrode is made of a lead-silver alloy (Pb-Ag, where oxygen gas is formed). Why these particular electrode metals are used in this 'electrowinning' process I'm not quite sure, but aluminium is so unreactive that it is effectively inert, and lead and silver are also of low activity, but ... ???

 

Advanced Inorganic Chemistry Page Index and Links

 

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

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