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Doc Brown's Chemistry  Advanced Level Inorganic Chemistry Periodic Table Revision Notes – Transition Metals

Part 10. Transition Metals 3d–block:   10.12 Zinc Chemistry

Zinc is a member of the 3d–block of elements BUT why isn't zinc a true transition metal? Zinc cannot form an ion with an incomplete d sub–shell and is therefore not a true transition element. Zinc's chemistry is determined solely by the formation of compounds in its +2 oxidation state, but it does form many complexes, though not as many as other transition metals.

principal oxidation states of zinc, redox reactions of zinc, ligand substitution displacement reactions of zinc, balanced equations of zinc chemistry, formula of zinc complex ions, shapes colours of zinc complexes, formula of compounds

(c) doc b GCSE/IGCSE Periodic Table Revision Notes * (c) doc b GCSE/IGCSE Transition Metals Revision Notes

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 

data comparison of zinc with the other members of the 3d–block and transition metals

Z and symbol 21 Sc 22 Ti 23 V 24 Cr 25 Mn 26 Fe 27 Co 28 Ni 29 Cu 30 Zn
property\name scandium titanium vanadium chromium manganese iron cobalt nickel copper zinc
melting point/oC 1541 1668 1910 1857 1246 1538 1495 1455 1083 420
density/gcm–3 2.99 4.54 6.11 7.19 7.33 7.87 8.90 8.90 8.92 7.13
atomic radius/pm 161 145 132 125 124 124 125 125 128 133
M2+ ionic radius/pm na 90 88 84 80 76 74 72 69 74
M3+ ionic radius/pm 81 76 74 69 66 64 63 62 na na
common oxidation states +3 only +2,3,4 +2,3,4,5 +2,3,6 +2,3,4,6,7 +2,3,6 +2,3 +2,+3 +1,2 +2 only
outer electron config. 3d14s2 3d24s2 3d34s2 3d54s1 3d54s2 3d64s2 3d74s2 3d84s2 3d104s1 3d104s2
Electrode potential M(s)/M2+(aq) na –1.63V –1.18V –0.90V –1.18V –0.44V –0.28V –0.26V +0.34V –0.76V
Electrode potential M(s)/M3+(aq) –2.03V –1.21V –0.85V –0.74V –0.28V –0.04V +0.40 na na na
Electrode potential M2+(aq)/M3+(aq) na –0.37V –0.26V –0.42V +1.52V +0.77V +1.87V na na na

Extended data table for ZINC

property of zinc/unit value for Zn
melting point Zn/oC 420
boiling point Zn/oC 907
density Zn/gcm–3 7.13
1st Ionisation Energy/kJmol–1 906
2nd IE/kJmol–1 1733
3rd IE/kJmol–1 3832
4th IE/kJmol–1 5730
5th IE/kJmol–1 7970
Zn atomic radius/pm 133
Zn2+ ionic radius/pm 74
Relative polarising power Zn2+ ion 2.7
oxidation state of Zn +2 only
simple electron configuration of Zn 2,8,18,2
outer electrons of Zn [Ar]3d104s2
Electrode potential Zn(s)/Zn2+(aq) –0.76V
Electronegativity of Zn 1.65

Advanced Inorganic Chemistry Page Index and Links

  • Uses of ZINC

    • Zinc is a greyish silvery white metal which is quite brittle at room temperature.

    • Zinc is a good conductor of heat and electricity.

    • Zinc slowly reacts with oxygen and water, but quite fast with acids.

    • Zinc is used in zinc–carbon batteries, as is zinc chloride, ZnCl2. (in the 'paste')

    • Zinc is alloyed with copper to make brass.

    • Zinc sulfide, ZnS, is used in paint manufacture.

    • Zinc oxide, ZnO is used in rubber manufacture.

    • Covalent organometallic zinc compounds (ZnR2) are used as catalysts in polymer production.

    • A solution of zinc sulphate, ZnSO4, is used in zinc plating as anti–corrosion treatment of other metals like steel.

    • Zinc chloride is also used in wood preservatives.

    • The phosphor Zn2SiO4:Mn is involved in the manufacture of night vision devices.

  • Biological role of zinc

    • Zinc is an essential trace element and is a co–factor in the operation of many enzymes such as lactic dehydrogenase.

    • In plants, zinc ions activate carboxylases and leaves may be malformed if there is a zinc deficiency in a plant.

  • The colour of zinc compounds

    • Most zinc compounds and complex ions (Zn only exhibits a +2 oxidation state in them) are white or colourless.

      • The lack of scope for a variety of coloured compounds arises from the fundamental electronic configuration of the Zn2+ ion, namely [Ar]3d10, giving a completely filled 3d sub–shell.

        • ie there is no electron that can be promoted to a higher level when the 3d sub–shell is split when the central metal ion interacts with the ligands.

        • Bottom right shows the ground state of the zinc(II) ion, and clearly, no electron can be promoted, so no absorption, no colour!

        • Even though zinc is a member of the 3d block of elements, this is why zinc is NOT a true member of the first transition metal series, it forms no ion with a partly filled 3d sub–shell.

        • For more details see Appendix 4. Electron configuration & complex ion colour theory

 

The Chemistry of ZINC

  • The electrode potential chart highlights the value for the one positive oxidation state of zinc.

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

  • Zinc metal readily dissolves in dilute hydrochloric acid or dilute sulfuric acid reducing hydrogen ions to hydrogen gas.

    • Zn(s) + 2H+(aq) ==> Zn2+(aq) + H2(g)

  • 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.

  • In aqueous solution zinc forms the colourless stable hydrated zinc ion, [Zn(H2O)6]2+(aq) and most complexes of the zinc ion have a co–ordination number of 6.

    • Solutions of zinc sulfate ZnSO4(aq) or zinc chloride ZnCl2(aq) are suitable for laboratory experiments for investigating the aqueous chemistry of the zinc ion..

  • (6 in crystals? or 4 in solution? [Zn(H2O)4]2+(aq)).

  • 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)

  • Zinc ions 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)

  • Zinc ions 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.

  • With aqueous of sodium carbonate zinc ion solutions produce 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) 

  • Some examples of zinc complex ion formation

    • The variation of the stability constant with change in ligand is illustrated with the zinc ion.

      • The data set for zinc compares five different monodentate ligands and the polydentate ligand EDTA.

        • Apart from the EDTA complex the stability constant (Kstab) equilibrium expression is

        • Kstab = {[ZnL4]2+/2–(aq)} / {[Zn(H2O)4]2+(aq)} {[L(aq)]4} mol–4dm12

    • Ligand substitution reaction to give new complex ion Kstab lg Kstab
      [Zn(H2O)4]2+(aq) + 4CN–(aq) ==> Zn(CN)4]2–(aq) + 4H2O(l) 5.0 x 1016 16.7
      [Zn(H2O)4]2+(aq) + 4NH3(aq) ==> Zn(NH3)4]2–(aq) + 4H2O(l) 3.8 x 109 9.58
      [Zn(H2O)4]2+(aq) + 4Cl–(aq) ==> [ZnCl4]2–(aq) + 4H2O(l) 1.0 0.0
      [Zn(H2O)4]2+(aq) + 4Br–(aq) ==> [ZnBr4]2–(aq) + 4H2O(l) 10–1 –1.0
      [Zn(H2O)4]2+(aq) + 4CN–(aq) ==> [Zn(CN)4]2–(aq) + 4H2O(l) 10–2 –2.0
      [Zn(H2O)4]2+(aq) + EDTA4–(aq) ==> [ZnEDTA]2–(aq) + 4H2O(l) 3.2 x 1016 16.5

    • The very value for the tetracyanozincate(II) in reflects the strong of central metal ion (Zn2+) – ligand (CN) bond.

    • The lower Kstab value for ammonia indicates on average a weaker dative covalent bond.

    • The ligand bonds are even weaker for the halide ions possibly due to their larger radius, since there is a steady decrease in Kstab as the halide radius increases, making the Zn–X dative covalent bond longer and weaker.

    • The stability constant for the zinc–EDTA complex is a very high value, typical for a polydentate ligand (see Appendix 8).

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

  • –

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 ... ???

–

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

keywords redox reactions ligand substitution displacement balanced equations formula complex ions complexes ligands colours oxidation states: zinc ions Zn(0) Zn2+ Zn(+2) ZnSO4 ZnCl2 ZnO [Zn(H2O)4]2+ + 4 OH– [Zn(OH)4]2– Zn(OH)2 + 2OH– [Zn(OH)4]2– [Zn(H2O)4]2+ + 4 NH3 [Zn(NH3)4]2+ + 4H2O Zn(OH)2 + 4NH3 [Zn(NH3)4]2+ + 2OH– Zn2+ + 2 HCO3– ==> ZnCO3 + H2O + CO2 Ligand substitution reaction to give new complex ion [Zn(H2O)4]2+ + 4CN– ==> Zn(CN)4]2– + 4H2O [Zn(H2O)4]2+ + 4NH3 ==> Zn(NH3)4]2– + 4H2O [Zn(H2O)4]2+ + 4Cl– ==> [ZnCl4]2– + 4H2O [Zn(H2O)4]2+ + 4Br– ==> [ZnBr4]2– + 4H2O [Zn(H2O)4]2+ + 4 CN– ==> [Zn(CN)4]2– + 4H2O [Zn(H2O)4]2+ + EDTA4– ==> [ZnEDTA]2– + 4H2O oxidation states of zinc, redox reactions of zinc, ligand substitution displacement reactions of zinc, balanced equations of zinc chemistry, formula of zinc complex ions, shapes colours of zinc complexes  Na2CO3 NaOH NH3

Advanced Level Inorganic Chemistry of Zinc – A level Revision notes to help revise for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Advanced Level Chemistry OCR GCE Advanced Level Chemistry Edexcel GCE Advanced Level Chemistry Salters AS A2 Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre–university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses)

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Introduction 3d–block Transition Metals * 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

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