transition metal chemistry of zinc complexes oxidation state +2 redox chemical reactions physical properties advanced inorganic chemistry of zinc

Revision notes on 3d block chemistry of zinc for Advanced A Level Inorganic Chemistry:

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colourless hexaaquazinc ion octahedral complex with water

colourless tetrahedral shape complex tetrahydroxozinc ion [Zn(OH)4]2-Periodic Table - 3d block - zinc Chemistry - Doc Brown's Chemistry  Revising Advanced Level Inorganic Chemistry Periodic Table Revision Notes

Part 10. Transition Metals 3d–block

10.12 Zinc Chemistry

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Zinc is a member of the 3d–block of elements BUT why isn't zinc a true transition metal?

Zinc cannot form a stable 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.

The only oxidation state of zinc is +2, so there is a much more limited chemistry in terms of 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 compared to true transition metals.

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. [Ar]... 3d14s2 3d24s2 3d34s2 3d54s1 3d54s2 3d64s2 3d74s2 3d84s2 3d104s1 3d104s2
Electrode pot'l M(s)/M2+(aq) na –1.63V –1.18V –0.90V –1.18V –0.44V –0.28V –0.26V +0.34V –0.76V
Electrode pot'l M(s)/M3+(aq) –2.03V –1.21V –0.85V –0.74V –0.28V –0.04V +0.40 na na na
Elect. pot'l M2+(aq)/M3+(aq) na –0.37V –0.26V –0.42V +1.52V +0.77V +1.87V na na na

Elect. pot. = standard electrode potential data for zinc (EŘ at 298K/25oC, 101kPa/1 atm.)

na = data not applicable to zinc

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 [beyond argon core] [Ar]3d104s2
Electrode potential Zn(s)/Zn2+(aq) –0.76V
Electronegativity of Zn 1.65
  • 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 sulfate, 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.

        • diagram explaining why zinc ion complexes compounds are colourless of white no 3d electrons can be promoted in the sub-orbital splitting

        • Bottom right shows the ground state of the zinc(II) ion, and clearly, no electron can be promoted in the 3d sub-levels, so no absorption of visible light photons, no colour from transmitted visible light!

        • 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

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

Pd s block d blocks (3d block zinc) and f blocks of metallic elements p block elements
Gp1 Gp2 Gp3/13 Gp4/14 Gp5/15 Gp6/16 Gp7/17 Gp0/18


2 3Li 4Be The modern Periodic Table of Elements

ZSymbol, z = atomic or proton number

3d block of metallic elements: Scandium to Zinc Sc-Zn focus on zinc

5B 6C 7N 8O 9F 10Ne
3 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar
4 19K 20Ca 21Sc







 [Ar] 3d34s2



[Ar] 3d54s1



   [Ar]   3d54s2



[Ar] 3d64s2



[Ar] 3d74s2



[Ar] 3d84s2



[Ar] 3d104s1



[Ar] 3d104s2


31Ga 32Ge 33As 34Se 35Br 36Kr
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe
6 55Cs 56Ba 57,58-71 72Hf 73Ta 74W 75Re 76Os 77Ir 78Pt 79Au 80Hg 81Tl 82Pb 83Bi 84Po 85At 86Rn
7 87Fr 88Ra 89,90-103 104Rf 105Db 106Sg 107Bh 108Hs 109Mt 110Ds 111Rg 112Cn 113Nh 114Fl 115Mc 116Lv 117Ts 118Os
  *********** *********** ************ ************ ************** ********** ********** ********** ********** **********  

Summary of oxidation states of the 3d block metals (least important) Ti to Cu are true transition metals

Sc Ti V Cr Mn Fe Co Ni Cu Zn
  (+2) (+2) (+2) +2 +2 +2 +2 +2 +2 (3d10)
+3 +3 +3 +3 (+3) +3 +3 (+3) (+3)  
  +4 +4   +4     (+4)    
      +6 (+6) (+6)        
3d14s2 3d24s2 3d34s2 3d54s1 3d54s2 3d64s2 3d74s2 3d84s2 3d104s1 3d104s2
The outer electron configurations beyond [Ar] and the (ground state of the simple ion)

Note that when 3d block elements form ions, the 4s electrons are 'lost' first.

The oxidation state (+2 only) and electron configuration of zinc in the context of the 3d block of elements

electrode potential chart diagram show position of Zn/Zn2+ in the context of the 3d block transition metals

  • 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..

  • 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]0

    • or  [Zn(H2O)6]2+(aq)  + 2OH(aq) rev Zn(OH)2(aq) +  6H2O(l)

    •  A precipitation reaction which you can expression via various equations!

  • Zinc ions with excess sodium hydroxide:

    • (i) [Zn(H2O)6]2+(aq)  + 4OH(aq) rev [Zn(OH)4]2–(aq) + 6H2O(l)  (from original aqueous ion)

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

      • For (i) the formation of tetrahydroxozincate ion is a ligand exchange reaction (hydroxide ion for water) with change in shape (octahedral to tetrahedral), change in co-ordination number (from 6 to 4), but no change in oxidation state of zinc (+2). However the overall charge on the zinc complex changes from 2+ to 2- (2+ 4x-1).

    • 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)6]2+(aq) + 4NH3(aq) rev [Zn(NH3)4]2+(aq) + 6H2O(l)  (formation from original aqueous ion)

      • The formation of the tetraammine zinc(II) ion is a ligand exchange reaction (ammonia for water) with change in shape (octahedral to tetrahedral), co-ordination number changes (from 6 to 4), but no change in the oxidation state of zinc (+2) or overall change in the net charge on the zinc complex ion (2+, since both ligands involved are neutral).

    • or  Zn(OH)2(s)  + 4NH3(aq) rev [Zn(NH3)4]2+(aq) + 2OH(aq) (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 zinc hydroxide, Zn(OH)2.

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

    • better prepared using less alkaline 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  (equations below)

        • Remember [H2O] is not included in the equilibrium expression.

        • The data also assumes a ligand coordination number of 4 for all the complexes involved.

        • Many of the species involving the hydrated zinc ion can be partially substituted and hydrated and exist as octahedral complexes, but I've ignored these complications and denoted the end product in terms of the ligand substitution i.e. the water ligand exchanged for CN-, NH3, X- (halide).

    • 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) + 4I(aq) ==> [ZnI4]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 high 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

The Extraction and Purification of Zinc

  • Zinc is extracted from either zinc blende/sphalerite ore (zinc sulfide) or sometimes calamine/Smithsonite ore (zinc carbonate).
  • (1) The zinc sulfide 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 sulfuric acid and purified electrolytically as described below.
    • (b) Two stages
      • (i) It is dissolved and neutralised with dilute sulfuric acid to form impure zinc sulfate 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 ... ???

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


GCSE Level Notes on Transition Metals (for the basics)

The chemistry of Scandium * Titanium * Vanadium * Chromium * Manganese

The chemistry of Iron * Cobalt * Nickel * Copper * Zinc * Silver & Platinum

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, Eř * 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 Some pages have a matching sub-index

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

Group numbering and the modern periodic table

The original group numbers of the periodic table ran from group 1 alkali metals to group 0 noble gases (= group 8). To account for the d block elements and their 'vertical' similarities, in the modern periodic table, groups 3 to group 0/8 are numbered 13 to 18. So, the p block elements are referred to as groups 13 to group 18 at a higher academic level, though the group 3 to 0/8 notation is still used, but usually at a lower academic level. The 3d block elements (Sc to Zn) are now considered the head (top) elements of groups 3 to 12.

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