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

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

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

na = data not applicable to zinc

Extended data table for ZINC

• 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.

      • 

      • 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

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

• Electrode potential chart for zinc 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) + H_2(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.

  •  See Appendix 4. complex ion colour theory

• 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 ZnSO_4(aq) or zinc chloride ZnCl_2(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) 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) [Zn(OH)₄]^2–_(aq) + 6H₂O_(l)  (from original aqueous ion)

  • or (ii) Zn(OH)_2(s) + 2OH^–_(aq) [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) ===> ZnCl_2(aq) + H₂O_(l)

• Zinc ions with excess ammonia:

  • [Zn(H₂O)₆]²⁺_(aq) + 4NH_3(aq) [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)  + 4NH_3(aq) [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) + CO₃^2–_(aq) ===> ZnCO_3(s) 

  • better prepared using less alkaline NaHCO₃: Zn²⁺_(aq) + 2HCO₃^–_(aq) ===> ZnCO_3(s) + H₂O_(l) + CO_2(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

      • K_stab = [[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 K_stab 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

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

WHAT NEXT? 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.

Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.