|
History and Production
Derived from German zink, meaning of obscure origin; or possibly zinke, meaning spike or tooth. Zinc has been used as an alloy, especially with copper to make brass thousands of years ago.
As early as 13th century AD reduction of calamine (ZnCO3) was already achieved in India. Nowaday, it is commercially obtained by roasting its ore followed by reduction of the oxide with carbon. The commerically
important oares are blende or sphalerite (ZnS), calamine and hemimorphite (Zn4Si2O7(OH)2.H2O). Zinc is important in making alloys such as brass,
bronze and nickel silver. It is also used to galvanize iron to prevent corrosion. It is also made into sheets for making roof. It is also used in making dry cells such as
the carbon-zinc cells, mercury cells and alkaline manganese cells.
Physical Data
It is bluish-white metallic in appearance but tarnish easily in air. It is brittle at room temperatue but malleable when heated to 100-150°C.
Interatomic distance: 267 pm
Melting point: 419.53°C
Boiling point: 907°C
Thermal conductivity/Wm-1K-1: 116 (27°C)
Density/kgm-3: 7133 (20°C), 6577 (m.p.)
Standard Thermodynamic Data (atomic gas)
Enthalpy of formation: 130.4 kJ/mol
Gibbs free energy of formation: 94.8 kJ/mol
Entropy: 161.0 J/mol K
Heat capacity: 20.8 J/mol K
Electronic data
Electronic configuration: [Ar] 3d10 4s2
Term symbol: 1S0
Electron affinity: (not stable) Electronegativity (Pauline): 1.65
Ionization energy (first, second, third): 906.403, 1733.30, 3832.69 kJ/mol
Chemical properties
when zinc is heated in air, it burns with a greenish-blue flame giving a fibrous deposit of zinc oxide, referred to as the 'philisopher's wool' by the alchemists.
Zinc combines directly with chlorine and sulfur but not with nitrogen. However, the compound zinc nitride, Zn3N2, can be obtained by passing ammonia
over red-hot zinc.
Zinc reacts with aqueous sulfuric acids and hydrochloric acids to liberate hydrogen gas, whereas with an alkaline solution
the metal dissolves to form zincate ions, [Zn(OH)4]2-.
Test for zinc ions:
(1) Potassiuma and sodium hydroxide solutions give a white precipitate soluble in excess.
(2) With ammonium sulfide, a white precipitate of zinc sulfide is obtained.
(3) With potassium cyanide, the white precipitate of zinc cyanide first formed, which dissolves in excess forming a complex ion of [Zn(CN)4]2-.
| 
