Introduction to Zinc

Zinc

The compounds of zinc are:

Hydrides

The term hydride is used to indicate compounds of the type M_xH_y and it is not necessarily that indicated any compounds listed would behave as same as hydrides chemically.

• Zinc hydride: ZnH₂

Fluorides

• Zinc difluoride: ZnF₂

Chlorides

• Zinc dichloride: ZnCl₂

Bromides

• Zinc dibromides: ZnBr₂

Iodides

• Zinc diiodide: ZnI₂

Oxides

• Zinc oxide: ZnO
• Zinc peroxide: ZnO₂

Sulfides

• Zinc sulphide: ZnS

Selenides

• Zinc selenide: ZnSe

Tellurides

• Zinc telluride: ZnTe

Nitrides

• Trizinc dinitride: Zn₃N₂

Complexes

• Hexaaquozinc dinitrate: Zn(NO₃)₂.6H₂O
• Zinc sulphate pentahydrate: ZnSO₄.7H₂O

Ores

Calamine → ZnCO₃

Zincite → ZnO

Zinc blende → ZnS

Willemite → ZnO.SiO₂

Extraction of zinc (Zn)

It is extracted from ZnS.

• Concentration: at first the ore is crushed into fine pieces and then powdered into the ball mills. The concentrated ore is then subjected to froth flotation method.
• Roasting: The concentrated ore is roasted in the presence of air at about 900 where zinc sulphide is turned to zinc oxide.

$$ZnS+O_2\longrightarrow ZnO+SO_2$$

During the process, the impurities like arsenic, phosphorus, sulphur etc. are expelled out in the form of volatile oxide.

$$S+O_2\longrightarrow SO_2$$

$$4P+5O_2\longrightarrow 2P_2O_5$$

$$4As+3O_2\longrightarrow 2As_2O_3$$

• Vertical retord processing: The obtained zinc oxide is mixed with coke dust in the ratio 2:1 then the small briquettes are made from it. These briquettes are put into vertical retord.

$$ZnO+C\longrightarrow Zn+CO$$

When the furnace is heated by burning to about 1400, the oxide ores in briquettes gets reduced and vapors of zinc and carbon monoxide are formed and carried to the condenser. The vapors of zinc condense in the condenser in the form of molten mass called zinc splinter.

1. Fractional distillation: At first, zinc splinter is distilled over 1000 where zinc and Cd distil over leaving behind the impurities. The impure zinc is then distilled leaving behind the zinc.
2. Electrolysis: In this method impure zinc is taken as the anode and pure zinc as the cathode. When the electric current is passed over, impure zinc from anode dissolve out and pure zinc is deposited at the cathode.

At anode

Zn – 2e^- → Zn²⁺

At cathode

Zn²⁺ + 2e^- → Zn

Physical properties

• It is bluish white shining metal.
• It is a good conductor of heat and electricity.
• It is malleable and ductile.

Chemical properties

• Action with air: when zinc reacts with moist air, it forms basic carbonate of zinc.

$$Zn+O_2+CO_2+H_2O\longrightarrow ZnCO_3.Zn(OH)_2$$

• Action with water: zinc reacts with water to form zinc hydroxide and hydrogen gas.

$$Zn+H_2O\longrightarrow Zn(OH)_2+H_2$$

• Action with heat: zinc when heated in the presence of air, it burns with the greenish-blue flame of zinc oxide. It is also known as philosopher’s wool.

$$2Zn+O_2\longrightarrow 2ZnO$$

• Action with NaOH: On boiling with the alkali solution (NaOH) it evolved out hydrogen gas.

$$Zn+NaOH\longrightarrow Na_2ZnO_2+H_2$$

• Action with metal salt: It displaces less electropositive metals from their salt.

$$Zn+CuSO_4\longrightarrow ZnSO_4+Cu$$

• Reaction with acid

$$Zn+HCl\longrightarrow ZnCl_2+H_2O (with\space HCl\space it\space forms\space zinc\space chlorise)$$

Uses of Zinc

• Mostly zinc is used to galvanize other metals in order to prevent them from rusting. Galvanized steel is used for car bodies, street lamp posts suspension bridges etc..

• Large quantities of zinc are used to produce metals by pouring molten metal into a mold, which is widely used in the automobile, electrical and hardware industries. Zinc is also used in alloys such as brass, nickel silver, and aluminium solder.

• Zinc oxide is used in the manufacture of many products such as paints, rubber, cosmetics, pharmaceuticals electrical equipment etc. Zinc sulfide is used in making luminous paints, fluorescent lights and x-ray screens.

Natural abundance

Zinc is found in several ores in the form of zinc blende and calamine. The principal mining areas are in China, Australia, and Peru. Commercially, zinc is obtained from its ores by concentrating and roasting the ore, then reducing it to zinc by heating with carbon or by electrolysis. World production is more than 11 million tons a year.

Reference

pulse, Tracy. Introduction to chemistry. u.s.a: flex book, 2010.

Pathak, Sita Karki. The Text Book of Chemistry. Kathmandu: Vidhyarthi Pustak Bhandar, 2012.