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Element Index
Zinc
Zirconium
Chemical's truth
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Zinc oxide
(Flowers of zinc, philosopher's wool, zinc white)
CAS no: 1314-13-2 Formula: ZnO Molecular weight: 81.39
Physical Data
Appearance: White or yellowish white odorless, hexagonal crystalline powder (wurtzite structure).
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Melting point: 1974°C (decomposes)
Boiling point: -
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Density: 5.67 g/cm3
Solubility: -
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Thermodynamic Data
All data are expressed in the standard conditions.
Enthalpy of formation: -350.5 kJ/mol
Gibbs energy of formation: -320.5 kJ/mol
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Entropy: 43.7 J/mol K
Heat capacity: 40.3 J/mol K
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Production and preparation
The archaic name for zinc oxide is philosopher's wool. It occurs in nature as zincite with characteristic blood-red color (due to the presence of manganese and iron) and orange-yellow streaks. It is also
commonly associated with franklinite and calcite minerals. Zinc oxide can be prepared, to a high purity, by burning zinc vapor in air.
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Behavior and Chemical Properties
Zinc oxide crystallizes in the wurtzite structure and its bonding characteristic is between that of ionic and covalent. When heated, the color changes to yellow and back to white when cooled.
The color change is due to the release of oxygen atoms from the crystal lattice to give nonstoichiometric phase, Zn(1+n)O. Different colors, such as green and brown, can be obtained
when Zn is doped in the lattice defects.
The oxide is amphoteric, meaning that it can react with both acids and alkalis. For example, with hydrochloric acid, soluble zinc chloride is formed.
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With aqueous sodium hydroxide, soluble, zincate ions are formed. The formers are complex ions with formulas such as [Zn(OH)3]- and [Zn(OH)4]2-.
With molten sodium hydroxide, the zincate ion may probably be in the form of ZnO22-.
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History and Uses
Zinc oxide has been known to exist even longer than the metallic zinc itself, as the former is an inevitable byproduct of production of brass (an alloy of zinc and tin) in ancient times.
As early as 200 BC, the Romans produced brass by heating in crucibles a mixture of zinc oxide and charcoal covered with lumps of metallic copper.
Zinc was not discovered at earlier times because, unlike copper and tin oxides, zinc oxide is very difficult to reduce. Nowadays, it is used in the production of zinc by heating with powdered
coke in the furnace at 1400°C. At this temperature, zinc vapor is produced which is then condensed to give solid zinc.
Zinc oxide had been widely used as a white pigment in paint (Chinese white). It was used extensively in a dense form of oxide in oil since 1845 in order to replace the toxic lead-based pigments.
However, zinc oxide is now largely being replaced by titanium(IV) dioxide, a whiter chemical substance.
Zinc oxide is the most piezoelectric of all materials and is widely used as a transducer in electronic devices. It is also used in semiconductor applications in the production of phosphers for television tubes
and fluorescent lamp.
In pharmaceuticals it is being used as a mild antiseptic ointment (calamine lotion), astringent and sun-blocking skin cream.
It is also incorporated in cross-linking rubber where it shortens the time of vulcanization and acts as heat dissipater.
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Hazard, Storage and Handling
Harmful if inhaled. Irritating to eyes and repiratory. Care should be taken to avoid inhaling the dust, especially working with a large quantity of the
materials.
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(Last update: November 2002) |
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