Compounds Of Zinc

Zinc oxide (ZnO)

The synonyms of zinc oxides are as follows:

• zinc oxide
• zinc(II) oxide

The oxidation number of zinc in zinc oxide is 2.

• Formula: ZnO
• Hill system formula: O₁Zn₁
• Formula weight: 81.389
• Class: oxide

• Colour: white
• Appearance: crystalline solid
• Melting point: 2248°C; 1975°C
• Boiling point: sublimes at 1800
• Density: 5600 kg m^-3

Element analysis

Zinc oxide constitutes 19.66% of oxygen and 80.34% of zinc in it.

Preparation

• It is prepared by heating zinc in the presence of air.

2Zn + O₂ → 2ZnO

• It is also prepared by heating the compound of zinc i.e. zinc carbonate (ZnCO₃), zinc hydroxide {Zn(OH)₂}, zinc nitrate {Zn(NO₃)₂}.

ZnCO₃ → ZnO + CO₂

Zn(OH)₂ → ZnO + H₂O

2Zn(NO₃)₂ → 2ZnO + 4NO₂ + O₂

Physical properties

• It is white fibrous powder.
• It is insoluble in water.

Chemical properties

• It reacts with both acid and base to form salt and water.

ZnO + HCl → ZnCl₂ + H₂O (reaction with acid)

ZnO + NaOH → Na₂ZnO₂ + H₂O (reaction with base)

• Reaction with cobalt nitrate: it reacts with cobalt nitrate to form cobalt zincate. It is known as Rinman’s green.

2ZnO + 2CO(NO₃)₂ → 2COZnO₂ + 4NO₂ + O₂

Rinman’s green

Uses

• Used as white paint.
• Used for preparing Rinman’s green pigment.
• Used for preparing other compounds of green.

Zinc sulphate (ZnSO₄.7H₂O) or White vitrol

The synonyms of zinc sulphate are as follows:

• zinc sulphate heptahydrate
• zinc(II) sulphate 7-water
• zinc sulfate 7-water
• zinc sulfate heptahydrate
• zinc sulphate 7-water
• zinc(II) sulfate 7-water

The oxidation number of zinc in zinc sulphate heptahydrate is 2.

• Formula: ZnSO₄.7H₂O
• Hill system formula: H₁₄O₁₁S₁Zn₁
• Formula weight: 287.561
• Class: complex

• Colour: white
• Appearance: crystalline solid
• Melting point: 100°C (dehydrates)
• Boiling point: decomposes at > 500
• Density: 1957 kg m^-3

Zinc sulphate constitutes 4.92% Hydrogen, 61.20% oxygen, 11.15% sulphur and 22.74% zinc in it.

Preparation

• It is prepared by heating zinc sulphide below 800 in the presence of air.

ZnS + 2O₂ → ZnSO₄

• White vitrol can also be prepared by heating zinc (Zn), zinc oxide (ZnO),zinc carbonate (ZnCO₃) with dilute H₂SO₄

Zn + H₂SO₄ → ZnSO₄ + H₂ ↑

ZnO + H₂SO₄ → ZnSO₄ + H₂O

ZnCO₃ + H₂SO₄ → ZnSO₄ + H₂ O + CO₂

On crystallization crystals of white vitrol can be obtained.

ZnSO₄ + 7H₂O → ZnSO₄.7H₂O

Crystals of white vitrol are obtained.

Physical properties

• It is a colorless crystalline solid.
• It is soluble in water.

Chemical properties

• Action with heat: On heating, it loses its water of crystallization and finally gets converted into zinc oxide.

ZnSO₄.7H₂O → ZnSO₄.6H₂O → ZnSO₄.H₂O → ZnSO₄ → ZnO + SO₂ + O₂

On heating white vitrol at 70, it loses one water of crystallization and on g=further heating at 100 loses five more water of crystallization and at 300 loses all seven water of crystallization and when heated further over 800 it gets converted into zinc oxide.

• Action with barium sulphate: Zinc sulphate when treated with barium sulphide, double salt called lithopone is obtained.

ZnSO₄ + BaS → BaSO₄ + ZnS (both products are combined called lithopone)

• Reaction with NaOH: when few amount of NaOH is used then white ppt. of zinc hydroxide is obtained and if excess NaOH is used sodium zincate is obtained.

ZnSO₄ + NaOH → Zn(OH)₂ + Na₂SO₄ (Limited amount of NaOH is used)

Zn(OH)₂ + NaOH → Na₂ZnO₂ (excess amount of NaOH is used)

• Reaction with H₂S: On passing H₂S gas over zinc sulphate solution white ppt. of zinc sulphide is obtained.

ZnSO₄ + H₂S → ZnS + H₂SO₄

Uses

• Used for making lithopone.
• Used for preparing oxides of zinc.
• Used in medicine as an eye lotion.

Galvanization

It is the process of coating zinc over iron to prevent from rusting galvanization prevents rusting of iron in following ways:

1. It acts as a barrier between iron and moisture.
2. By sacrificial or cathodic protection: in his process zinc act as anode and iron acts as the cathode. They form a galvanic cell. In this process, Zn alone gets corroded preventing iron from rusting. So, this method is also called sacrificial protection.

Methods of galvanization

1. Pickling: At first the iron articles to be galvanized are cleaned and then washed with dilute HCl to remove oxides films if present. After this galvanization is done by following methods:
2. Hot dip galvanization: iron articles are dipped into a hot bath of molten zinc by which thin coating of zinc is made of the articles.
3. Sherardising: The iron article is mixed with zinc dust heated for several hours n rotating the drum. Bolts, screws etc can be sheradised by zinc.

• Metallising: In this method, the zinc is coated over the iron article by spraying molten zinc over them. This method is used for ship, bridge etc.