Revision Guide: Important Compounds of Zinc (Zn)

Introduction to the Major Compounds of Zinc

Zinc (Zn) is a d-block element, primarily exhibiting a +2 oxidation state in its compounds. Its compounds are widely used in various industrial processes, medicine, and as reagents in chemistry. Key compounds of zinc commonly tested in high school examinations include Zinc Oxide, Zinc Sulphate, and Zinc Hydroxide, all exhibiting distinct chemical properties and applications.

Zinc Oxide (ZnO)

Zinc Oxide is one of the most significant inorganic compounds of zinc.

Chemical Formula

ZnO

Common Names

Zinc White, Philosopher’s Wool

Laboratory Preparation

1. By Heating Zinc Carbonate: When zinc carbonate is heated, it decomposes to form zinc oxide and carbon dioxide. ZnCO₃(s) \xrightarrow\{\Delta\} ZnO(s) + CO₂(g)

2. By Heating Zinc Metal in Air (Industrial Method): Zinc metal reacts with oxygen when heated, producing zinc oxide. 2Zn(s) + O₂(g) \xrightarrow\{\Delta\} 2ZnO(s)

Properties and Reactions

• Appearance: White solid at room temperature, turns yellow on heating, and reverts to white upon cooling (thermochromism).
• Amphoteric Nature: Zinc oxide reacts with both acids and strong bases.
  • Reaction with Acids: ZnO(s) + 2HCl(aq) \rightarrow ZnCl₂(aq) + H₂O(l)
  • Reaction with Strong Bases (e.g., Sodium Hydroxide): ZnO(s) + 2NaOH(aq) + H₂O(l) \rightarrow Na₂[Zn(OH)₄](aq) (Sodium Tetrahydroxozincate(II)) Alternatively, in concentrated conditions: ZnO(s) + 2NaOH(aq) \rightarrow Na₂ZnO₂(aq) + H₂O(l) (Sodium Zincate)
• Reduction: Can be reduced to zinc metal by carbon at high temperatures. ZnO(s) + C(s) \xrightarrow\{\Delta\} Zn(s) + CO(g)

Zinc Sulphate (ZnSO₄·7H₂O)

Zinc Sulphate is a common salt of zinc, widely used in various applications.

Chemical Formula

ZnSO₄·7H₂O (Heptahydrate, commonly known as White Vitriol) Anhydrous form: ZnSO₄

Preparation Process

1. From Zinc Metal and Dilute Sulphuric Acid: Zinc metal reacts with dilute sulphuric acid to produce zinc sulphate and hydrogen gas. Zn(s) + H₂SO₄(aq) \rightarrow ZnSO₄(aq) + H₂(g)

2. From Zinc Oxide and Dilute Sulphuric Acid: Zinc oxide reacts with dilute sulphuric acid to yield zinc sulphate and water. ZnO(s) + H₂SO₄(aq) \rightarrow ZnSO₄(aq) + H₂O(l)

Properties and Exam-Relevant Reactions

• Appearance: White crystalline solid, efflorescent (loses water of crystallisation on exposure to air).
• Thermal Decomposition:
  • Upon heating, the heptahydrate loses its water of crystallization to form anhydrous zinc sulphate. ZnSO₄·7H₂O(s) \xrightarrow\{\Delta, \text\{ ~280°C\}\} ZnSO₄(s) + 7H₂O(g)
  • Further strong heating of anhydrous zinc sulphate decomposes it into zinc oxide, sulphur dioxide, and oxygen. 2ZnSO₄(s) \xrightarrow\{\Delta, \text\{ ~750°C\}\} 2ZnO(s) + 2SO₂(g) + O₂(g)
• Reaction with Alkalis: When an alkali is added to a zinc sulphate solution, white precipitate of zinc hydroxide is formed. ZnSO₄(aq) + 2NaOH(aq) \rightarrow Zn(OH)₂(s) + Na₂SO₄(aq) This precipitate is amphoteric and dissolves in excess strong alkali to form a soluble complex: Zn(OH)₂(s) + 2NaOH(aq) \rightarrow Na₂[Zn(OH)₄](aq) (Sodium Tetrahydroxozincate(II))
• Qualitative Analysis (Test for Zn²⁺ ions):
  • With Ammonium Hydroxide: Addition of ammonium hydroxide to a zinc sulphate solution gives a white gelatinous precipitate of zinc hydroxide, which dissolves in excess ammonium hydroxide forming a soluble tetraamminezinc(II) complex. ZnSO₄(aq) + 2NH₄OH(aq) \rightarrow Zn(OH)₂(s) + (NH₄)₂SO₄(aq) Zn(OH)₂(s) + 4NH₄OH(aq) \rightarrow [Zn(NH₃)₄](OH)₂(aq) + 4H₂O(l)
  • With Hydrogen Sulphide (H₂S): No precipitate in acidic medium. In basic or ammoniacal medium, a white precipitate of zinc sulphide (ZnS) is formed. Zn²⁺(aq) + H₂S(g) + 2NH₄OH(aq) \rightarrow ZnS(s) + 2NH₄⁺(aq) + 2H₂O(l)

Zinc Hydroxide (Zn(OH)₂)

Zinc Hydroxide is an important amphoteric hydroxide of zinc.

Chemical Formula

Zn(OH)₂

Preparation

1. By Adding Soluble Alkali to a Zinc Salt Solution: When a soluble base like sodium hydroxide or ammonium hydroxide is added to a solution of a zinc salt (e.g., zinc chloride, zinc sulphate), a white gelatinous precipitate of zinc hydroxide is formed. ZnCl₂(aq) + 2NaOH(aq) \rightarrow Zn(OH)₂(s) + 2NaCl(aq) ZnSO₄(aq) + 2NH₄OH(aq) \rightarrow Zn(OH)₂(s) + (NH₄)₂SO₄(aq)

Properties and Reactions

• Appearance: White gelatinous precipitate.
• Amphoteric Nature: Zinc hydroxide reacts with both acids and strong bases.
  • Reaction with Acids: Zn(OH)₂(s) + 2HCl(aq) \rightarrow ZnCl₂(aq) + 2H₂O(l)
  • Reaction with Strong Bases (e.g., excess NaOH): Zn(OH)₂(s) + 2NaOH(aq) \rightarrow Na₂[Zn(OH)₄](aq) (Sodium Tetrahydroxozincate(II), soluble)
  • Reaction with Ammonia (excess NH₄OH): Zn(OH)₂(s) + 4NH₄OH(aq) \rightarrow [Zn(NH₃)₄](OH)₂(aq) + 4H₂O(l) (Tetraamminezinc(II) Hydroxide, soluble complex)

Comparative Properties: Key Zinc Compounds

Compound	Chemical Formula	Nature	Solubility (in water)	Key Characteristics
Zinc Oxide	ZnO	Amphoteric	Insoluble	White solid, turns yellow on heating; pigment
Zinc Sulphate	ZnSO₄·7H₂O (hydrate)	Salt of strong acid/base	Soluble	White Vitriol; efflorescent; source of Zn²⁺ ions
Zinc Hydroxide	Zn(OH)₂	Amphoteric	Insoluble (precipitate)	White gelatinous precipitate; dissolves in excess alkali/ammonia
Zinc Chloride	ZnCl₂	Acidic (aqueous)	Highly soluble	Deliquescent; Lewis acid; used as a flux in soldering