Zinc (Zn): Properties, Reactions & Uses | CBSE/JEE Revision

Introduction to Zinc (Zn)

Zinc (Zn) is a bluish-white, lustrous metal, critical to modern industry and biological systems. Its applications range from corrosion protection of steel to its role as an essential micronutrient in living organisms. Understanding its chemical and physical properties is fundamental for high school chemistry examinations.

CBSE/JEE Quick Revision Notes

• Atomic Symbol: Zn
• Atomic Number: 30
• Atomic Mass: 65.38 u
• Group: 12 (IIB)
• Period: 4
• Block: d-block element (though often considered a post-transition metal due to its full d-orbital in its common oxidation state).
• Common Oxidation State: +2 (Always forms Zn²⁺ ions).
• Physical State at Room Temperature: Solid
• Nature: Brittle metal at room temperature; becomes malleable at 100-150°C.
• Standard Electrode Potential (Zn²⁺/Zn): -0.76 V (highly electropositive, readily loses electrons).

Electron Configuration & Bonding Behavior

• Ground State Electron Configuration: [Ar] 3d¹⁰ 4s²
• Electronic Configuration of Zn²⁺ Ion: [Ar] 3d¹⁰
  • The complete 3d¹⁰ orbital in both its atomic and common ionic state (Zn²⁺) distinguishes Zinc from typical transition metals, leading some classifications to term it a post-transition metal or main group metal.
• Bonding: Primarily forms ionic compounds with non-metals (e.g., ZnO, ZnS, ZnCl₂). It can also form covalent compounds and a variety of complex ions due to the availability of empty orbitals and its small size for a d-block element.
• Coordination Number: Commonly exhibits coordination numbers of 4 (tetrahedral complexes, e.g., [Zn(NH₃)₄]²⁺) and sometimes 6 (octahedral complexes, e.g., [Zn(H₂O)₆]²⁺).

Crucial Chemical Reactions

Zinc’s reactivity is characterized by its electropositive nature and amphoteric behavior.

1. Reaction with Air/Oxygen

• At Room Temperature: Forms a protective passive layer of zinc oxide (ZnO) on its surface, preventing further corrosion.
• Upon Heating: Burns in air with a bluish-green flame to form zinc oxide. 2Zn(s) + O₂(g) → 2ZnO(s)

2. Reaction with Acids

• With Dilute Non-oxidizing Acids (e.g., HCl, H₂SO₄): Liberates hydrogen gas. Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) Zn(s) + H₂SO₄(dilute) → ZnSO₄(aq) + H₂(g)
• With Concentrated Sulfuric Acid: Produces sulfur dioxide gas. Zn(s) + 2H₂SO₄(conc) → ZnSO₄(aq) + SO₂(g) + 2H₂O(l)
• With Dilute Nitric Acid: Forms nitric oxide or nitrous oxide depending on concentration. 3Zn(s) + 8HNO₃(dilute) → 3Zn(NO₃)₂(aq) + 2NO(g) + 4H₂O(l)
• With Concentrated Nitric Acid: Forms nitrogen dioxide. Zn(s) + 4HNO₃(conc) → Zn(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)

3. Reaction with Bases (Amphoteric Nature)

Zinc is an amphoteric metal, reacting with strong bases to form zincates. Zn(s) + 2NaOH(aq) + 2H₂O(l) → Na₂[Zn(OH)₄](aq) + H₂(g) (Sodium tetrahydroxozincate(II))

4. Reaction with Water/Steam

• With Cold Water: No reaction.
• With Steam (at red heat): Produces zinc oxide and hydrogen gas. Zn(s) + H₂O(g) → ZnO(s) + H₂(g)

5. Displacement Reactions

Zinc, being more reactive than hydrogen and many other metals, can displace them from their salt solutions.

• Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
• Zn(s) + FeSO₄(aq) → ZnSO₄(aq) + Fe(s) (Requires heating for appreciable reaction)

6. Formation of Complexes

Zinc ions readily form stable complexes, particularly with ammonia and hydroxide ions.

• Zn²⁺(aq) + 4NH₃(aq) → [Zn(NH₃)₄]²⁺(aq) (Tetraamminezinc(II) ion)
• Zn²⁺(aq) + 4OH⁻(aq) → [Zn(OH)₄]²⁻(aq) (Tetrahydroxozincate(II) ion)

Industrial and Biological Importance

Industrial Importance

1. Galvanization: The most significant use. Zinc coating protects steel and iron from rusting by providing both a physical barrier and cathodic protection (sacrificial anode).
2. Alloys:
   • Brass: Alloy with copper (Cu), widely used for plumbing, musical instruments, and decorative items.
   • German Silver: Alloy with copper and nickel (Ni), used in cutlery and ornaments.
3. Batteries:
   • Dry Cells (Leclanché Cell): Zinc acts as the anode.
   • Alkaline Batteries: Zinc powder is used as the anode.
   • Nickel-Cadmium (Ni-Cd) Batteries: Electrodes may contain zinc.
4. Zinc Oxide (ZnO):
   • Pigment (Chinese white) in paints.
   • Filler in rubber and plastics.
   • Sunscreen and cosmetic ingredient (UV blocker).
   • Antiseptic in ointments.
5. Zinc Sulfide (ZnS):
   • Used in phosphors for cathode ray tubes (TV screens) and fluorescent lights.
   • Luminescent material.
6. Zinc Dust: Reducing agent in organic synthesis.

Biological Importance

1. Essential Trace Element: Zinc is vital for all forms of life, including humans, animals, and plants.
2. Enzyme Cofactor: Over 300 enzymes require zinc for their catalytic activity. Key examples include:
   • Carbonic Anhydrase: Essential for CO₂ transport in blood.
   • Alcohol Dehydrogenase: Involved in alcohol metabolism.
   • DNA Polymerase: Crucial for DNA replication and repair.
   • RNA Polymerase: Essential for gene expression.
3. Protein Structure: Plays a structural role in many proteins, particularly in “zinc finger” motifs that bind to DNA and regulate gene expression.
4. Immune Function: Supports a healthy immune system.
5. Wound Healing: Essential for tissue repair and cell growth.
6. Sensory Function: Critical for taste and smell perception.

A deficiency of zinc can lead to impaired growth, immune dysfunction, skin problems, and neurological disorders.