Copper Compounds: Revision Guide

Introduction to Major Compounds of Copper

Copper (Cu) is a transition metal commonly found in +1 (cuprous) and +2 (cupric) oxidation states. In high school chemistry, most important compounds of copper studied are derived from the more stable +2 oxidation state. These compounds play significant roles in various industrial, agricultural, and analytical applications.

Copper(II) Oxide (CuO)

Chemical Formula: CuO
Common Name: Cupric oxide, Black copper oxide
Laboratory Preparation:
- From Copper Metal: Heating copper metal in the presence of air or oxygen.
```
2Cu (s) + O₂ (g) → 2CuO (s)
```
- From Copper(II) Nitrate: Thermal decomposition of copper(II) nitrate.
```
2Cu(NO₃)₂ (s) → 2CuO (s) + 4NO₂ (g) + O₂ (g)
```
- From Copper(II) Hydroxide or Carbonate: Thermal decomposition of copper(II) hydroxide or copper(II) carbonate.
```
Cu(OH)₂ (s) → CuO (s) + H₂O (g)
CuCO₃ (s) → CuO (s) + CO₂ (g)
```
Properties and Exam-Relevant Reactions:
- Physical: Black solid, insoluble in water.
- Nature: Basic oxide, but exhibits slight amphoteric character with concentrated strong alkalis.
- Reaction with Acids: Reacts with acids to form corresponding copper(II) salts and water.
```
CuO (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l)
CuO (s) + 2HCl (aq) → CuCl₂ (aq) + H₂O (l)
```
- Reaction with Concentrated Alkalis:
```
CuO (s) + 2NaOH (aq) + H₂O (l) → Na₂[Cu(OH)₄] (aq) (Sodium tetrahydroxocuprate(II))
```
- Reduction: Reduced to metallic copper by reducing agents like hydrogen, carbon, or carbon monoxide at high temperatures.
```
CuO (s) + H₂ (g) → Cu (s) + H₂O (g)
CuO (s) + C (s) → Cu (s) + CO (g)
CuO (s) + CO (g) → Cu (s) + CO₂ (g)
```

Copper(II) Sulfate Pentahydrate (CuSO₄·5H₂O)

Chemical Formula: CuSO₄·5H₂O (Hydrated form), CuSO₄ (Anhydrous form)
Common Name: Blue vitriol (for the pentahydrate)

Preparation Process:

From Copper Metal: Reaction of copper with hot, concentrated sulfuric acid.
```
Cu (s) + 2H₂SO₄ (conc.) → CuSO₄ (aq) + SO₂ (g) + 2H₂O (l)
```

From Copper(II) Oxide/Hydroxide/Carbonate: Reaction with dilute sulfuric acid.

CuO (s) + H₂SO₄ (dil.) → CuSO₄ (aq) + H₂O (l)
Cu(OH)₂ (s) + H₂SO₄ (dil.) → CuSO₄ (aq) + 2H₂O (l)
CuCO₃ (s) + H₂SO₄ (dil.) → CuSO₄ (aq) + H₂O (l) + CO₂ (g)

Properties and Exam-Relevant Reactions:
- Physical: Blue crystalline solid (pentahydrate), readily soluble in water. Anhydrous CuSO₄ is white.
- Dehydration/Efflorescence: Loses water of crystallization upon heating, changing color from blue to white.
```
CuSO₄·5H₂O (s) <div className="text-sm"><sup>100°C</sup></div> → CuSO₄·H₂O (s) + 4H₂O (g)
CuSO₄·H₂O (s) <div className="text-sm"><sup>200°C</sup></div> → CuSO₄ (s) + H₂O (g)
```
- Hydrolysis: Solutions are acidic due to the hydrolysis of the Cu²⁺ ion.
```
Cu²⁺ (aq) + 2H₂O (l) ⇌ Cu(OH)₂ (s) + 2H⁺ (aq)
```
- Reaction with Alkalis: Forms a pale blue precipitate of copper(II) hydroxide.
```
CuSO₄ (aq) + 2NaOH (aq) → Cu(OH)₂ (s) ↓ + Na₂SO₄ (aq)
CuSO₄ (aq) + 2NH₄OH (aq) → Cu(OH)₂ (s) ↓ + (NH₄)₂SO₄ (aq)
```
- Reaction with Excess Ammonia Solution: The initially formed Cu(OH)₂ dissolves to form a deep blue soluble complex, tetraamminecopper(II) ion.
```
Cu(OH)₂ (s) + 4NH₃ (aq) → [Cu(NH₃)₄]²⁺ (aq) + 2OH⁻ (aq)
```
  or directly from CuSO₄ solution with excess ammonia:
```
CuSO₄ (aq) + 4NH₃ (aq) → [Cu(NH₃)₄]SO₄ (aq)
```
- Displacement Reactions: More reactive metals (e.g., Fe, Zn) displace copper from its solution.
```
Fe (s) + CuSO₄ (aq) → FeSO₄ (aq) + Cu (s)
Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)
```

Copper(II) Hydroxide (Cu(OH)₂)

Chemical Formula: Cu(OH)₂
Common Name: Cupric hydroxide
Preparation Process:
- From Copper(II) Salt Solution: Precipitation by adding a soluble base (e.g., sodium hydroxide, ammonium hydroxide) to an aqueous solution of a copper(II) salt.
```
CuSO₄ (aq) + 2NaOH (aq) → Cu(OH)₂ (s) ↓ + Na₂SO₄ (aq)
CuCl₂ (aq) + 2NH₄OH (aq) → Cu(OH)₂ (s) ↓ + 2NH₄Cl (aq)
```
Properties and Exam-Relevant Reactions:
- Physical: Pale blue, gelatinous precipitate, insoluble in water.
- Thermal Decomposition: Decomposes readily upon gentle heating to black copper(II) oxide and water.
```
Cu(OH)₂ (s) → CuO (s) + H₂O (g)
```
- Amphoteric Nature:
  - Reaction with Acids (basic property):
```
Cu(OH)₂ (s) + H₂SO₄ (aq) → CuSO₄ (aq) + 2H₂O (l)
Cu(OH)₂ (s) + 2HCl (aq) → CuCl₂ (aq) + 2H₂O (l)
```
  - Reaction with Concentrated Strong Alkalis (acidic property): Forms a soluble complex.
```
Cu(OH)₂ (s) + 2NaOH (aq) → Na₂[Cu(OH)₄] (aq)
```
- Reaction with Excess Ammonia Solution: Forms a deep blue soluble tetraamminecopper(II) complex.
```
Cu(OH)₂ (s) + 4NH₃ (aq) → [Cu(NH₃)₄]²⁺ (aq) + 2OH⁻ (aq)
```

Copper(II) Carbonate (CuCO₃)

Chemical Formula: CuCO₃ (Note: Pure CuCO₃ is unstable. Often encountered as basic copper carbonate, e.g., malachite Cu₂(OH)₂CO₃)
Common Name: Cupric carbonate
Brief Guide:
- Preparation: While direct precipitation of pure CuCO₃ is challenging, it can be conceptually formed by adding a soluble carbonate to a copper(II) salt solution. In practice, basic copper carbonates are more commonly formed.
```
CuSO₄ (aq) + Na₂CO₃ (aq) → CuCO₃ (s) ↓ + Na₂SO₄ (aq)
```
- Properties: Green solid, insoluble in water.
- Thermal Decomposition: Decomposes upon heating to black copper(II) oxide and carbon dioxide gas.
```
CuCO₃ (s) → CuO (s) + CO₂ (g)
```
- Reaction with Acids: Reacts with acids to produce a copper(II) salt, water, and carbon dioxide gas (effervescence).
```
CuCO₃ (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l) + CO₂ (g)
```

Comparative Properties of Key Copper(II) Compounds

Property	Copper(II) Oxide (CuO)	Copper(II) Sulfate (CuSO₄·5H₂O)	Copper(II) Hydroxide (Cu(OH)₂)	Copper(II) Carbonate (CuCO₃)
Color	Black	Blue (hydrated), White (anhydrous)	Pale blue precipitate	Green
Solubility in Water	Insoluble	Soluble	Insoluble	Insoluble
Nature (Acid/Base)	Basic (Amphoteric)	Salt (solution is acidic)	Basic (Amphoteric)	Basic
Effect of Heat	Stable (reduces)	Dehydrates, then decomposes	Decomposes to CuO + H₂O	Decomposes to CuO + CO₂
Reaction w/ Dilute Acids	Forms Cu(II) salts	No reaction (is a salt)	Forms Cu(II) salts	Forms Cu(II) salts, CO₂
Reaction w/ Excess Aq. NH₃	No reaction	Forms deep blue complex	Forms deep blue complex	No reaction

Copper Compounds Revision Guide