Copper (Cu): Chemical Reactions and Properties

Chemical Properties Overview

Copper (Cu) is a transition metal with atomic number 29. It exhibits characteristic properties typical of d-block elements, including variable oxidation states (+1 and +2, with +2 being more stable in aqueous solutions) and complex formation.

Reactivity Series Position

Copper is positioned below hydrogen in the electrochemical series (standard electrode potential Cu²⁺/Cu = +0.34 V). This indicates its relatively low reactivity compared to metals above hydrogen (e.g., Fe, Zn) and its inability to displace hydrogen from dilute non-oxidizing acids.

Electronegativity

Copper has an electronegativity of approximately 1.90 on the Pauling scale, indicating moderate electronegativity.

General Reactivity

Copper is a relatively unreactive metal. It does not readily corrode in dry air but tarnishes in moist air. Its reactions often require higher temperatures or the presence of oxidizing agents.

Action of Air and Oxygen

In Dry Air

Copper does not react with dry air at room temperature.

In Moist Air

In the presence of moist air and carbon dioxide over extended periods, copper slowly tarnishes, forming a greenish layer known as verdigris (basic copper carbonate).

2 Cu(s) + O₂(g) + CO₂(g) + H₂O(l) → CuCO₃·Cu(OH)₂(s) (Basic Copper Carbonate, Verdigris)

Heating in Air/Oxygen

1. At moderate temperatures (~300-800°C): Copper reacts with oxygen to form black copper(II) oxide (cupric oxide). 2 Cu(s) + O₂(g) → 2 CuO(s) (Black)

2. At higher temperatures (~1000°C) with limited oxygen: Copper can react to form red copper(I) oxide (cuprous oxide). 4 Cu(s) + O₂(g) → 2 Cu₂O(s) (Red)

Action of Water and Steam

Copper does not react with cold water, hot water, or steam, even at high temperatures, due to its position below hydrogen in the reactivity series.

Action of Acids and Bases

With Dilute Non-oxidizing Acids (e.g., HCl, dilute H₂SO₄)

Copper does not react with dilute hydrochloric acid or dilute sulfuric acid in the absence of air, as it cannot displace hydrogen. Cu(s) + 2 HCl(aq) → No reaction Cu(s) + H₂SO₄(dilute) → No reaction

However, in the presence of air, slow oxidation can occur: 2 Cu(s) + 4 HCl(aq) + O₂(g) → 2 CuCl₂(aq) + 2 H₂O(l)

With Oxidizing Acids

1. With Dilute Nitric Acid (HNO₃): Copper reacts to produce copper(II) nitrate and nitric oxide gas. 3 Cu(s) + 8 HNO₃(dilute) → 3 Cu(NO₃)₂(aq) + 2 NO(g) + 4 H₂O(l)

2. With Concentrated Nitric Acid (HNO₃): Copper reacts vigorously to produce copper(II) nitrate, nitrogen dioxide gas (a brown gas), and water. Cu(s) + 4 HNO₃(conc.) → Cu(NO₃)₂(aq) + 2 NO₂(g) + 2 H₂O(l)

3. With Hot Concentrated Sulfuric Acid (H₂SO₄): Copper reacts to produce copper(II) sulfate, sulfur dioxide gas, and water. Cu(s) + 2 H₂SO₄(conc.) → CuSO₄(aq) + SO₂(g) + 2 H₂O(l)

With Alkalis/Bases

Copper does not react with aqueous solutions of strong alkalis like sodium hydroxide (NaOH) or potassium hydroxide (KOH), as it is not an amphoteric metal. Cu(s) + NaOH(aq) → No reaction

Key Laboratory Test/Identification Reactions (for Cu²⁺ ions)

These tests are crucial for identifying Cu²⁺ ions in qualitative analysis.

1. Action of Ammonium Hydroxide (NH₄OH)

• Small amount of NH₄OH: A pale blue precipitate of copper(II) hydroxide forms. Cu²⁺(aq) + 2 OH⁻(aq) → Cu(OH)₂(s) ↓ (Pale blue precipitate)

• Excess NH₄OH: The pale blue precipitate dissolves to form a deep blue, soluble complex, tetraamminecopper(II) ion. Cu(OH)₂(s) + 4 NH₃(aq) → [Cu(NH₃)₄]²⁺(aq) + 2 OH⁻(aq) (Deep blue solution)

2. Action of Sodium Hydroxide (NaOH)

• Small amount/Excess NaOH: A pale blue precipitate of copper(II) hydroxide forms, which is insoluble in excess alkali. Cu²⁺(aq) + 2 NaOH(aq) → Cu(OH)₂(s) ↓ + 2 Na⁺(aq) (Pale blue precipitate)

3. Action of Potassium Iodide (KI)

When potassium iodide is added to a solution containing Cu²⁺ ions, a redox reaction occurs. White cuprous iodide precipitates, and iodine (I₂) is liberated, imparting a brown color to the solution. 2 Cu²⁺(aq) + 4 KI(aq) → 2 CuI(s) ↓ + I₂(aq) (White precipitate, brown solution)

4. Action of Potassium Ferrocyanide (K₄[Fe(CN)₆])

A chocolate brown precipitate of copper(II) ferrocyanide is formed. This is a highly sensitive test for Cu²⁺ ions. 2 Cu²⁺(aq) + K₄[Fe(CN)₆](aq) → Cu₂[Fe(CN)₆](s) ↓ + 4 K⁺(aq) (Chocolate brown precipitate)