Nickel (Ni) - Exam Study Guide

Introduction

Nickel (Ni) is a silvery-white, lustrous metal with a slight golden tinge, belonging to the d-block transition elements. It is highly valued for its corrosion resistance, strength, and ability to impart these properties to alloys. Its diverse applications range from industrial catalysts and battery components to coinage and critical structural materials, making it a crucial element in modern technology and daily life.

CBSE/JEE Quick Revision Notes

• Symbol: Ni
• Atomic Number (Z): 28
• Atomic Mass (A): 58.69 u
• Group: 10
• Period: 4
• Block: d-block (Transition Metal)
• Common Oxidation States: +2 (most common), +3 (less stable, often in complexes)
• Nature: Ferromagnetic metal
• Density: 8.90 g/cm³
• Melting Point: 1455 °C
• Boiling Point: 2913 °C
• Standard Electrode Potential (E° Ni²⁺/Ni): -0.25 V
• Corrosion Resistance: High, due to passivation in air.

Electron Configuration & Bonding Behavior

Electron Configuration

• Ground State: [Ar] 3d⁸ 4s²
• Ni²⁺ Ion: [Ar] 3d⁸ (loses two 4s electrons)
• Ni³⁺ Ion: [Ar] 3d⁷ (loses two 4s electrons and one 3d electron)

Bonding Behavior

• Nickel typically forms ionic compounds in its +2 oxidation state (e.g., NiCl₂).
• It readily forms numerous coordination compounds (complexes) where it acts as a central metal ion.
• Common geometries for Ni(II) complexes include:
  • Square Planar: Often with strong field ligands, involving dsp² hybridization (e.g., [Ni(CN)₄]²⁻). Diamagnetic.
  • Tetrahedral: Often with weak field ligands, involving sp³ hybridization (e.g., [NiCl₄]²⁻). Paramagnetic.
  • Octahedral: Common with six ligands, involving sp³d² or d²sp³ hybridization (e.g., [Ni(NH₃)₆]²⁺). Paramagnetic.
• Nickel also forms organometallic compounds, notably nickel tetracarbonyl, Ni(CO)₄, where nickel exhibits a formal oxidation state of zero.

Crucial Chemical Reactions

1. Reaction with Dilute Acids

Nickel reacts slowly with dilute non-oxidizing acids (like HCl, H₂SO₄) to liberate hydrogen gas. Ni(s) + 2HCl(aq) → NiCl₂(aq) + H₂(g)

2. Reaction with Oxidizing Acids

Concentrated nitric acid passivates nickel due to the formation of a protective, impervious oxide layer on its surface, preventing further reaction. Dilute nitric acid can react, but the products depend on concentration and temperature.

3. Reaction with Oxygen

At high temperatures, nickel reacts with oxygen to form nickel(II) oxide. 2Ni(s) + O₂(g) --(heat)--> 2NiO(s)

4. Reaction with Halogens

Nickel reacts directly with halogens upon heating to form nickel halides. Ni(s) + Cl₂(g) --(heat)--> NiCl₂(s)

5. Mond Process (Refining of Nickel)

This process utilizes the formation and decomposition of volatile nickel tetracarbonyl.

• Formation: Nickel reacts with carbon monoxide at 50-60 °C to form volatile nickel tetracarbonyl. Ni(s) + 4CO(g) --(50-60°C)--> Ni(CO)₄(g)
• Decomposition: Nickel tetracarbonyl decomposes at 180-200 °C to yield pure nickel. Ni(CO)₄(g) --(180-200°C)--> Ni(s) + 4CO(g)

6. Complex Formation

Nickel(II) forms various stable coordination complexes.

• Hexaamminenickel(II) ion: Ni²⁺(aq) + 6NH₃(aq) → [Ni(NH₃)₆]²⁺(aq) (Octahedral, paramagnetic)
• Tetracyanonickelate(II) ion: Ni²⁺(aq) + 4CN⁻(aq) → [Ni(CN)₄]²⁻(aq) (Square planar, diamagnetic)

Industrial and Biological Importance

Industrial Importance

• Alloys: Nickel is a primary component in numerous high-performance alloys.
  • Stainless Steel: Improves corrosion resistance, strength, and ductility (e.g., austenitic steels contain 8-12% Ni).
  • Nichrome: An alloy of nickel and chromium, used in heating elements due to its high electrical resistance and stability at high temperatures.
  • Monel: An alloy of nickel and copper, highly resistant to corrosion by seawater and various acids.
  • Invar: An alloy of nickel and iron, known for its extremely low coefficient of thermal expansion, used in precision instruments.
• Catalyst: Used extensively in industrial processes.
  • Hydrogenation: Catalyzes the hydrogenation of unsaturated vegetable oils to produce margarine and other solid fats.
  • Methane Production: Used in the steam reforming of natural gas to produce hydrogen and synthesis gas.
• Batteries: Essential component in rechargeable batteries.
  • Nickel-Cadmium (NiCd) batteries.
  • Nickel-Metal Hydride (NiMH) batteries.
  • Lithium-ion batteries (newer generation).
• Electroplating: Nickel plating provides a decorative finish and corrosion resistance to other metals.
• Coinage: Historically used in coins due to its durability and resistance to wear.

Biological Importance

• Trace Element: Nickel is an essential trace element for several enzymes in certain organisms, primarily bacteria, fungi, and some plants.
• Enzyme Cofactor: It acts as a cofactor for enzymes like urease (involved in urea metabolism) and hydrogenase (involved in hydrogen metabolism).
• Toxicity: While essential in trace amounts, high concentrations of nickel can be toxic and carcinogenic, particularly through inhalation of dust containing nickel compounds.