Manganese (Mn): Comprehensive Study Guide

Introduction to Manganese (Mn)

Manganese (Mn) is a silvery-grey, hard, and brittle transition metal. Its importance spans various fields, from strengthening steel alloys and powering batteries to its crucial role as a biological trace element. Its diverse oxidation states are central to its chemical utility and reactivity.

CBSE/JEE Quick Revision Notes

• Atomic Number: 25
• Atomic Mass: 54.938 u
• Symbol: Mn
• Group: 7 (d-block, Transition Metal)
• Period: 4
• Block: d-block
• Nature: Hard, brittle, silvery-grey metal.
• Magnetic Property: Paramagnetic.
• Common Oxidation States: +2, +3, +4, +6, +7. The +2 and +7 states are particularly stable or widely utilized.

Electron Configuration & Bonding Behavior

Electronic Configuration

• Ground State: [Ar] 3d⁵ 4s²
• Explanation: The half-filled 3d⁵ subshell contributes to the stability observed in many Manganese compounds, particularly in its +2 oxidation state (Mn²⁺), where the configuration is [Ar] 3d⁵.

Bonding and Oxidation States

Manganese exhibits a wide range of oxidation states, characteristic of transition metals, due to the participation of both 4s and 3d electrons in bonding.

• +2 Oxidation State (Mn²⁺): Most stable in aqueous solutions, often forming ionic compounds. Example: MnCl₂, MnSO₄. The d⁵ configuration gives it relative stability.
• +3 Oxidation State (Mn³⁺): Less stable than +2, readily disproportionates or gets reduced. Example: Mn₂O₃.
• +4 Oxidation State (MnO₂): Found in manganese dioxide (pyrolusite), a black solid, which is industrially important. It is amphoteric in nature.
• +6 Oxidation State (MnO₄²⁻): The manganate ion, typically green. Stable only in strongly alkaline solutions; otherwise, it disproportionates. Example: K₂MnO₄.
• +7 Oxidation State (MnO₄⁻): The permanganate ion, typically purple. A very strong oxidizing agent, especially in acidic medium. Example: KMnO₄.

Crucial Chemical Reactions

1. Redox Reactions of Potassium Permanganate (KMnO₄)

Potassium permanganate is a powerful oxidizing agent. Its reduction products vary with the pH of the medium.

a) In Acidic Medium (e.g., with dilute H₂SO₄):

Permanganate ion is reduced to the colourless Mn²⁺ ion. MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Examples:

• Oxidation of Ferrous salts to Ferric salts: 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
• Oxidation of Oxalate ions to CO₂: 5(COO)₂²⁻ + 2MnO₄⁻ + 16H⁺ → 10CO₂ + 2Mn²⁺ + 8H₂O
• Oxidation of Sulphite ions to Sulphate ions: 5SO₃²⁻ + 2MnO₄⁻ + 6H⁺ → 5SO₄²⁻ + 2Mn²⁺ + 3H₂O
• Oxidation of Iodide ions to Iodine: 10I⁻ + 2MnO₄⁻ + 16H⁺ → 5I₂ + 2Mn²⁺ + 8H₂O
• Oxidation of Hydrogen Sulphide to Sulphur: 5H₂S + 2MnO₄⁻ + 6H⁺ → 5S + 2Mn²⁺ + 8H₂O

b) In Neutral or Weakly Alkaline Medium:

Permanganate ion is reduced to Manganese dioxide (MnO₂). MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻

Examples:

• Oxidation of Iodide ions to Iodate ions: 2MnO₄⁻ + I⁻ + H₂O → 2MnO₂ + IO₃⁻ + 2OH⁻
• Oxidation of Thiosulphate ions: 8MnO₄⁻ + 3S₂O₃²⁻ + H₂O → 8MnO₂ + 6SO₄²⁻ + 2OH⁻

c) In Strongly Alkaline Medium:

Permanganate ion is reduced to the manganate ion (MnO₄²⁻). MnO₄⁻ + e⁻ → MnO₄²⁻

2. Disproportionation of Manganate (MnO₄²⁻) Ion

The green manganate ion disproportionates in neutral or acidic medium to form permanganate and manganese dioxide. 3MnO₄²⁻ + 2H₂O → 2MnO₄⁻ + MnO₂ + 4OH⁻

3. Preparation of Chlorine Gas from Manganese Dioxide

Manganese dioxide reacts with concentrated hydrochloric acid upon heating to produce chlorine gas. MnO₂ (s) + 4HCl (aq) → MnCl₂ (aq) + Cl₂ (g) + 2H₂O (l)

Industrial and Biological Importance

Industrial Importance

• Steel Industry: Manganese is critical for steel production. It acts as a deoxidizer and desulfurizer, removing impurities. It also enhances the strength, hardness, and wear resistance of steel when alloyed (e.g., ferromanganese).
• Batteries: Manganese dioxide (MnO₂) is a key component in dry cell batteries (Leclanché cells) as a depolarizer.
• Chemicals:
  • Potassium permanganate (KMnO₄) is widely used as a strong oxidizing agent in organic synthesis, a disinfectant, an antiseptic, and for water purification.
  • Manganese compounds are used as catalysts in various chemical processes.
• Pigments: Manganese compounds are used to produce pigments, such as manganese violet (Mn(NH₄)₂P₂O₇) and brown pigments.
• Ceramics and Glass: Used as a colorant in ceramics and to decolorize glass by counteracting the green tint caused by iron impurities.

Biological Importance

• Essential Trace Element: Manganese is an essential trace element for all known life forms.
• Enzyme Cofactor: It acts as a cofactor for numerous enzymes, playing vital roles in:
  • Metabolism: Carbohydrate, lipid, and protein metabolism (e.g., pyruvate carboxylase).
  • Antioxidant Defense: Superoxide dismutase (Mn-SOD), crucial for mitigating oxidative stress.
  • Bone Development: Essential for bone formation and connective tissue synthesis.
  • Nerve Function: Involved in neurotransmitter synthesis.
• Photosynthesis: In plants, manganese is crucial for the oxygen-evolving complex during photosynthesis.
• Deficiency: Can lead to impaired growth, skeletal abnormalities, and reproductive issues.
• Toxicity: Excessive intake can be neurotoxic, leading to symptoms similar to Parkinson’s disease.