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Important equations in d block elements — KMnO₄ and K₂Cr₂O₇

Potassium Dichromate (K₂Cr₂O₇) & Potassium Permanganate (KMnO₄)

Verified from the image and converted to HTML. Minor typos corrected: “20H⁻” → “2OH⁻” (appears in pH / basic-medium equations).

Potassium Dichromate [K₂Cr₂O₇]

Preparation:

It is prepared from chromite ore: FeCr₂O₄ / FeO·Cr₂O₃

Step 1:

4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂

Step 2:

2Na₂CrO₄ (Yellow) + H₂SO₄ → Na₂Cr₂O₇ (Orange) + Na₂SO₄ + H₂O

Step 3:

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Orange → Orange

Effect of pH:

Cr₂O₇²⁻ (Orange) + 2OH⁻ → 2CrO₄²⁻ (Yellow) + H₂O
2CrO₄²⁻ (Yellow) + 2H⁺ → Cr₂O₇²⁻ (Orange) + H₂O

Oxidizing Character:

Dichromate ion acts as strong oxidizing agent in acidic medium.

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
Cr₂O₇²⁻ + 14H⁺ + 6I⁻ → 2Cr³⁺ + 7H₂O + 3I₂
Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺
Cr₂O₇²⁻ + 14H⁺ + 3Sn²⁺ → 2Cr³⁺ + 7H₂O + 3Sn⁴⁺
Cr₂O₇²⁻ + 8H⁺ + 3H₂S → 2Cr³⁺ + 7H₂O + 3S

Potassium Permanganate [KMnO₄]

Preparation:

from pyrolucite ore [MnO₂]

Step 1:

2MnO₂ (Black) + 4KOH + O₂ → 2K₂MnO₄ (Green) + 2H₂O

Step 2:

3MnO₄²⁻ (Green) + 4H⁺ → 2MnO₄⁻ (Purple) + MnO₂ + 2H₂O

Oxidizing Character: (Oxidation in acidic medium)

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Eq. mass of KMnO₄ = 158/5 = 31.6

MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
2MnO₄⁻ + 10Cl⁻ + 16H⁺ → 2Mn²⁺ + 8H₂O + 5Cl₂
2MnO₄⁻ + 5C₂O₄²⁻ + 16H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O
2MnO₄⁻ + 5SO₃²⁻ + 6H⁺ → 2Mn²⁺ + 5SO₄²⁻ + 3H₂O
2MnO₄⁻ + 5NO₂⁻ + 6H⁺ → 2Mn²⁺ + 5NO₃⁻ + 3H₂O
2MnO₄⁻ + 5S²⁻ + 16H⁺ → 2Mn²⁺ + 5S + 8H₂O
2MnO₄⁻ + 3H₂O + I⁻ → 2MnO₂ + IO₃⁻ + 2OH⁻
8MnO₄⁻ + 3S₂O₃²⁻ + H₂O → 8MnO₂ + 6SO₄²⁻ + 2OH⁻
2MnO₄⁻ + 3Mn²⁺ + 2H₂O → 5MnO₂ + 4H⁺