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ChemistryMediumClass 12
Important Compounds — KMnO₄, K₂Cr₂O₇ — reactions
d & f-block Elements
9
JEE Qs
8%
Hard
75
min
Focus on memorizing the reduction products and color changes of KMnO₄ and K₂Cr₂O₇ in different media, and practice balancing complex redox reactions to master this topic.
🧮 Key Formulas
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (Acidic medium)
MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻ (Neutral/Faintly alkaline medium)
MnO₄⁻ + e⁻ → MnO₄²⁻ (Strongly alkaline medium)
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (Acidic medium)
2CrO₄²⁻ + 2H⁺ ⇌ Cr₂O₇²⁻ + H₂O (Chromate to Dichromate conversion in acid)
Cr₂O₇²⁻ + 2OH⁻ ⇌ 2CrO₄²⁻ + H₂O (Dichromate to Chromate conversion in base)
✅ Key Points for JEE
- 1Potassium permanganate (KMnO₄) is a very strong oxidizing agent; its reduction products and the number of electrons accepted depend critically on the pH of the medium.
- 2Potassium dichromate (K₂Cr₂O₇) is also a strong oxidizing agent, predominantly used in acidic solutions, where it gets reduced to Cr³⁺.
- 3Characteristic color changes (e.g., purple MnO₄⁻, orange Cr₂O₇²⁻, green Cr³⁺, colorless Mn²⁺, brown MnO₂) are crucial for identifying products and understanding qualitative tests.
- 4The interconversion between yellow chromate (CrO₄²⁻) and orange dichromate (Cr₂O₇²⁻) is a pH-dependent equilibrium which is important for understanding their properties.
- 5Mastering the balancing of redox equations in acidic and basic media is fundamental for predicting reaction stoichiometry and solving quantitative problems.
⚠️ Common Mistakes
- ✕Incorrectly identifying the specific reduction product of KMnO₄ in different pH conditions (e.g., Mn²⁺ in acid, MnO₂ in neutral/faintly alkaline, MnO₄²⁻ in strong base).
- ✕Failing to correctly balance redox reactions, specifically neglecting H⁺/OH⁻ ions and H₂O molecules, leading to errors in stoichiometric calculations.
- ✕Confusing the characteristic color changes associated with manganese and chromium species (e.g., during titrations or qualitative analysis).
NCERT Chapters
- Class 12 Chemistry Part 1 Ch 8: The d- and f-Block Elements
- Class 11 Chemistry Part 1 Ch 8: Redox Reactions