Important Compounds of Transition Metals
Preparation and properties of potassium dichromate and potassium permanganate. (Chemistry › d- and f-Block Elements, NEET UG syllabus.)
What is Important Compounds of Transition Metals?
An orange crystalline inorganic compound of chromium, widely used as a strong oxidizing agent, especially in acidic solutions.
Key formula / rule: Preparation of Sodium Chromate from Chromite Ore
Key points
- Describe the preparation methods for potassium dichromate and potassium permanganate.
- Explain the oxidizing properties of K₂Cr₂O₇ in acidic medium.
- Explain the oxidizing properties of KMnO₄ in acidic, neutral, and alkaline media.
- Write and balance chemical equations for the reactions involving these compounds.
Common exam trap
Confusing the oxidation states of Cr and Mn in different compounds and reaction products.
Definitions
- Term
Potassium Dichromate (K₂Cr₂O₇)
- Meaning
An orange crystalline inorganic compound of chromium, widely used as a strong oxidizing agent, especially in acidic solutions.
- Term
Potassium Permanganate (KMnO₄)
- Meaning
A dark purple crystalline inorganic compound of manganese, known for its powerful oxidizing properties, which vary significantly with the pH of the solution.
- Term
Oxidizing Agent
- Meaning
A substance that gains electrons and is reduced in a chemical reaction, thereby causing another substance to be oxidized.
Learning objectives
Describe the preparation methods for potassium dichromate and potassium permanganate.
Explain the oxidizing properties of K₂Cr₂O₇ in acidic medium.
Explain the oxidizing properties of KMnO₄ in acidic, neutral, and alkaline media.
Write and balance chemical equations for the reactions involving these compounds.
Identify the color changes associated with the reactions of K₂Cr₂O₇ and KMnO₄.
State the important applications of K₂Cr₂O₇ and KMnO₄.
Formulae
- Name
Preparation of Sodium Chromate from Chromite Ore
- Note
Fusion with alkali and air
- Expression
4FeCr₂O₄ + 8Na₂CO₃ + 7O₂ → 8Na₂CrO₄ + 2Fe₂O₃ + 8CO₂
- Name
Conversion of Chromate to Dichromate
- Note
Acidification
- Expression
2Na₂CrO₄ + 2H⁺ → Na₂Cr₂O₇ + 2Na⁺ + H₂O
- Name
Preparation of Potassium Dichromate
- Note
Precipitation due to lower solubility
- Expression
Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl
- Name
Oxidizing action of Dichromate (acidic medium)
- Note
Cr(VI) to Cr(III)
- Expression
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
- Name
Preparation of Potassium Manganate
- Note
Fusion with alkali and air/oxidizing agent
- Expression
2MnO₂ + 4KOH + O₂ → 2K₂MnO₄ + 2H₂O
- Name
Oxidation of Manganate to Permanganate (electrolytic)
- Note
Or by disproportionation: 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O
- Expression
2K₂MnO₄ + Cl₂ → 2KMnO₄ + 2KCl
- Name
Oxidizing action of Permanganate (acidic medium)
- Note
Mn(VII) to Mn(II)
- Expression
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
- Name
Oxidizing action of Permanganate (neutral/weakly alkaline medium)
- Note
Mn(VII) to Mn(IV)
- Expression
MnO₄⁻ + 2H₂O + 3e⁻ → MnO₂ + 4OH⁻
- Name
Oxidizing action of Permanganate (strongly alkaline medium)
- Note
Mn(VII) to Mn(VI)
- Expression
MnO₄⁻ + e⁻ → MnO₄²⁻
Prerequisites
Basic knowledge of d-block elements and their general characteristics.
Understanding of oxidation states and redox reactions.
Ability to balance redox reactions using oxidation number or ion-electron method.
Concepts of acidity and basicity (pH).
Common mistakes
Confusing the oxidation states of Cr and Mn in different compounds and reaction products.
Incorrectly balancing redox reactions involving K₂Cr₂O₇ and KMnO₄, especially the number of electrons transferred and H⁺/OH⁻ ions.
Forgetting the different products of KMnO₄ reduction in acidic, neutral, and alkaline media.
Not knowing the colors of the various chromium and manganese species (e.g., CrO₄²⁻, Cr₂O₇²⁻, Cr³⁺, MnO₄⁻, MnO₄²⁻, MnO₂).
Keywords
Potassium Dichromate
Potassium Permanganate
Oxidizing Agent
Chromite Ore
Pyrolusite Ore
Redox Reactions
Manganate
Chromate
Dichromate
Transition Metals
Practice preview
Which of the following is the starting material for the preparation of potassium permanganate (KMnO4)?…
easy
What is the characteristic colour of potassium dichromate (K2Cr2O7) solution?…
easy
When potassium permanganate (KMnO4) solution is added to an acidic solution containing Fe2+ ions, what is the product formed from Fe2+?…
medium