Properties, Reactions, and Applications of Potassium Dichromate and Permanganate
Potassium Dichromate and Potassium Permanganate are two vital inorganic compounds widely used as powerful oxidizing agents in laboratories and industries. Understanding their formulas, structures, color, and roles in redox titrations is essential for mastering transition element chemistry. This article covers the formulas, preparation, key differences, and uses of these compounds, making it easy to distinguish between potassium dichromate and potassium permanganate in chemical experiments and exam questions.
Formulas, Structure, and Color Characteristics
The fundamental identities of potassium dichromate and potassium permanganate lie in their unique chemical formulas and vivid colors, which serve as indicators during redox reactions.
Potassium Dichromate and Potassium Permanganate Formula & Structure
- Potassium Dichromate formula: \( K_2Cr_2O_7 \)
- Potassium Permanganate formula: \( KMnO_4 \)
- In \( K_2Cr_2O_7 \), there are two potassium ions (\( K^+ \)) and a dichromate ion (\( Cr_2O_7^{2-} \)) with chromium in the +6 oxidation state.
- In \( KMnO_4 \), one potassium ion combines with a permanganate ion (\( MnO_4^- \)), with manganese in the +7 state.
Colour and Appearance
- Potassium Dichromate: Orange-red crystalline solid.
- Potassium Permanganate: Deep purple/violet crystals.
Preparation of Potassium Dichromate and Potassium Permanganate
Both compounds are synthesized from metal ores using oxidation processes. Here’s an overview for Class 12 chemistry and lab applications:
- Potassium Dichromate is made from chromite ore (\( FeCr_2O_4 \)):
- Ore is roasted with alkaline carbonates to form sodium chromate.
- Acidification with sulphuric acid and addition of potassium chloride crystallizes potassium dichromate.
- Potassium Permanganate derives from manganese dioxide (\( MnO_2 \)):
- Oxidation with potassium hydroxide and a suitable oxidant yields potassium manganate (\( K_2MnO_4 \)).
- Subsequent oxidation (chemical or electrolytic) converts this to potassium permanganate (\( KMnO_4 \)).
Physical and Chemical Properties
Both compounds are strong oxidizing agents, but show different physical features and chemical behavior in reactions.
Physical Properties Comparison
- Potassium dichromate: Orange, soluble in water, non-hygroscopic, high melting point.
- Potassium permanganate: Intensely purple, water soluble, also has a high melting point, acts as a self-indicator in titrations.
Chemical Properties & Redox Reactions
- Potassium dichromate and potassium permanganate are generally used in redox titration because of their:
- Predictable oxidizing strengths
- Distinct color transitions at the end point
- Well-defined chemical equations for redox analysis
- In redox titrations, KMnO4 is a self-indicator (purple to colorless), while K2Cr2O7 requires an external indicator (usually diphenylamine).
- Potassium permanganate has a stronger oxidizing potential and works effectively in acidic, neutral, or alkaline media.
For example, heating potassium permanganate gives:
$$ 2KMnO_4 \rightarrow K_2MnO_4 + MnO_2 + O_2 $$
Main Differences: Potassium Dichromate vs Potassium Permanganate
- Color: K2Cr2O7 is orange; KMnO4 is purple.
- Self-indicating property: Only potassium permanganate is a self-indicator.
- Oxidizing Strength: KMnO4 is stronger than K2Cr2O7.
- Oxidation states: Chromium (+6) in dichromate, manganese (+7) in permanganate.
Key Uses in Analytical Chemistry and Industry
- Common as oxidizing agents in redox titrations and volumetric analysis
- Water purification (KMnO4)
- Dye, tanning, cement, and photographic industries (K2Cr2O7)
- Oxidation of alcohols and organic compounds in synthesis
To further explore chemical structures and physical properties in detail, visit material properties in chemistry or the fundamentals of matter on Vedantu.
Common Errors to Avoid
- Confusing the indicator properties during titration (KMnO4 needs no external indicator; K2Cr2O7 does).
- Mixing up color transitions at the endpoint.
- Overlooking toxicity risks, especially for potassium dichromate.
Summary
In conclusion, potassium dichromate and potassium permanganate play indispensable roles in analytical chemistry due to their predictable chemical behavior and vibrant color changes. They differ in formula, structure, and oxidizing power. Their uses—from redox titrations to industry—emphasize the importance of recognizing the difference between potassium dichromate and potassium permanganate, especially when handling redox reactions. Mastery of these concepts enhances practical skills and exam success in chemistry.
FAQs on Comparison of Potassium Dichromate and Potassium Permanganate
1. What is potassium dichromate and what are its main uses?
Potassium dichromate is a bright orange inorganic chemical compound used mainly as an oxidizing agent in laboratories and industries.
Key uses include:
- Laboratory reagent for oxidation reactions
- Component in cleaning solutions and dyes
- In the tanning of leather
- For the preparation of other chromium compounds
2. What is the difference between potassium dichromate and potassium permanganate?
Potassium dichromate (K2Cr2O7) and potassium permanganate (KMnO4) are both oxidizing agents but differ in color, composition, and specific applications.
Main differences:
- Potassium dichromate: Orange, contains chromium, highly toxic and carcinogenic
- Potassium permanganate: Deep purple, contains manganese, less toxic, antiseptic properties
- Use in titrations: Dichromate for redox titrations in acidic medium, permanganate for volumetric analysis and as a disinfectant
3. What are the physical properties of potassium permanganate?
Potassium permanganate appears as dark purple or almost black, odorless crystals.
Key physical properties:
- Soluble in water, forms purple solutions
- High melting point (~240°C)
- Acts as a strong oxidizing agent
4. How is potassium dichromate prepared in the laboratory?
Potassium dichromate can be prepared from sodium dichromate by reaction with potassium chloride.
Preparation steps:
- Roast chromite ore with sodium carbonate and air to obtain sodium chromate
- Treat sodium chromate with sulphuric acid to get sodium dichromate
- React sodium dichromate with potassium chloride to yield potassium dichromate as orange crystals
5. What is the role of potassium permanganate in redox titrations?
Potassium permanganate acts as a self-indicator and a strong oxidizing agent in redox titrations.
Main roles:
- Oxidizes analytes such as iron(II), oxalate, and hydrogen peroxide
- Does not require external indicator; the purple color disappears at endpoint
- Commonly used in acidic medium for volumetric analysis
6. Why is potassium dichromate considered hazardous?
Potassium dichromate is hazardous due to its toxicity, corrosiveness, and carcinogenicity.
Hazards include:
- Causes severe skin, eye, and respiratory irritation
- Can induce allergic reactions and ulcers
- Carcinogenic due to hexavalent chromium content
- Harmful to the environment and aquatic life
7. Write the balanced chemical equation for the reaction of potassium permanganate in acidic medium.
In acidic medium, potassium permanganate acts as a strong oxidizer, following this balanced equation:
- 2 KMnO4 + 3 H2SO4 + 5 H2C2O4 → K2SO4 + 2 MnSO4 + 10 CO2 + 8 H2O
8. What are some important uses of potassium permanganate?
Potassium permanganate has multiple uses due to its oxidizing and disinfectant properties.
Main uses:
- Water purification and disinfection
- Treatment of skin conditions in medicine
- In redox titrations and qualitative analysis
- Bleaching agent and deodorizer
9. What safety precautions should be observed while handling potassium dichromate and potassium permanganate?
Both potassium dichromate and potassium permanganate require careful handling due to their oxidizing and toxic properties.
Safety precautions:
- Wear gloves, protective clothing, and eye protection
- Avoid inhaling dust or ingesting chemicals
- Use in a well-ventilated area or fume hood
- Dispose of wastes properly as per chemical guidelines
10. How does the color change indicate the endpoint in potassium permanganate titrations?
The endpoint in potassium permanganate titrations is signaled by a persistent pink or purple color.
Process:
- During titration, the purple color disappears as KMnO4 is reduced
- Endpoint is reached when a faint pink color persists, indicating no more analyte to react
