Preparation and Properties of K2Cr2O7

Preparation and Properties of Potassium Dichromate (K2Cr2O7)

Potassium dichromate (K2Cr2O7) is a bright orange-red crystalline chemical compound that is widely used in various industrial processes, such as leather tanning, wood staining, and as an oxidizing agent in organic synthesis. It is also used in the preparation of chromic acid, which is utilized in cleaning laboratory glassware.

Preparation of K2Cr2O7

Potassium dichromate is commonly prepared from chromite ore (FeCr2O4) through a series of steps. The overall process involves the extraction of chromium as sodium chromate from the ore, which is then converted to sodium dichromate. Finally, potassium dichromate is obtained by the reaction of sodium dichromate with potassium chloride.

Step-by-Step Preparation:

  1. Conversion of Chromite Ore to Sodium Chromate: The chromite ore is finely powdered and mixed with sodium carbonate (Na2CO3). This mixture is roasted in the presence of air at around 1100°C, which converts the chromite ore to sodium chromate.

$$ 4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \rightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2 $$

  1. Leaching: The roasted mass is leached with water, and the solution is filtered to remove insoluble impurities.

  2. Conversion to Sodium Dichromate: The filtered solution of sodium chromate is treated with concentrated sulfuric acid (H2SO4), which converts it to sodium dichromate.

$$ 2Na_2CrO_4 + H_2SO_4 \rightarrow Na_2Cr_2O_7 + Na_2SO_4 + H_2O $$

  1. Crystallization: The sodium dichromate is crystallized from the solution.

  2. Conversion to Potassium Dichromate: The crystallized sodium dichromate is then treated with potassium chloride (KCl), resulting in the formation of potassium dichromate, which is less soluble in water and crystallizes out.

$$ Na_2Cr_2O_7 + 2KCl \rightarrow K_2Cr_2O_7 + 2NaCl $$

The potassium dichromate crystals are then separated, washed, and dried.

Properties of K2Cr2O7

Potassium dichromate has several important physical and chemical properties:

Physical Properties:

  • Appearance: Bright orange-red crystalline solid
  • Solubility: Soluble in water; the solubility increases with temperature
  • Melting Point: 398°C (decomposes)
  • Molar Mass: 294.185 g/mol
  • Density: 2.676 g/cm³

Chemical Properties:

  • Oxidizing Agent: K2Cr2O7 is a strong oxidizing agent in acidic medium. It can oxidize a variety of inorganic and organic substances.
  • Acidic Nature: In aqueous solution, K2Cr2O7 is acidic due to the formation of chromic acid (H2CrO4).
  • Redox Reactions: It undergoes redox reactions, where chromium is reduced from the +6 oxidation state to +3.

Chemical Reactions:

  1. Reaction with Alcohols: Potassium dichromate oxidizes primary alcohols to aldehydes and then to carboxylic acids, and secondary alcohols to ketones.

For example, the oxidation of ethanol to acetic acid:

$$ 3CH_3CH_2OH + K_2Cr_2O_7 + 4H_2SO_4 \rightarrow 3CH_3COOH + Cr_2(SO_4)_3 + K_2SO_4 + 7H_2O $$

  1. Reaction with Sulfur-Containing Compounds: It oxidizes sulfides to sulfates and thiosulfates to tetrathionates.

  2. Reaction with Halides: Potassium dichromate can oxidize iodides to iodine and bromides to bromine in acidic medium.

  3. Dichromate-Cr(VI) to Cr(III) Reduction: In acidic solution, dichromate ions can be reduced to Cr(III) ions, changing the color from orange to green.

$$ K_2Cr_2O_7 + 14HCl \rightarrow 2KCl + 2CrCl_3 + 3Cl_2 + 7H_2O $$

Safety and Environmental Concerns

Potassium dichromate is highly toxic and carcinogenic. It is also a strong oxidizer, which can cause fires if it comes into contact with combustible materials. Proper handling, storage, and disposal are essential to minimize the risks associated with its use.

Summary Table

Property/Aspect Description
Chemical Formula K2Cr2O7
Appearance Bright orange-red crystalline solid
Preparation Derived from chromite ore through a series of chemical reactions involving sodium carbonate, sulfuric acid, and potassium chloride
Physical Properties Soluble in water, high melting point, and dense
Chemical Properties Strong oxidizing agent, forms chromic acid in water, participates in redox reactions
Uses Oxidizing agent in organic synthesis, cleaning laboratory glassware, leather tanning, wood staining
Safety Toxic, carcinogenic, strong oxidizer

This comprehensive overview of the preparation and properties of potassium dichromate should provide a solid foundation for understanding this important chemical compound in the context of exams and practical applications.