Potassium Dichromate and Potassium Permanganate

1.0Potassium Dichromate K₂Cr₂O₇ and Potassium Permanganate KMnO₄  

Potassium dichromate and potassium permanganate are among the most important oxidising agents used in chemistry, particularly in volumetric analysis and organic synthesis. Their unique properties and preparation methods are crucial topics for JEE, as they are part of the d- and f-block elements chapter. This guide covers their industrial preparation and key chemical properties in detail.

2.0Potassium Dichromate K₂Cr₂O₇

Potassium dichromate is a powerful oxidizing agent that appears as an orange crystalline solid.

Preparation from Chromite Ore $(FeC{r}_2{O}_4)$

The preparation of K₂Cr₂O₇ ​ involves a three-step process:

1. Conversion of Chromite Ore to Sodium Chromate:
   
   Chromite ore is fused with sodium carbonate  (Na_2CO_3) in the presence of air. The iron is oxidized to iron(III) oxide, and the chromium is oxidized to soluble sodium chromate.
   
   $4FeC{r}_2{O}_4+8N{a}_{2}C{O}_3+7O_2\stackrel{\Delta }{\to }8N{a}_{2}Cr{O}_4+2F{e}_2{O}_3+8C{O}_2$
2. Conversion of Sodium Chromate to Sodium Dichromate:
   
   The yellow solution of sodium chromate is acidified with sulfuric acid (H_2SO_4) Chromate ions are converted to orange dichromate ions.
   
   $2N{a}_{2}Cr{O}_4+H_{2}S{O}_4\to N{a}_{2}C{r}_2{O}_7+N{a}_{2}S{O}_4+H_{2}O$
   
3. Conversion of Sodium Dichromate to Potassium Dichromate:
   
   Potassium chloride (KCl) is added to the solution of sodium dichromate. Sodium dichromate is highly soluble, while potassium dichromate is less soluble and precipitates out.
   
   $N{a}_{2}C{r}_2{O}_7+2KCl\to K_{2}C{r}_2{O}_7\downarrow +2NaCl$

Properties

• Oxidizing Nature:(K_2Cr_2O_7)​ acts as a strong oxidizing agent in acidic medium. The chromium(VI) is reduced to chromium(III).
  
  $C{r}_2{O}_7^{2-}+14H^{+}+6e^{-}\to 2C{r}^{3+}+7H_{2}O(E^{\circ }=+1.33\text{ V})$
  
  One mole of K₂Cr₂O₇​ accepts 6 electrons, so its equivalent weight is 6Molar Mass​.
• Action of Alkali: In an alkaline medium, the orange dichromate ion converts to the yellow chromate ion. This reaction is reversible and pH-dependent.
  
  $C{r}_2{O}_7^{2-}\text{ (Orange)}+2O{H}^{-}\rightleftharpoons 2Cr{O}_4^{2-}\text{ (Yellow)}+H_{2}O$
• Reaction with Halides: In an acidic medium, it oxidizes iodide ions to iodine and bromide ions to bromine.
  
  $C{r}_2{O}_7^{2-}+14H^{+}+6I^{-}\to 2C{r}^{3+}+3I_2+7H_{2}O$

3.0Potassium Permanganate KMnO₄  

Potassium permanganate is a powerful oxidizing agent that exists as a dark purple crystalline solid.

Preparation from Pyrolusite Ore (MnO_2)

The industrial preparation of KMnO4​ is also a multi-step process:

Fusion of Pyrolusite Ore to Potassium Manganate:

$2Mn{O}_2+4KOH+O_2\stackrel{\text{fusion}}{\to }2K_{2}Mn{O}_4\text{ (green)}+2H_{2}O$

Finely powdered pyrolusite ore is fused with potassium hydroxide (KOH) and an oxidizing agent like potassium nitrate (KNO_3). Manganese(IV) is oxidized to green manganate(VI) ion.

Oxidative Electrolysis of Potassium Manganate:

• The green solution of potassium manganate is electrolytically oxidized. Manganate(VI) ions are converted to permanganate(VII) ions at the anode.
• $AtAnode:Mn{O}_4^{2-}\to Mn{O}_4^{-}+e^{-}$
• $AtCathode:2H_{2}O+2e^{-}\to H_2+2O{H}^{-}$
• The green solution turns purple due to the formation of permanganate ions.

Properties

• Oxidizing Nature: The oxidizing power of KMnO₄ depends on the medium (acidic, basic, or neutral).
• Acidic Medium: It is a powerful oxidizing agent. Manganese(VII) is reduced to manganese(II).
  
  $Mn{O}_4^{-}+8H^{+}+5e^{-}\to M{n}^{2+}+4H_{2}O(E^{\circ }=+1.51\text{ V})$
  
  One mole of  KMnO₄  accepts 5 electrons, so its equivalent weight is 5Molar Mass​.
• Neutral or Faintly Alkaline Medium: Manganese(VII) is reduced to manganese(IV).
  
  $Mn{O}_4^{-}+2H_{2}O+3e^{-}\to Mn{O}_2\downarrow +4O{H}^{-}$
  
  One mole of KMnO₄ accepts 3 electrons.
• Strongly Alkaline Medium: Manganese(VII) is reduced to manganese(VI).
  
  $Mn{O}_4^{-}+e^{-}\to Mn{O}_4^{2-}$
  
  One mole of KMnO₄  accepts 1 electron.
• Colour: KMnO₄  solutions are intensely purple due to the charge transfer spectrum. It does not show d-d transition because Mn has a d0 configuration in the MnO_4^- ion.

4.0Key Differences Between K₂Cr₂O₇ and KMnO₄