Inorganic chemistry is of paramount importance for the preparation of the IIT JEE for all the 2 stages, i.e., the JEE Mains and JEE Advance. Realizing the significance of this topic we treat Inorganic chemistry as a separate subject, as its relevance is more than 40% of the IIT JEE syllabus. 

In this chapter, you will study Potassium dichromate or K2Cr2O7 and KMno4 is one of the most important chemical compounds of inorganic chemistry. It is produced by applying neutral yellow chromate of potassium to the solution, which is a moderate form of one of the stronger classes of acids.

Study the complete notes to get full information about preparation, properties, and uses of K2Cr2O7, and KMnO4.

Preparation, properties, and uses of K2Cr2O7, and KMnO4

Potassium dichromate, which is also known as anhydrochromate, can be produced by applying neutral yellow chromate of potassium to the solution, which is a moderate form of one of the stronger classes of acids. Commercially, potassium permanganate is prepared by mixing the potassium hydroxide sol. and powdered manganese oxide with oxidizing agents such as potassium chlorate.

Preparation of Potassium dichromate or KMnO4

Potassium permanganate is prepared from the pyrolusite ore or MnO2. Potassium hydroxide or KOH also takes an important part in the process. The preparation includes two steps.

Step 1

The powdered ore of MnO2 is treated with Potassium hydroxide (KOH). The reaction is done in the presence of air (i.e., oxygen). But other than oxygen, we can use an oxidizing agent like Potassium Nitrate (KNO₃) or Potassium Chlorate(KClO3).

 The reaction that takes place in the first place is 

6KOH + 3MnO2 + KClO3 → 3K2MnO4 + KCl + 3H2O 

The K2MnO4 or potassium manganate that we get is green-coloured. It is important regarding KMnO4 color.

Step 2

The obtained K2MnO4 is now mixed with water. The temperature should be kept high and trigger the boiling process. Along with water, Carbon Dioxide, ozone air, and chlorine current should be passed into that. This continues till it becomes permanent. We must remove the formed manganese dioxide because it can break down the Permanganate.

 The reaction of this step is 

6K2MnO4 + 3Cl2 → 6KMnO4 (Potassium Permanganate) + 6KCl 

That’s how we get our KMnO4.

Properties of KMnO4 

Physical Properties

Potassium permanganate can be seen as monoclinic crystals, practically obscure with a blue metallic gloss. It gets broken up in ethanol and natural solvents.

• It appears in the shape of dull purple crystals, which liquefies at 513 degrees Kelvin
• It is sparingly dissolvable in cool water at the same time, genuinely solvent in steaming hot water
• It is a scentless, purple to fuchsia glasslike strong
• It is dissolvable in water,acetone, acetic acid, methanol, and pyridine
• It is scentless. An aqueous solution has a sweetish astringent taste. 

Chemical Properties

Oxidation

In an acidic, alkaline, or neutral medium, it works as a very powerful oxidizing agent. The reaction for the neutral or alkaline medium is

2KMnO4 + H2O → 2KOH + 2MnO2 + 3[O] MnO4– + 2H2O + 3e– → MnO2 + 4OH– 

The reaction for acidic medium is-

2KMnO4 + H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5[O] MnO4– + 8H+ + 5e– → Mn2+ + 4H2O 

Reaction With Heat

If we heat Potassium permanganate, we can observe a decomposition. 

The reaction is – 

2KMnO4 → K2MnO4 + MnO2(s) + O2 

Potassium permanganate is an extremely impressive oxidizing specialist and can, for this reason, be utilized as an oxidant in a wide range of substance responses. 

The oxidizing force of potassium permanganate can be seen while playing out a redox response with it. The dark purple arrangement transforms dismal and afterwards into a brown-coloured solution.  

Use of potassium permanganate

• The KMnO4 is used in quantitative chemical analysis. For example, volumetric analysis is a sector where we can see its use
• It is mainly used as an antiseptic. We can use it on any bacterial infections
• It is used in the leather industry. The chrome tanning process involves the use of KMnO4
• It is also used by experts for unsaturation tests in the research facility
• It is utilized for distinguishing halides in a subjective investigation
• As it is an oxidizing agent, it is utilized as an amazing oxidizing specialist in the lab and industry. It is significant regarding the use of potassium permanganate.

Preparation of K2Cr2O7

Steps involved in preparation of potassium  dichromate as follows :

Step 1

The chromite ore or FeCr2O4 reacts with sodium carbonate or Na2CO3 to form sodium chromate. 

4FeO.Cr2O3 + 7O2+ 8Na2CO3  –> 8Na2CrO4 + 2Fe2O3 + 8CO2 

Step 2

This step involves reaction of sodium chromate( Na2CrO4), with concentrated sulphuric acid(H2SO4) to get sodium dichromate solution. 

2Na2CrO4 + 2H+  →  Na2Cr2O7 + 2Na+ + H2O 

We also get Sodium ions from this reaction. 

Step 3

In the final step sodium dichromate solution is mixed with potassium chloride or KCl to form  potassium dichromate or K2Cr2O7.

Na2Cr2O7 + 2KCl →  K2Cr2O7 + 2NaCl

From this reaction, we also get Sodium chloride ( salt ).

Properties of K2Cr2O7

For the chemical compounds, we can divide their properties into two parts- physical and chemical properties. 

Physical Properties

• Potassium dichromate is relatively soluble in water which is cold. The solubility is not that high for cold water
• For hot water, Potassium dichromate is easily mixed. The solubility is very high when it comes to hot water
• The color of the crystal formation of potassium dichromate is red-ish orange
•  The crystal can be melted at a temperature of 396° centigrade.

 Chemical Properties 

Reaction With Alkalis

You will get a yellow-coloured solution if you add alkali with the orange red K2Cr2O7 solution. Potassium chromate is the solution we get, and it is the reason behind the yellow color.

The reaction of it  is

K2Cr2O7 + 2KOH →  2K2CrO4 + H2O (here K2Cr2O7 is orange and K2CrO4 is yellow)

Again if the Potassium chromate is treated with acid, the color will shift to red-orange again. It happens because the reaction produces potassium dichromate again. 

The reaction is

2K2CrO4 + H2SO4 →  K2Cr2O7 + K2SO4 + H2O

Chromyl Chloride

If we treat chloride salts with the Potassium dichromate and the presence of concentrated sulphuric acid, we get Chromyl Chloride. The Chromyl Chloride will appear in vapor, and the color will be brown.

The reaction involved is 

K2Cr2O7 + 4KCl + 6H2SO4 → 2CrO2Cl2 + 6 KHSO4 + 3H2O ( CrO2Cl2 is Chromyl Chloride) 

Oxidation

The oxidation property is important because potassium dichromate is an active agent of oxidation. If we treat it with diluted Sulphuric acid, it gives three moles of oxygen. 

The reaction is 

K2Cr2O7 + 4 dil.H2SO4 →  K2SO4 + Cr2 (SO4)3 + 4H2O + 3(O)

Uses of Potassium Dichromate

• Potassium dichromate is used in a large amount in the leather industry. The chrome tanning process involves K2Cr2O7.
• It is also used in quantitative chemical analysis just like the use of potassium permanganate.
• In the photography industry, Potassium dichromate is used. Also for gelatin film solidification, potassium dichromate is a vital compound.
• K2Cr2O7 is used in dyeing and the calico type of printing in the printing industry, same as the use of KMnO4. 

Conclusion

IIT JEE aspirants have a lot of syllabus to cover while preparing for their entrance exam. Aspirants must focus on organic chemistry in their preparation for the JEE Mains and JEE Advanced. K2Cr2O7 is a crucial element to remember. Potassium permanganate is prepared from the pyrolusite ore or MnO2. Potassium hydroxide or KOH also plays an important part in the process. Aspirants must memorize all the chemical reactions, compositions, and formulas to secure a good score in their entrance examinations.