Brief Notes on Properties of Potassium Dichromate

Potassium Dichromate is a hexavalent chromium compound. The chemical formula of potassium dichromate is given as  K2Cr2O7. It is an inorganic chemical reagent generally used for oxidising in various laboratories and for industrial use. It is a hexavalent compound, it is very harmful to the skin and body. Potassium dichromate is crystalline and appears to be in a very bright red and orange colour. This salt is highly used in laboratories because it cannot easily absorb the moisture in the air and cannot easily get dissolved in it; this property is called deliquescent. This salt is preferable to other dichromate salts in industries.

Physical properties of Potassium Dichromate(K2Cr2O7):

Potassium Dichromate appears to be in bright red-orange colour crystals. The molar mass of K2Cr2O7 is 294.185g/mol. Potassium Dichromate has a density of 2.676g/cm3. Potassium Dichromate has a melting point of 398℃, showing polymorphic transformation.500℃ is the boiling point of K2Cr2O7, and from 1360℃, it starts to decompose. It is insoluble in alcohol as well as acetone, and also, as the temperature increases, its solubility in water increases. It can easily be soluble in hot water than in cold water. It has no smell meaning it is odourless. The heat capacity of K2Cr2O7 is 219J/mol.K.Potassium Dichromate has a refractive index of 1.738. It has a highly corrosive nature and is also prone to combustion. Potassium Dichromate is an ionic compound. It has two potassium ions, K+, and  negatively charged dichromate ion which represents Cr2O72- in this kind of geometry, the two hexavalent chromium atoms, which have an oxidation number +6, are attached to three oxygen atoms. Potassium Dichromate has a crystalline structure, whereas the coordinate geometry of the central atom, which is chromium, is tetrahedral.

Chemical properties of Potassium Dichromate(K2Cr2O7):

1. Oxidation Reactions:

• Compared to Potassium permanganate(KMnO4), it is a mild oxidising agent, so producing aldehydes from alcohols is proven to be good.
• It liberated three atoms of oxygen when reacted with dil. H2SO4.

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

• K2Cr2O7 liberates I2 from Kl

  K2Cr2O7 +7H2SO4 +6KI → 4K2SO4 + Cr2(SO4)3 + 3I2 + 7H2O

• It is also used to oxidise H2S to sulphur

  K2Cr2O7 + 4H2SO4 + 3H2S → K2SO4 + Cr2(SO4)3 +7H2O +3S

• Potassium Dichromate converts secondary alcohol to ketones. On the other hand, tertiary alcohol can not be oxidised.
• K2Cr2O7  can also be used to check aldehydes or ketones. When potassium Dichromate is reacted with an alcohol, it changes its colour due to the reduction of dichromate ion. So when ketones are formed, there is no change of co. This colour-changing property can be used as a test.
• It also helps in the oxidation of ferrous sulphate to ferric sulphate.

   K2Cr2O7 +7H2SO4 + 6FeSO4 → K2SO4  + Cr2(SO4)3 +3Fe2(SO4)3 + 2H2O 

2. When heated:

When Potassium Dichromate is heated, it decomposes to form potassium chromate, and oxygen gas is released when heated strongly.

 4K2Cr2O7  → 4K2CrO4 + 2Cr2O3 + 3O2 

3. When reacted with an alkali

• When  K2Cr2O7 reacts with an alkali solution, it shows changes in colour from orange to yellow because of the formation of chromate ion (CrO4-2) 

    K2Cr2O7 + K2CO3 →  2 K2CrO4 +CO2

• When K2Cr2O7  reacts with KOH, a yellow solution can be observed because of the formation of chromate ion (CrO4-2) 

        K2Cr2O7 + 2KOH → 2 K2CrO4 + H2O 

          Orange                                yellow

When K2CrO4  is acidified, an orange, red solution is obtained.

    2K2CrO4 + H2SO4 → K2Cr2O7 + K2SO4 + H2O 

          Yellow                                Orange 

4. Reaction with Sulphuric Acid:

• When Potassium Dichromate and cold sulphuric acid are reacted, crystals of Chromium anhydride (CrO3)of red colour are formed.

   K2Cr2O7 + 2H2SO4 → 2CrO3 + 2KHSO4 + H2O

• When K2Cr2O7 is heated with concentrated sulphuric acid, oxygen gas is released.

 K2Cr2O7 + 8H2SO4 → 2K2SO4 + 2Cr2(SO4)3+ 8H2O +3O2

• Chromyl Chloride Test:  When a chloride that contains salt reacts with sulphuric acid and K2Cr2O7, a reddish-brown vapour of chromyl chloride is formed. This reaction is also used for the presence of chloride ions in salts.

  K2Cr2O7 + 4KCl + 8H2SO4 →  2 CrO2Cl2 + 6KHSO4 + 3 H2O

This CrO2Cl2  is known as chromyl chloride.

Conclusion

Potassium Dichromate is the most widely used chemical reagent in organic chemistry. The formula of Potassium Dichromate is  K2Cr2O7. Dichromate salts are formed from chromate salts. It is a very strong oxidising agent used in many industrial areas and laboratories. It is also known as dichromatic acid. It is orange-red, very attractive, but at the same time is very corrosive and can even cause severe damage to your skin and eyes or can also be the cause of your infertility. It is mostly used for alum production in leather tanning, photography, and construction. It can also be used to create negatives used in photography.