K2Cr2O7 (Potassium dichromate): Structure, Molecular Mass, Properties and Uses

Potassium Dichromate - K₂Cr₂O₇-

Potassium dichromate is a very important chemical used in the leather industry and as an oxidant for the preparation of many azo compounds. Dichromates are generally prepared from chromate, which in turn are obtained by the fusion of chromite ore (FeCr₂O₄) with sodium or potassium carbonate in free access of air. The reaction with sodium carbonate occurs as follows:

4FeCr₂O₄+8Na₂CO₃+7O₂→8Na₂CrO₄+2Fe₂O₃+8CO₂

The yellow solution of sodium chromate is filtered and acidified with sulphuric acid to give a solution from which orange sodium dichromate, Na₂Cr₂O₇.2H₂O can be crystallized.

2Na₂CrO₄+2H⁺→Na₂Cr₂O₇+2Na⁺+H₂O

Sodium dichromate is more soluble than potassium dichromate. The latter is, therefore, prepared by treating the solution of sodium dichromate with potassium chloride.

Na₂Cr₂O₇+2KCl→K₂Cr₂O₇+2NaCl

Orange crystals of potassium dichromate crystallize out. The chromates and dichromates are interconvertible in an aqueous solution depending upon the pH of the solution. The oxidation state of chromium in chromate and dichromate is the same.

2CrO₄²⁻+2H⁺→Cr₂O₇²⁻+H₂O+Cr₂O₇²⁻+2OH⁻→2CrO₄²⁻+H₂O

The structures of the chromate ion, CrO₄^2– and the dichromate ion, Cr₂O₇^2– are shown below. The chromate ion is tetrahedral whereas the dichromate ion consists of two tetrahedra sharing one corner with Cr–O–Cr bond angle of 126°. Sodium and potassium dichromates are strong oxidizing agents; sodium salt has a greater solubility in water and is extensively used as an oxidizing agent in organic chemistry. Potassium dichromate is used as a primary standard in volumetric analysis. In an acidic solution, its oxidizing action can be represented as follows:

Cr₂O₇²⁻+14H⁺+6e⁻→2Cr³⁺+7H2O

Thus, acidified potassium dichromate will oxidize iodides to iodine, sulfides to sulfur, tin(II) to tin(IV), and iron(II) salts to iron(III). The half-reactions are noted below:

6I→3I₂+6e⁻+3H₂ S→6H⁺+3S+6e⁻ +3Sn²⁺→3Sn⁴⁺+6e⁻+6Fe²⁺→6Fe³⁺+6e⁻

The full ionic equation may be obtained by adding the half-reaction for potassium dichromate to the half-reaction for the reducing agent, for e.g.,

Cr₂O₇²⁻+14H⁺+6Fe²⁺→2Cr³⁺+6Fe³⁺+7H₂O

Recommended topic video on(Potassium Dichromate - K₂Cr₂O₇)

Some Solved Examples

Q.1 Which of the following compounds is formed when a mixture of K₂Cr₂O₇ and NaCl is heated with conc. H₂SO₄?

1) CrO₂Cl₂

2) CrCl₂

3) Cr₂(SO₄)₃

4) Na₂CrO₄

Solution-

As we learned,

In the Chromyl chloride test for detecting Chloride ions, orange-red fumes of Chromyl chloride are formed when any ionic chloride like NaCl is allowed to react with K₂Cr₂O₇ in the presence of H₂SO₄. The reaction is given as

K₂Cr₂O₇+4NaCl+6H₂SO₄→2KHSO₄+4NaHSO₄+2CrO₂Cl₂+3H₂O

Hence, the answer is the option (1).

Q.2 When SO₂ is passed through acidified K₂Cr₂O₇ solution-

1) the solution turns blue

2) the solution is decolourised

3) SO₂ is reduced.

4) green Cr₂(SO₄)₃ is formed

Solution:

As we learned,

In the Chromyl chloride test for detecting Chloride ions, orange-red fumes of Chromyl chloride are formed when any ionic chloride like NaCl is allowed to react with K₂Cr₂O₇ in the presence of H₂SO₄. The reaction is given as

K₂Cr₂O₇+4NaCl+6H₂SO₄→2KHSO₄+4NaHSO₄+2CrO₂Cl₂+3H₂O

Hence, the answer is the option (1).

Conclusion

Potassium dichromate is a chemical compound that has significant industrial and laboratory applications due to its oxidizing properties. It is commonly used in laboratories for oxidation reactions, as a cleaning agent for glassware, and in the preparation of various chromium compounds. However, potassium dichromate is highly toxic and carcinogenic, requiring careful handling and disposal to prevent environmental contamination and health hazards.