Potassium Dichromate

Question 1

What is the main industrial use of potassium dichromate (K₂Cr₂O₇)?

Answer 1

Potassium dichromate is used as an oxidant in leather processing and the preparation of azo compounds.

Question 2

How are dichromates typically prepared?

Answer 2

Dichromates are obtained by the fusion of chromate ore (FeCr₂O₄) with sodium or potassium carbonate in the presence of air.

Question 3

Write the reaction of chromate ore with sodium carbonate to produce sodium chromate.

Answer 3

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

Question 4

What is the process to convert sodium chromate to sodium dichromate?

Answer 4

The yellow solution of sodium chromate is filtered and acidified with sulfuric acid to produce sodium dichromate.

Question 5

Write the chemical equation for the conversion of sodium chromate to sodium dichromate.

Answer 5

2Na₂CrO₄ + 2H⁺ → Na₂Cr₂O₇ + H₂O

Question 6

What is the color of sodium dichromate?

Answer 6

Sodium dichromate is orange.

Question 7

What is potassium dichromate commonly used for besides leather processing?

Answer 7

It is used in the preparation of azo compounds.

Question 8

What is the main ore used to prepare dichromates?

Answer 8

Chromate ore, also known as FeCr₂O₄.

Question 9

What is the role of sodium or potassium carbonate in the preparation of dichromates?

Answer 9

It reacts with chromate ore during the fusion process to produce chromates.

Question 11

What happens to the solution of sodium chromate after acidification?

Answer 11

It is converted to sodium dichromate, which can then be crystallized.

Question 12

Write the complete process to prepare potassium dichromate (K₂Cr₂O₇) from chromate ore.

Answer 12

1. Chromate ore reacts with sodium carbonate and oxygen to form sodium chromate. 2. Sodium chromate is acidified with sulfuric acid to form sodium dichromate. 3. Sodium dichromate is treated with potassium chloride to produce potassium dichromate.

Question 13

Why is potassium dichromate preferred over sodium dichromate in some reactions?

Answer 13

Potassium dichromate is less hygroscopic and more stable.

Question 14

What is the chemical equation for the preparation of potassium dichromate from sodium dichromate?

Answer 14

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Question 15

What is the color of potassium dichromate?

Answer 15

Potassium dichromate is orange.

Question 16

What type of compound is potassium dichromate?

Answer 16

It is an oxidizing agent.

Answer 17

.

Question 18

Why is sodium dichromate more soluble than potassium dichromate?

Answer 18

Sodium dichromate has higher solubility due to its ionic nature and smaller cation size compared to potassium dichromate.

Question 19

What is the chemical reaction for preparing potassium dichromate from sodium dichromate?

Answer 19

Na₂Cr₂O₇ + 2KCl → K₂Cr₂O₇ + 2NaCl

Question 20

What is the effect of pH on the interconversion of chromates and dichromates?

Answer 20

• Acidic medium: Dichromates are formed. - Basic medium: Chromates are formed.

Question 21

Write the equation for the interconversion of chromates to dichromates in acidic medium.

Answer 21

2CrO₄²⁻ + 2H⁺ → Cr₂O₇²⁻ + H₂O

Question 22

Write the equation for the interconversion of dichromates to chromates in basic medium.

Answer 22

Cr₂O₇²⁻ + 2OH⁻ → 2CrO₄²⁻ + H₂O

Question 23

What happens to the color of the solution during the interconversion of chromates and dichromates?

Answer 23

• Chromates (CrO₄²⁻) are yellow. - Dichromates (Cr₂O₇²⁻) are orange.

Question 24

What remains constant during the interconversion of chromates and dichromates?

Answer 24

The oxidation state of chromium remains constant at +6.

Question 25

Why are dichromates and chromates strong oxidizing agents?

Answer 25

They readily accept electrons, reducing chromium from +6 to +3 oxidation state.

Question 27

What are the chemical formulas for chromate and dichromate ions?

Answer 27

Chromate ion: CrO4²⁻, Dichromate ion: Cr2O7²⁻

Question 28

What is the structural difference between chromate and dichromate ions?

Answer 28

The chromate ion is tetrahedral, while the dichromate ion consists of two tetrahedra sharing one corner with a Cr-O-Cr bond angle of 126°.

Question 29

What is the reaction between dichromate and hydroxide ions?

Answer 29

Cr2O7²⁻ + 2OH⁻ → 2CrO4²⁻ + H2O

Question 30

What are the solubility and common uses of sodium and potassium dichromates?

Answer 30

Sodium dichromate has greater solubility in water and is used as an oxidizing agent in organic chemistry. Potassium dichromate is used as a primary standard in volumetric analysis.

Question 31

Write the reduction reaction of dichromate ion in acidic solution.

Answer 31

Cr2O7²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H2O (E° = 1.33V)

Question 32

How does potassium dichromate oxidize iodides?

Answer 32

6I⁻ → 3I2 + 6e⁻

Question 33

How does potassium dichromate oxidize sulfides?

Answer 33

3H2S → 6H⁺ + 3S + 6e⁻

Question 34

How does potassium dichromate oxidize tin(II) ions?

Answer 34

3Sn²⁺ → 3Sn⁴⁺ + 6e⁻

Question 35

How does potassium dichromate oxidize iron(II) ions?

Answer 35

6Fe²⁺ → 6Fe³⁺ + 6e⁻

Question 36

Write the full ionic equation for the reaction of potassium dichromate with ferrous ion (Fe²⁺).

Answer 36

Cr2O7²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 6Fe³⁺ + 7H2O

Question 37

Why are dichromates strong oxidizing agents?

Answer 37

Because they readily accept electrons and have a high standard reduction potential (E° = 1.33V).