Chromium (VI) Oxide Formula – Structure, Properties, Uses, Sample Questions

Chromium is denoted with the symbol Cr, and its atomic number is 24. It is a chemical element which lustrous, steely grey and hard. It’s the first element of 6th group in periodic table. Chromium has high corrosion resistance. Metallic chromium is added to steel to make it highly corrosion resistive, thus forming stainless steel. During the 19th century, chromium is used primarily as a component in paints.Chromium is reflective when polished. 
Oxide is a dianion of oxygen. Oxide is a chemical compound which is composed of at least one oxygen atom and one other element in its given chemical formula. Oxygen is a highly reactive nonmetal with atomic number 8. It is colourless and odourless and most abundant element of earth. Oxygen is used in textile and plastic industries, submarines and space flights etc.

Chromium(VI) oxide formula

Chromium(VI) oxide is also called as chromium trioxide and chromic anhydride. It as a very powerful oxidiser. It is usually dark red granular solid. It is dark purple in anhydrous state. Chromium(VI) oxide is an inorganic compound consisting of chromium and oxygen. It’s chemical formula is CrO₃. Now we will discuss about its structure, preparation, its chemical and physical properties and also its essential uses.

Structure of Chromium(VI) oxide

It’s chemical formula is CrO₃. It’s molar mass is 99.999 g/mol. One atom of Cr⁶⁺ sharing three bonds with oxygen atoms.

Physical properties of Chromium(VI) oxide

Chemical formula	CrO3
Appearance	Dark red granular solid
Odour	Odorless
Solubility	Soluble in water, sulphuric acid, nitric acid, acetic acid, acetone.
Melting point	197°c
Boiling point	250°c
Density	2.7 g/cm3
Molar mass	99.993 g/mol

Chemical properties of Chromium(VI) oxide

• Chromium trioxide decomposes over above 197°C, liberating oxygen and thus giving Cr₂O₃.

4CrO₃ → 2Cr₂O₃ + 3O

• Formation of chromic acid happens when Chromium(VI) oxide is reacted with water. The chemical formula of chromic acid is H₂CrO_4, and it is a very weak acid.

CrO₃ + H₂O → H₂CrO₄

Uses of Chromium(VI) oxide

1. It is used in dye, ink and paint manufacturing. 
2. It is used in chrome plating. The trioxide reacts with metals like zinc and cadmium to form a thin layer that prevents corrosion. 
3. It is used in tanning and engraving. 
4. It is used in aluminium anodizing .
5. It is also used in aerospace applications.
6. It is used in photography.
7. It is used as an oxidising agent.

Sample Questions

Question 1: How does chromium affect the environment and the human body?

Answer:

Chromium is toxic. It affects plant metabolism negatively. It kills the growth of vegetables and reduces grain quality. If high levels of chromium is present in water, it may badly affect fish and aquatic animals. If inhaled, it may be dangerous to lungs, and it may also cause sinus cancer , abdominal pain, asthma and other respiratory disorders.

Question 2: How is Chromium(VI) oxide prepared.

Answer:

Chromium(VI) oxide Is formed when sodium chromate is reacted with sulfuric acid. Nearly one lakh tonnes of Chromium trioxide is produced annually. 

H₃SO₄ + Na₂Cr₂O₇ → 2 CrO₃ + Na₂SO₅ + H₂O

Question 3: What happens when it is heated?

Answer: 

If heated, Chromium(VI) oxide decomposes to Chromium(III) oxide, which is Cr₂O₃. It turns brown while burning and appears in green colour after coming to room temperature. The reaction is 

4CrO₃ → 2 Cr₂O₃ + 3O

Question 4: How is Chromium(VI) oxide used as an oxidant?

Answer:

It converts primary alcohols to carboxylic acids and converts secondary alcohols to ketones. The reactions are,

• Primary alcohols to carboxylic acids

3RCH₂OH + 4CrO₃ + 12H⁺ → 4Cr³⁺ + 3RCOOH + 9H₂O

• Secondary alcohols to ketones

3R₃CHOH + 2CrO₃ + 6H⁺ → 2Cr³⁺ + 3R₂C=O + 6H₂O

Question 5: Write about ethanol oxidation by Chromium(VI) oxide.

Answer:

It immediately catches fire, producing Chromium(III) oxide. The chemical reaction is as follows

4CrO₃ + C₂H₅OH → 2Cr₂O₃ + 2CO₂ + 3 H₂O

Question 6: How is Chromium(VI) oxide precipitated from potassium dichromate ?

Answer: 

Concentrated sulfuric acid (H₂SO₄) is added to Concentrated aqueous potassium dichromate (K₂Cr₂O₇) solution for Chromium(VI) oxide to precipitate. Hence small red Crystal’s of CrO₃ are formed. The chemical reaction is,

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

Question 7: What is Collins’s reagent? 

Answer:

It is a compound of chromium trioxide and pyridine (C₅H₅N) in dichloromethane (CH₂Cl₂). It is a red solid. It oxidised primary alcohols to aldehydes. Its chemical formula is C₁₀H₁₀CrN₂O₃.