What is Chromic Oxide Cr2O3 Definition Formula Preparation Reactions and Uses
Cr2O3 is an inorganic compound that goes by the chemical name Chromic Oxide. The alternative names of the compound are Dichromium Trioxide or Chromium Sesquioxide or Chromium (III) Oxide or Chrome green or Chromia. This is one of the major Oxides of Chromium. Here, the metal has a +3 oxidation state. The monoisotopic mass of the Chromic Oxide (Cr2O3) is 151.866 Da. The molar mass is 151.9904 g/Mol. It appears as crystals or in fine crystalline powder form of light to dark green colouration. The rare natural mineral of Cr2O3 is Eskolaite. It is named after a Finnish geologist Pentti Eskola.
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The mineral is extracted from Chromium rich tremolite skarns, Metaquartzite, and Chlorite veins. A trace amount can also be found in Chondrite meteorites.
Properties of Chromic Oxide (Cr2O3)
The compound has a hexagonal crystal structure with nearly spherical morphology. Cr2O3 is a hard and brittle material. The density of the compound is 5.22 g/cm3.
It is insoluble in water, alcohol, and acetone and not very reactive to acids. It is very slightly soluble in alkalis.
It has a boiling point of 40000 C or 4270 K and a melting point of 24350C or 2708 K.
It is canonicalized with five covalent bonds. There are three hydrogen bond acceptors and zero hydrogen bond donors.
The chemical composition of chromic oxide has 68.46% of Chromium and 31.58% Oxygen content.
It has a refractive index of 2.551. The magnetic susceptibility is +1960.0×10-6 cm3/mol. It is antiferromagnetic up to 307K.
Production of Chromic Acid
The very first transparent hydrated form of Chromium (+3) Oxide was prepared by The Parisians Pannetier and Binet in 1838. The mineral Chromite like (Fe, Mg)CR2O4 is used to derive Cr2O3. The conversion process takes place via Na2Cr2O7, which gets reduced with Sulfur at high temperatures.
Na2Cr2O7 + S → Na2SO4 + Cr2O3
Chromia can also be formed by the decomposition of Chromium salts like Chromium nitrate or Ammonium dichromate by an exothermic reaction.
(NH4)2Cr2O7 → Cr2O3 + N2 + 4H2O
Since the traditional process used for chromic oxide production discharges large quantities of solid waste and has low energy efficiency, a cleaner process is developed by the Chinese Academy of Sciences in Beijing. This new cleaner process promises advancement for industrial production is based on the 3Rs principle.
Uses of Chromic Oxide (Cr2O3)
Chromic oxide is quite stable and is used as a green pigment in paints, inks, and glasses for its stability. Originally, it was called viridian.
It is also used as a colourant for ceramics and produces a green tinge in ‘chrome green’ and ‘institutional green’.
It is used as a surface coating on food-processing and food packaging equipment to prevent abrasive wear.
In refractory materials, electric semiconductors it is used as a pigment. At times, the compound has also been used in printing banknotes and fabrics.
It is a catalyst for organic and inorganic reactions.
It has metallurgical uses, like manufacturing chromium metal and aluminium-chromium master alloys.
Another special use is in colouring cement and granules for asphalt roofing and camouflage painting.
Anhydrous chromic oxide is used for its heat, light, and chemical resisting properties in applications.
It is a catalyst in the preparation of methanol, butadiene, and high-density polyethene.
Stainless steel polishing is also done by chromic oxide.
It is used as a green pigment in automotive finishes.
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Chemical Nature of Cr2O3
Chromium being a d-block element exhibits a variation in oxidation numbers of its oxides. Many d-block elements exhibit both basic or acidic properties in different oxide forms. The higher the oxidation number the more acidic its corresponding oxide. Chromium has three different oxides. CrO has an oxidation number +2, and is a basic oxide, while CrO3 is acidic with a +6 oxidation number. Cr2O3 is amphoteric. Even if it is insoluble in water, it dissolves in acid to give hydrated chromium ions [Cr(H2O)6]3+. It reacts with alkali to yield chromite ions.
FAQs on Chromic Oxide Cr2O3 Structure Properties and Applications
1. What is chromic oxide (Cr2O3)?
Chromic oxide is a green, stable inorganic compound with the chemical formula Cr2O3, also known as chromium(III) oxide. It consists of chromium in the +3 oxidation state combined with oxygen.
- Molar mass: 151.99 g·mol-1
- Appearance: Dark green crystalline solid
- Nature: Amphoteric oxide
- Structure: Corundum-type crystal structure (similar to Al2O3)
2. What is the oxidation state of chromium in Cr2O3?
The oxidation state of chromium in Cr2O3 is +3. Oxygen has an oxidation state of −2, and since there are three oxygen atoms (total −6), the two chromium atoms must together contribute +6 to balance the compound.
- 3 O atoms × (−2) = −6
- Total charge = 0 (neutral compound)
- 2 Cr atoms = +6 total
- Each Cr = +3
3. Is Cr2O3 acidic, basic, or amphoteric?
Chromic oxide (Cr2O3) is an amphoteric oxide, meaning it reacts with both acids and bases.
- With acids: Cr2O3(s) + 6HCl(aq) → 2CrCl3(aq) + 3H2O(l)
- With strong bases (on fusion): forms chromite salts such as sodium chromite
4. How is chromium(III) oxide prepared?
Chromium(III) oxide is commonly prepared by the thermal decomposition of ammonium dichromate. The balanced reaction is:
- (NH4)2Cr2O7(s) → Cr2O3(s) + N2(g) + 4H2O(g)
5. What are the uses of Cr2O3?
Chromium(III) oxide is widely used as a pigment, polishing agent, and refractory material. Major uses include:
- Green pigment in paints, ceramics, and glass (chrome green)
- Polishing compound for metals and optical lenses
- Refractory material due to high melting point
- Component in metallurgy and chromium plating processes
6. What is the molar mass of Cr2O3?
The molar mass of Cr2O3 is 151.99 g·mol-1. It is calculated as:
- Cr: 2 × 52.00 = 104.00 g·mol-1
- O: 3 × 16.00 = 48.00 g·mol-1
- Total = 152.00 g·mol-1 (≈151.99 g·mol-1)
7. How do you balance the formation reaction of Cr2O3 from chromium and oxygen?
The balanced formation reaction of chromium(III) oxide from its elements is 4Cr(s) + 3O2(g) → 2Cr2O3(s).
- Start with Cr + O2 → Cr2O3
- Balance chromium atoms (4 Cr on left, 4 on right)
- Balance oxygen atoms (6 O on both sides)
8. What is the difference between CrO3 and Cr2O3?
The main difference between CrO3 and Cr2O3 is the oxidation state of chromium and their chemical behavior.
- CrO3: Chromium is +6, strongly acidic oxide, powerful oxidizing agent
- Cr2O3: Chromium is +3, amphoteric oxide, relatively stable
9. Is chromium(III) oxide soluble in water?
Chromium(III) oxide (Cr2O3) is practically insoluble in water. It is a stable, crystalline solid with strong ionic and lattice forces.
- Does not dissolve in cold or hot water
- Reacts with strong acids to form Cr3+ salts
- Can react with strong bases under fusion conditions
10. Why is Cr2O3 green in color?
Cr2O3 is green due to d–d electronic transitions of Cr3+ ions in its crystal field. Chromium(III) has a 3d3 electronic configuration.
- Crystal field splitting occurs in the oxide lattice
- Certain wavelengths of visible light are absorbed
- The reflected light appears green
