Key Properties and Chemical Behavior of Cr₂O₃
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): Complete Guide
1. What is Chromic Oxide and what is its chemical formula?
Chromic Oxide is an inorganic compound composed of chromium and oxygen. Its chemical formula is Cr₂O₃. It is also commonly known as chromium(III) oxide or chromia. It is one of the principal oxides of the element chromium and is found in nature as the rare mineral eskolaite.
2. What are the main industrial uses of Chromic Oxide (Cr₂O₃)?
Due to its significant stability and colour, Chromic Oxide has several important industrial applications. Key uses include:
Pigment: It is widely used as a green pigment in paints, inks, and glasses, often under the name viridian.
Abrasive: Because of its hardness, it is used for polishing or grinding metal and optical surfaces (stropping).
Refractory Material: Its high melting point (2435 °C) makes it suitable for lining furnaces and kilns.
Catalyst: It serves as a catalyst in various organic synthesis processes.
3. What is the characteristic colour and physical state of Chromic Oxide?
Chromic Oxide (Cr₂O₃) is a fine, crystalline powder that is typically light to dark green in colour. It is odourless and insoluble in water, acids, and alkalis. Its green colour is very stable and does not fade with exposure to light or heat, which is why it's a popular pigment.
4. How is Chromic Oxide (Cr₂O₃) prepared?
Chromic Oxide can be prepared through several methods. A common laboratory method involves the thermal decomposition of ammonium dichromate, which produces Cr₂O₃, nitrogen gas, and water vapor in an exothermic reaction, often demonstrated as the 'volcano experiment'.
The reaction is: (NH₄)₂Cr₂O₇(s) → Cr₂O₃(s) + N₂(g) + 4H₂O(g).
5. What are the health and safety considerations when handling Chromic Oxide?
While Cr₂O₃ is significantly less toxic than hexavalent chromium compounds, it is still considered a hazardous substance. Inhalation of the dust can cause respiratory irritation. Direct contact may lead to skin or eye irritation. For safety, it is essential to use personal protective equipment (PPE) like gloves, safety glasses, and respirators in well-ventilated areas when handling the powder.
6. Why is Chromic Oxide (Cr₂O₃) considered an amphoteric oxide?
Chromic Oxide (Cr₂O₃) is classified as an amphoteric oxide because it exhibits both acidic and basic properties. It reacts with acids to form chromium(III) salts (acting as a base) and with strong alkalis to form chromites (acting as an acid). For example:
- Reaction with acid (HCl): Cr₂O₃ + 6HCl → 2CrCl₃ + 3H₂O
- Reaction with base (NaOH): Cr₂O₃ + 2NaOH → 2NaCrO₂ + H₂O
7. How does the crystal structure of Chromic Oxide contribute to its properties?
Chromic Oxide (Cr₂O₃) adopts the corundum crystal structure, similar to that of alumina (Al₂O₃). In this structure, the oxide anions form a hexagonal close-packed lattice, and the chromium(III) cations (Cr³⁺) occupy two-thirds of the octahedral interstices. This rigid, tightly packed lattice is responsible for many of its key properties, including its high melting point, significant hardness, and chemical inertness.
8. How does the passive layer of Chromic Oxide protect stainless steel?
Stainless steel's remarkable resistance to corrosion is due to a process called passivation. When exposed to oxygen, the chromium in the steel alloy rapidly forms an extremely thin, invisible, and chemically inert surface layer of Chromic Oxide (Cr₂O₃). This passive layer acts as a barrier, shielding the underlying iron from reacting with oxygen and water, thus preventing rust and other forms of corrosion.
9. What is the difference in properties between Chromium(III) Oxide (Cr₂O₃) and Chromium(VI) Oxide (CrO₃)?
The properties of chromium oxides vary significantly with the oxidation state of chromium. The key differences are:
Nature: Cr₂O₃ (oxidation state +3) is amphoteric and is a stable green solid.
Nature: CrO₃ (oxidation state +6) is strongly acidic, forming chromic acid in water. It is a highly toxic, corrosive, and powerful oxidizing agent, appearing as a dark red-brown solid.
Stability & Use: Cr₂O₃ is very stable and used as a pigment, while CrO₃ is a hazardous and reactive compound primarily used in chrome plating and as a strong oxidant in organic chemistry.
