Chromium trioxide

Chemical compound From Wikipedia, the free encyclopedia

Chromium trioxide (also known as chromium(VI) oxide or chromic anhydride) is the inorganic compound with the formula CrO3.[7] It is the acidic anhydride of chromic acid, and is sometimes marketed under the same name.[6]

Quick facts Names, Identifiers ...
Chromium trioxide
Ball-and-stick model of chromium trioxide
Ball-and-stick model of chromium trioxide
  Chromium, Cr
  Oxygen, O
Powder of chromium trioxide
Powder of chromium trioxide
Names
IUPAC name
Chromium trioxide
Other names
Chromic anhydride, Chromium(VI) oxide, Chromic acid (misnomer)
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.014.189 Edit this at Wikidata
RTECS number
  • GB6650000
UNII
UN number 1463
  • InChI=1S/Cr.3O checkY
    Key: WGLPBDUCMAPZCE-UHFFFAOYSA-N checkY
  • InChI=1/Cr.3O/rCrO3/c2-1(3)4
    Key: WGLPBDUCMAPZCE-YFSAMUSXAF
  • O=[Cr](=O)=O
Properties
CrO3
Molar mass 99.993 g·mol−1
Appearance Dark red granular solid, deliquescent
Odor Odorless
Density 2.7 g/cm3 (20 °C)[1]
Melting point 197 °C (387 °F; 470 K)[1]
Boiling point 250 °C (482 °F; 523 K)
decomposes[1]
  • 164.8 g/(100 mL) (0 °C)
  • 169 g/(100 mL) (25 °C)[1]
  • 172.6 g/(100 mL) (40 °C)
  • 198.1 g/(100 mL) (100 °C)[2]
Solubility Soluble in H2SO4, HNO3, (CH3CH2)2O, CH3COOH, (CH3)2CO
+40·10−6 cm3/mol[1]
Thermochemistry
73.2 J/(mol·K)[3]
−589.3 kJ/mol[4]
Hazards
GHS labelling:
GHS03: OxidizingGHS05: CorrosiveGHS06: ToxicGHS08: Health hazardGHS09: Environmental hazard[5]
Danger
H271, H301+H311, H314, H317, H330, H334, H335, H340, H350, H361f, H372, H410[5]
P210, P260, P280, P303+P361+P353, P304+P340+P310, P305+P351+P338[5]
NFPA 704 (fire diamond)
[6]
NFPA 704 four-colored diamondHealth 4: Very short exposure could cause death or major residual injury. E.g. VX gasFlammability 0: Will not burn. E.g. waterInstability 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g. white phosphorusSpecial hazard OX: Oxidizer. E.g. potassium perchlorate
4
0
2
OX
Lethal dose or concentration (LD, LC):
80 mg/kg (rats, oral)[6]
Safety data sheet (SDS) ICSC 1194
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)
Close

This compound is a dark-purple solid. Millions of kilograms are produced annually, mainly for electroplating.[8] Chromium trioxide is a powerful oxidiser, a mutagen, and a carcinogen.[9]

Production and structure

Chromium trioxide is generated by treating sodium dichromate with sulfuric acid:[6]

H2SO4 + Na2Cr2O7 → 2 CrO3 + Na2SO4 + H2O

Approximately 100,000 tonnes are produced annually by this or similar routes.[8]

The solid consists of chains of tetrahedrally coordinated chromium atoms that share vertices. Each chromium center therefore shares two oxygen centers with neighbors. Two oxygen atoms are not shared, giving an overall stoichiometry of 1:3.[10][11]

Ball-and-stick model of chains in the crystal structure of CrO3
  Chromium, Cr
  Oxygen, O

The structure of monomeric CrO3 has been calculated using density functional theory, and is predicted to be pyramidal (point group C3v) rather than planar (point group D3h).[12]

Ball-and-stick model of the DFT-calculated structure of the CrO3 monomer

Reactions

Chromium trioxide decomposes above 197 °C, liberating oxygen and eventually giving Cr2O3:[7]

4 CrO3 → 2 Cr2O3 + 3 O2

With water it converts to "chromic acid", which includes red-orange species such as H2CrO4 and H2Cr2O7.[7]

Chromium trioxide is a Lewis acid, forming adducts with many (non-oxidizable) bases, such as chloride:[13]

CrO3 + Cl− → CrO3Cl−

Applications

Chromium trioxide is mainly used in chrome plating. It is typically employed with additives that affect the plating process but do not react with the trioxide. The trioxide reacts with cadmium, zinc, and other metals to generate passivating chromate films that resist corrosion. It is also used in the production of synthetic rubies. Chromic acid solution is also used in applying types of anodic coating to aluminium, which are primarily used in aerospace applications. On the International Space Station, it is used to control bacteria growth in the wastewater storage tank. A chromic acid/phosphoric acid solution is also the preferred stripping agent of anodic coatings of all types.

Organic chemistry

Main article: Oxidation with chromium(VI) complexes § Inventory of Cr(VI)-pyridine and pyridinium reagents

Chromium trioxide and a variety of its derivatives are used in organic chemistry. Some of these reagents:

Typically these reagents convert alcohols to carbonyls:[16]

  • 4 CrO3 + 3 RCH2OH + 12 H+ → 3 RCOOH + 4 Cr3+ + 9 H2O
    2 CrO3 + 3 R2CHOH + 6 H+ → 3 R2C=O + 2 Cr3+ + 6 H2O

Safety

Chromium trioxide is highly toxic, corrosive, and carcinogenic.[17] It is the main example of hexavalent chromium, an environmental hazard.[18] The related chromium(III) derivatives are not particularly dangerous; thus, reductants are used to destroy chromium(VI) samples.

Chromium trioxide, being a powerful oxidizer, will ignite organic materials such as alcohols on contact.

Images

  • A concentrated solution of potassium dichromate in water.
    A concentrated solution of potassium dichromate in water.
  • Addition of sulfuric acid to the solution.
    Addition of sulfuric acid to the solution.
  • Crystallization of chromium trioxide from the reaction.
    Crystallization of chromium trioxide from the reaction.
  • Reaction between chromium trioxide and ethanol
    Reaction between chromium trioxide and ethanol

References

External links

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