Chromium III Chloride: Formula, Structure, Properties, Preparation

Chromium (III) Chloride also referred to as chromic chloride describes any of several purple solid compounds with the formula CrCl₃ • x H₂O, where x can be 0, 5, and 6. The most common form of Chromium (III) Chloride is the dark green hexahydrate, CrCl₃ · 6 H₂O. It is also used as a catalyst and as a precursor to many inorganic compounds of chromium. It is produced by the direct chlorination of chromium metal. Despite CrCl₃ being ionic, the solid-state structure is kinetically inert so that anhydrous chromic chloride is reluctant to dissolve in H₂O. Further, we will discuss the Chromium (III) Chloride formula along with its chemical structure, preparation, properties and uses.

Key Takeaways: Chromium, Lewis acid, Cr³⁺, Hexahydrate, Trimethylsilyl, Chlorine, Chloride, Inorganic compounds, Catalyst, Chlorination, Solid state, Solid

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Structure of Chromium (III) Chloride

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Anhydrous Chromium (III) Chloride has the YCI₃ structure along with the Cr³⁺ that covers one-third of the octahedral interstices inside the alternating layers of a pseudo-cubic close-packed lattice of Cl^- ions. Due to the absence of cations in alternate layers, the bonding between adjacent layers weakens. Because of this, crystals of CrCl₃ cleave easily along the planes between layers, which results in the flaky appearance of samples of chromium (III) chloride.

The inertness indicates that CrCl₃ is generally sluggish to react without the presence of a reducing agent. However, when it does react, it undergoes ligand substitution reactions to form other complexes of chromium (III). Chromium Chloride reacts as a Lewis Acid, forming stable chloro complexes such as [CrCl₆]₃^-.

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Properties of Chromium (III) Chloride

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• Formula – CrCl₃
• Appearance – Purple solid (anhydrous); Dark green (hexahydrate)
• Boiling Point – 1300 °C
• Melting Point – 1152 °C (anhydrous); 83 °C (hexahydrate)
• Density - 2.87 g/cm³ (anhydrous); 1.760 g/cm³ (hexahydrate)
• Molar Mass - 158.36 g/mol (anhydrous); 266.45 g/mol (hexahydrate)
• Solubility in water – Slightly soluble

Chromium (III) chloride is a type of Lewis Acid, categorized as "hard" according to the Hard Soft Acid Base theory. The general way of opening CrCl₃ up to substitution in solution is to reduce even a trace amount to CrCl₂, for example using Zn in HCL. The chromium compound undergoes substitution easily, and it can exchange electrons with CrCl₃ via a chloride bridge, allowing all of the CrCl₃ to react quickly.

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Preparation of Chromium (III) Chloride

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Chromium (III) chloride can be prepared from chromium metal and chlorine directly, or indirectly using Chromium (III) oxide with carbon and chlorine at 800 °C.

Cr₂O₃ (s) + 3 C(s) + 3 Cl₂(g)→ 2 CrCl₃(s)+ 3 CO(g)

It may also be prepared by chlorination of chromium metal directly, or indirectly by carbothermic chlorination of Chromium (III) oxide at 650–800 °C

Cr₂O₃ + 3 C + 3 Cl₂ → 2 CrCl₃ + 3 CO

The solvate can be derived by dehydration with trimethylsilyl chloride in THF:

CrCl₃ · 6 H₂O + 12 Me₃SiCl → CrCl₃(THF)₃ + 6 (Me₃Si)₂O + 12 HCl

Chromium chloride can also be prepared by treating the hexahydrate with thionyl chloride:

CrCl₃ · 6 H₂O + 6 SOCl₂ → CrCl₃ + 6 SO₂ + 12 HCL

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Things to Remember

• The Chromium (III) Chloride formula is CrCl₃.
• Despite trivalent chromium being far less poisonous than hexavalent, it should be handled carefully as it can cause an allergic skin reaction to a living being.
• CrCl₃.4H₂O precipitates the silver chloride when treated with silver nitrate.
• Cr₂O₃ + 3 C + 3 Cl₂ → 2 CrCl₃ + 3 CO
• CrCl₃ • 6H₂O + 12 Me₃SiCl → CrCl₃(THF)₃ + 6 (Me₃Si)₂O + 12 HCl
• CrCl₃ • 6H₂O + 6 SOCl₂ → CrCl₃ + 6 SO₂ + 12 HCl
• Chromium (III) Chloride reacts as a Lewis Acid.
• It is useful for the purpose of chrome plating in the oilfield industries.

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