University:
Seattle Pacific University
Course:
CHM 1211 | General Chemistry I
Academic year:

2021
Views:

326

Pages:

6
Author:

Carissa Thomas

Chromium(III) Chloride CrCl3·6H2O anhydrous CrCl3 Obtaining Non-hydrated Metal Halides • Many ionic metal halides and other ionic salts are hydrated with a specific number of water molecules per formula unit. • Water molecules may occupy specific sites within the crystal lattice or may be directly coordinated to the cation. • CaCl2∙2H2O lattice H2O BeCl2∙4H2O coordinated H2O CrCl3∙6H2O coordinated H2O CuSO4∙5H2O lattice H2O + coordinated H2O Salts with lattice waters of hydration can often be dehydrated by simply heating or heating in vacuum. CaCl2∙2H2O + ∆ → CaCl2 + 2H2O • Salts with coordinated water usually decompose to a mixture of oxides or oxohalides, often with no specific stoichiometry. • Sometimes anhydrous halides can be formed from the hydrated salts by chemically destroying the water of hydration with a reagent such as thionyl chloride. [Cr(H2O)6]Cl3 + 6SOCl2 + ∆ → CrCl3 + 12 HCl + 6SO2 • An alternative approach is to use either the metal or metal oxide and a halogenating agent at high temperature; e.g., Cr2O3 + 3CCl4 + ∆ → 2CrCl3 + 3COCl2 • This is the method used in today’s synthesis. Structure of CrCl3 • CrCl3 is not ionic, but rather has a network solid structure with bridged Cl– ions and octahedrally coordinated Cr(III) ions, which form layers. • Although CrCl3∙6H2O is green, solid CrCl3 is purple. • The solid consists of shiny flakes, owing to its layered structure. • The stability of the network structure makes CrCl3 insoluble in water and most other solvents. Notes on Synthesis • The reaction is carried out at 700 °C in a Vycor (quartz) tube in a tube furnace. • Nitrogen is bubbled through liquid CCl4, and the stream of N2/CCl4 is passed over a pile of green Cr2O3 in the center of the tube. • As CrCl3 is formed it sublimes and is carried in the nitrogen stream to the cooler far end of the tube, where it condenses. • A by product of the reaction is poisonous phosgene gas, COCl2. Cr2O3 + 3CCl4 + ∆ → 2CrCl3 + 3COCl2 • Be sure the fume hood is working properly! • Adjusting the N2(g) flow rate and maintaining a consistent flow of CCl4 is critical to successful synthesis. • Too slow a rate will cause the reaction to take excessively long time. • Too fast a rate causes product to be blown out the end of the tube without condensing. (Watch for wisps of white smoke.) • The reaction proceeds until nearly all of the Cr2O3 in the center of the tube is gone, about 3 hours. • If available, product from previous years will be provided for you to carry out the confirming tests, rather than waiting for your own product.