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In this issue: CO₂ emissions and hydration isomers of chromium(III) chloride

Question

From P. A. Barker of Rosemount, Blairgowrie

Can anybody tell me if there are any power stations that absorb CO₂ from the emissions?

Answer

From Dr Jeff Hardy, manager, environment, sustainability and energy forum at the RSC

Carbon dioxide (CO₂) capture has been demonstrated in large-scale processes such as the purification of natural gas and in the production of synthesis gas for processes such as ammonia production. To my knowledge there is currently no operational example of a full-scale power station fitted with carbon capture technology. However, this is set to change over the next 20 years with a number of pilot plants planned for construction in the UK, EU and overseas to test the viability of the technology. Carbon dioxide can essentially be captured from a pre- or post-combustion process and this dictates the capture technology that is required. The Intergovernmental Panel on Climate Change special report on carbon dioxide capture and storage provides an excellent review on this technology see Website.

Question

From David Pritchard at Stevenson College Edinburgh

I want to make the three hydrate isomers of chromium(III) chloride - hexaaquachromium(III) chloride [Cr(H₂O)₆]Cl₃, pentaaquachloro-chromium(III) chloride-1-water [Cr(H₂O)₅Cl]Cl₂^•H₂O, and tetraaquadi-chlorochromium(III) chloride-2-water [Cr(H₂O)₄Cl₂]Cl^•2H₂O. I have tried the usual sources of information (the Internet and various books) without any success. My aim is to make sufficient for a class to analyse the complexes for the chloride content using silver nitrate etc.

Answer

From Dr Chris Adams of the school of chemistry at Bristol University

One of the rarer types of isomerism encountered in transition metal chemistry is that of 'hydration isomerism', where complexes of identical stoichiometry differ in the way that the ligands are attached to the central metal. The classic case of this occurs in chromium(III) chemistry, where isomeric compounds with the formulae:
(a) [Cr(H₂O)₄Cl₂]Cl^•2H₂O; (b) [Cr(H₂O)₅Cl]Cl₂^•H₂O; and (c) [Cr(H₂O)₆]Cl₃ can be isolated. In solution, isomers (a) and (b) are slowly converted to isomer (c), but the kinetic inertness of the d³ chromium(III) ion makes this process slow enough so that a and b can be isolated under suitable conditions. There is a description of the preparation by Barbier, Kappenstein and Hugel in the Journal of Chemical Education.¹