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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 7 Issue 3 (2002) pp 142-145

Effects of Concentration on Hexaaquacobalt(II)/Tetrachlorocobalt(II) Equilibrium. A Discovery-Oriented Experiment for Chemistry Students

Noelle M. Barrera, Jillian L. McCarty, and Veljko Dragojlovic*

Oceanographic Center, Nova Southeastern University, 8000 North Ocean Avenue, Fort Lauderdale, FL, 33004,, and Department of Math, Science and Technology, Nova Southeastern University, 3301 College Avenue, Fort Lauderdale, FL 33314
Received January 10, 2002. Accepted March 3, 2002

Published online: 3 May 2002

Abstract. A solution of cobalt(II) chloride in HCl is commonly used to examine various effects, such as changes in temperature and concentration, on the Co(H2O)62+/CoCl42– equilibrium (Le Châtelier’s principle). In aqueous solution the cobalt(II) ion exists as a mixture of two complex ions at equilibrium, blue CoCl42– and pink Co(H2O)62+. As the ions have different colors, it is easy to determine the position of the equilibrium. A series of experiments was designed to allow students to examine the concentration of chloride ion, the dehydration effect, and how dissociation effects the equilibrium. Different compounds were added to the aqueous solution of cobalt(II) ion, and the position of the equilibrium was determined either visually or, more quantitatively, by means of UV–vis spectroscopy. This exercise is suitable for general chemistry students and is designed to introduce them to the complexity of the actual chemical reaction rather than presenting them with a simplistic model.

Key Words:  Laboratories and Demonstrations; physical/inorganic chemistry; aqueous solution chemistry; chemical equilibrium; discovery-based experiment; coordination chemistry trichloride

(*) Corresponding author. (E-mail:

Article in PDF format (234 KB) HTML format

Supporting Materials:

Instructor notes and spectra ( 363 KB ZIP file) ( 10.1007/s00897020559b

Issue date: June 7, 2002

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