Temperature Effect on Cobalt(II)-Chloride Complex Equilibrium in Aqueous Solution

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 14 Issue 5 (2009) pp 225-228

Temperature Effect on Cobalt(II)-Chloride Complex Equilibrium in Aqueous Solution

Nobuyoshi Koga*, Takaaki Kimizu, Masatora Sakamoto, and Yoshihiro Furukawa

Chemistry Laboratory, Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan, nkoga@hiroshima-u.ac.jp
Received October 9, 2008. Accepted May 22, 2009

Published online: 3 November 2009

Abstract. For the equilibrium system of cobalt(II) chloride in hydrochloric acid, a simple student experiment to discuss quantitatively the equilibrium shift depending on temperature was examined. It was found that the equilibrium constant at the temperature of the color change between the pink octahedral complex and deep-blue tetrahedral complex can be expressed approximately as a function of the concentration of hydrochloric acid. Through a simple experiment of measuring the color change temperatures for a series of cobalt(II) chloride solutions with different concentrations of hydrochloric acid, the temperature dependence of the equilibrium constant is estimated. Using the measured data for color- change temperatures and the calculated equilibrium constants at those temperatures, the standard enthalpy change DHº is evaluated by a van’t Hoff plot. The present experiment is useful to deepen student understanding of the temperature effect on the equilibrium shift and to introduce chemical thermodynamics at equilibrium.

Key Words: Laboratories and Demonstrations; general chemistry; thermodynamics

(*) Corresponding author. (E-mail: nkoga@hiroshima-u.ac.jp)

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