Thermochromism: Theory Directed Design of a Thermochromic Thermometer

Abstract

The dissolution of cobalt(II) chloride in an appropriate binary alcohol solvent yields an equilibrium mixture, consisting of the differently colored octahedral (pink in color) and tetrahedral (blue in color) cobalt(II) coordination complexes, which can exhibit a brilliant and reversible pink-to-blue color transition over a ∼10 °C window─i.e., function as a thermochromic thermometer─where the dynamic equilibrium is described by CoCl2(R1OH)4+2R2OH⇄CoCl2(R2OH)2+4R1OH. In this experiment, students are challenged to exploit the principles of equilibrium in order to exhibit control over the chemical system and design, model, and implement a thermochromic thermometer which, when correctly prepared, exhibits a specifically targeted composition and color at a unique temperature assigned to each student. By employing fundamental principles of physical chemistry in conjunction with spectrophotometric analysis, students quantify pertinent thermodynamic parameters for the chemical system, and then use them to predictively model/calculate (via Excel) the exact initial composition required to enable the specifically desired equilibrium composition and color to emerge at precisely the target temperature. Completely directed by their modeled prescriptions, students then validate their theoretical predictions by implementing the functional thermochromic thermometer in the laboratory.