edited by Denison University Granville. Ohio 43023
Inorganic Thermochromism: A Lecture Demonstration of A Solid State Phase Transition Roger D. Willett
Washington Stale University Pullman, WA 99164 CHECKED BY:
Arnold George Mansfield State College Mansfield, PA Students in introductory chemistry courses are introduced to the concept of phase transitions using the familiar examples of solid-liquid, liquid-vapor, and solid-vapor transitions. These are conveniently demonstrated in the lecture situation. The concept of transitions between two different solid phases, an important aspect of solid state chemistry and solid state physics, is not so adequately treated. In part, this is due to the lack of suitable means to readily demonstrate such transitions. An easy way to demonstrate a solid state phase transition is with the thermochromic material [(C2H&NH&CuCL. This is a low melting solid (T,, = 93°C) which undergoes a phase transition at 50°C.' The salt is easily prepared by fusing together stoichiometric amounts of anhydrous cupric chloride and anhydrous diethylamine hydrochloride. Prolonged or excessive heating should be avoided to prevent formation of [(CzHs)2NHz]zCu4Cllo0(this can easily be identified by its red color). The material is hygroscopic, so it must he stored, accordingly, in areas of high humidity. The phase transition is easily observed since the room temperature phase is a deep ereen and the hieh temnerature ohase is a brieht vellow and can be convenienTly demonstrated by the follo&gprocedure. Melt a small amount in a Petri dish and, using a wooden splint, spread the liquid around so that it is smeared thinly on the surface. Place the Petri dish in an inverted position on the overhead projector. Cooling can be facilitated by placing an ice cube on top of the inverted Petri dish for a short period of time. As the orange melt cools, the yellow high temperature Dhase will first crvstallize, followed a short time later by the
An alternative method of demonstrating the phase transition may be preferable if audiovisual equipment is routinely used. From a clear plastic sheet, two pieces of appropriate size to fit in a 35 mm slide are cut. A small amount of [(C2H&NHz]zCuC4 is melted between the two sheets. The edges are then sealed with tape, and placed in the slide holder. When placed in a conventional slide projector, the heat from the lamp is sufficient to induce the phase transition. The demonstration can also be utilized to demonstrate the interconnection between coordination eeometrv and color in of isolated transition metal salts. The coordination CuCli" ions is a delicate balance between crystal field stabilization, favoring a square planar geometry, and ligandlieand electrostatic re~ulsions,favorine a tetrahedral peomegy. Crystal structure analyses have shown that theireen ohase contains nearlv ~ l a n a CuCla2r anions? while in the .~ ;ellow phase the geom&y is closer t e t r a h e d ~ a lUnfortunatelv. the structure of both uhases are c o m ~ l e xand the s t r u c k e of the high temperatire phase has not been solved in detail. Similar thermochromic behavior is exhibited by [ $ C H Z C H ~ N H ~ C H ~ ] &and U C the ~ ~ .structures of both of its phases have been dete~mined.~ In the low temperature phase, ~ ~ - is planar (nearly D4h symmetry). The the C U C ~ anion C U Cion~ in~the~ high temperature phase has approximate DZdsymmetry with a trans C1-Cu-C1angle of 138". It has been shown that the driving. force of these thermochromic phase the tran.iti
