In the Classroom Tested Demonstrations
Thermochromic Solids submitted by:
Jeffrey G. Hughes Department of Applied Chemistry, RMIT University, Melbourne 3001, Australia
checked by:
George L. Gilbert Department of Chemistry, Denison University, Granville, OH 43023
Abstract Thermochromism is the reversible change of color of a solid when it is heated or cooled. Previous reported examples of thermochromic solids involve color changes due to changes of stereochemistry. Salts of mercuric iodide, M2HgI4 [M=Cu(I), Ag(I)] are easily prepared and exhibit sharp, reversible thermochromic transitions at moderately low temperatures. The Ag(I) solid changes color from yellow to orange at 50 °C and the Cu(I) solid at 67 °C. The color changes are due to subtle changes in crystal structure. Signs can be prepared for lecture demonstrations using pieces of filter paper saturated in the solids Keywords Demonstrations Coordination Chemistry Phase Transitions/Diagrams Supplementary Materials No supplementary material available.
Full Text
Return to Table of Contents
JChemEd.chem.wisc.edu • Vol. 75 No. 1 January 1998 • Journal of Chemical Education
Abstract
In the Classroom Tested Demonstrations
Thermochromic Solids submitted by:
Jeffrey G. Hughes Department of Applied Chemistry, RMIT University, Melbourne 3001, Australia
checked by:
George L. Gilbert Department of Chemistry, Denison University, Granville, OH 43023
Thermochromism is the reversible change of color of a solid when it is heated or cooled. A previous article in this Journal (1) described the preparation of the thermochromic solid bis(diethylammonium)tetrachlorocuprate(II). This compound has a low-temperature green form and a hightemperature orange form. The thermochromism is due to a change from square planar to tetrahedral geometry. In this demonstration the preparation of thermochromic compounds M 2HgI4 [M = Cu(I), Ag(I)] is described. These compounds are very easily prepared, can be stored for long periods, and exhibit sharp thermochromic transitions. Preparation
M = Ag(I) Dissolve 3.25 g of Hg(NO3 )2 in boiling water. Add 10% KI solution till the initial precipitate of HgI2 dissolves to give a clear solution. To this solution add 50 mL of a solution of 3.4 g of AgNO3 and boil for a few minutes. Filter off the yellow Ag2HgI 4 and dry in a desiccator.
M = Cu(I) Prepare a solution of Cu(I) by adding 25 mL of a solution of 2.5 g CuSO4 to a boiling solution of 6 g Na2SO3 ?7H2 O and 5 g NaCl in 50 mL of water. A greenish precipitate forms initially but redissolves to give a yellow solution. Dissolve 1.6 g of Hg(NO3)2 in 25 mL of boiling water. Form the HgI42{ ion as in the Ag(I) preparation, using 10% KI. To this solution add the hot Cu(I) solution. A deep purple solid precipitates, which changes color to red on cooling and filtering. The solid should be dried in a desiccator.
Demonstration The Ag(I) solid changes color from yellow to orange at 50 °C. The Cu(I) solid changes from red to purple at 67 °C. Both changes are quite sharp. Making a paste of the solids, smearing on a piece of filter paper, and heating on a hot-plate or with a hair dryer is a convenient way of demonstrating the thermochromic change. Another good variation is to cut up pieces of filter paper saturated with the solids (the papers used in the preparation are quite good for this) and use them to make an appropriate sign. They can be fixed to white poster card with clear adhesive tape. Using these compounds and the copper(II) compound mentioned previously gives a variety of color changes. One example of a sign might be to use the pieces of paper to form the chemical formulae. In a classroom or lecture theater the poster can be heated with a hair dryer to demonstrate the color changes. Discussion The color change is due to a subtle change in structure. The β or low-temperature form is tetragonal with tetrahedral HgI4 2{ units occupying the corners of the unit cell, but the M+ ions occupy only 4 of the 6 face-centered positions. In the cubic α form (high-temperature) all 6 faces, as well as the corners, are occupied at random by M or Hg in the ratio 2:1. In both α and β forms the positions of the iodide ions are fixed in a cubic close-packed array (2, 3). Literature Cited 1. Van Oort, M. J. M. J. Chem. Educ. 1988, 65, 84. 2. Day, J. H. Chem. Rev. 1968, 68, 649. 3. Palmer, W. G. Experimental Inorganic Chemistry; University Press: Cambridge, 1954; pp 170–171.
JChemEd.chem.wisc.edu • Vol. 75 No. 1 January 1998 • Journal of Chemical Education
57
