A Further Improvement on the Copper Cycle Experiment The experiment described by Umans and deVosl accomplishes its purpose of showing students in beginning chemistry that a chemical which is put through a series of reactions does not change its identity, and neither gains nor loses weight. d : is Its simolicitv. .. however, will make many students ask. "So what?" The comer .. is dissolved and then r e.~ r e c.i ~ i t a t rthis not an exciting or very interesting result. A series of reactions which involves several visible changes but finally yields the original material is more interesting and mare convincing t o the students. This article describes such a demonstration. A coil of thin copper wire is placed in a flask containing bromine water where it slowly dissolves (overnight).The resulting solution may be blue or brown, depending upon the copper concentration (the blue is due to [CuiHz0)4I2+,the brown t o [CuBrd2-. If insufficient bromine water has been used, some insoluble, white CuBr may also he formed. When the solution is evaporated i t becomes a very dark brown, with finally only solid CuBr, remaining. On heating to about 500°C this decomposes to white CuBr and free bromine (hood). The eopper(1) bromide reacts readily with concentrated nitric acid to give Cu(NO&, BIZ and NO2 (hood). The copper(I1) nitrate is converted to CuO by heating, and this is reduced to metallic copper by hydrogen. (Of course, care must he taken to conduct hydrogen through the flask to drive out all of the air hefore the copper oxide is heated.) All of these observations can be performed in a single flask, so the chance of losing anything is minimal. If the flask and copper are weighed a t the beginning and a t the end, it will be seen that no weight has been gained or lost. The pertinent equations are:
.
Cu
-
+ Br, + 4 H ~ 0
+
[ C ~ ( H Z ~ ) ~ ] B ~4Hz0 ~ ~ C U B ~ ~
ired)
blue 2CuBrZA 2 C u B r white
+ Brz
blue ~ [ C U ( H ~ O ) ~ ] ( N O ~ )2Cu0 Z (black) CuO
+ Hz
-
Cu (red)
' Umans, Ton, and deVos, Wobhe, J. CHEM.EDUC.,59.52 (1982). John C. Bailar, Jr. University of Illinois Urbana. iL 61801
Volume 60
Number 7
July 1983
583
