An Improved Copper Cycle Experiment Ton Umans Gymnasium Sorghvliet, The Hague, The Netherlands Wobbe de Vos Ooststelllngwerf State School, Oosterwolde, The Netherlands The conservation of elements is an imnortant touic in the introductory stage of chemical education. Even before particle theories have been introduced, students can be made aware that, after carrying out a series of chemical reactions with a substance like copper, it is alwayspossihle to recover the metal from one of the reaction products. If a particle model has been taught, students can interpret the conservation of elements as the immutability of atoms, as postulated by Dalton. C o ~ n eis r ~articularlvsuitable for the demonstration of the princfip~eof Eonservatiin. ~ o scopper t cycle experiments are purely qualitative; an experiment showing the quantitative recovery of copper was published earlier in THIS JOURNAL.' We have been working on an improved copper cycle experiment. The aim in our lessons is to encourage students to discover pri~ic~ple im t h t . ~ n ~ e l w rat11er s than t u tell tht R ~nl.iiio~i them the l~rinripltand then aali rhem r,, writ'? it. I)i.cowry i, mwh murc d i t i i ~ u tl h~ m \.trit'ir;~tiun, and st\.cr~lt u d t n t s . after correctly carrying out the sequence of reactions of a classical comer cvcle. failed to recognize the brown nowder obtained at't'he end as'copper. ~henguestioned,the majority did not know what the brown substance could be, and many denied that it could be a metal. Rust was often suggested, and one student insisted on havine observed the odor of cocoa powder. The problem seems to be that students know copper in daily life as a bright, shiny metal while the product of the cycle is a dull brown powder, and there is no simple way to change the powder back into a shiny metallic form. The alternative of starting the cycle with copper powder, although it makes recognition a t t h e end easier, ikrodnces an element of unfamiliarity from the beginning.and still does not exclude rust as a final product. T o overcome this problem we have used the decomposition of copper(I1) acetate (copper(I1) ethanoate) under carefully controlled conditions, as the final stage of the cycle. This nroduces comer as a shinv metal. A blue soi;tion of cop& acetate can be obtained by dissolving about 2 g of copper(I1) oxide, which is an intermediate in most copper cycle experiments, in 10 ml of a 10%acetic acid solution. However, a much more elegant method is to treat
52
Journal of Chemical Education
pieces of copper with a solution of acetic acid and hydrogen peroxide. Equipment:
a wide test tube (2-3 cm) a CooDer coil or small nieces of comer olatine
Procedure:
Mix the acetic acid and hydrogen peroxide solutions
(cotton wool) in the test tube and warm gently without boiling. Then start the reaction by adding the copper. After a few minutes remove unreacted copper from the test tube and evaporate the blue solution slowly to dryness. Then decornnose the bluish ereen salt bv stroneer
The width of the test tube is important. In a narrower tube the students will be unable to prevent the liquid from boiling over, and in a beaker or porcelain dish copper oxide is formed instead of comer. The reducing atmosnhere created bv the gaseous decomposition productshf the acetate in the testtube seems to be essential to ensure the formation of copper. Relatively small amounts of copper acetate produce the best results. With larger amounts, say a few hundred milligrams, the formation of copper is accompanied by a thick hrown smoke, and t h e use of a fume cupboard becomes advisable, although some cotton wool at the end of the tuhe will prevent the emission of most of the smoke. We introduced the acetate route in the copper cycle last year, and as a result most students recognized the end product as comer. For the first time there were snontaneous discussionsLkthe classes on the question of whether this was the same copper with which they had started the experiment. In our experiments we emphasize the qualitative aspect of the copper cycle but a quantitative approach is also possible using the acetic acid-hydrogen peroxide solution. Clearly this would reauire weiahina the comer coil and test tube before and after the expegmeit and e;&orating the solvent without the loss of material.
' Condike, G. F., J. CHEM.EDUC., 52, 615 (1975).
