edited by GEORGEL. GILBERT Denison University Granville. Ohio 43023
SUSMmED BY William H. Batschelet 3915 Starhlll S m Antonio, TX 78218 CHECKED BY
Arnold Georga Manstiaid Unlverslly Manalield, PA 18933 I n discussing the conversion of energy from one form to another, we allstate that it is possible tdinterconvert chemical and light energy and provide examples. However, there are timeswhen it is desirable actuallv to show that this can be done. This is especially true now 'that there is increased oublic interest in the ~ossibilitvof solar e n e r w conversion. ?'he luminol reactiodl and ~;alume' sticksuare effective wavs t o demonstrate the conversion of chemical energy to light. This demonstration goes the other direction and-dramaticallv shows the use of light - energy .. to drive a chemical reaction: T h e demonstration is based on the photochemical decomposition of tris(oxalato)iron(III) to yield iron(I1) and carbon dioxide. This is the basic reaction of ferric oxalate actinometry,2 and the overall reaction is
T h e reaction has previously been the subject of a demonstrations and a laboratory experiment4, and this demonstration incorporates the best features of each. T h e uniaue features are (1)the iron(I1) oroduced in the photochemical reaction is detected using p;tassium ferricyanide which is alreadv oresent in the reaction mixture, and (2) a thickening agent is added to the reaction mix to limit molecular diffusion. The advantages of using potassium ferricyanide to develop the color over the l , l ~ ~ ~ h e n a n t h r o l i n e previouslv usedZare that the color change is essentially in&ntane&s and does not require time todevelop a s does the iron(I1)-phenanthroline system," and that the color change the dark blue of the from the yellow of the F e ( C ~ 0 ~ ) 3to- ~ ferric ferrocyanide complex (Turnbull's blue)5 is easier to see than the yellow red-orange color change in thephenanthroline-containing system.
Solution A
1.2 g FelNO,) 1.9H20in 100 ml. water 10 rnl. Cab-O-Spew', a 15wr w/w suspension of fumed siiica6. (optional thirk~ningagent)
Solution B 0.8 g axalic acid in 100 mL water 10 mL 3%KzFe(CN)csolution 2 mL Triton' X-100, a uonionic surfactant, (optional thickening agent)
Pour the two solutions together and mix gently. When exposed to intense light, the resulting mixture immediately turns from the yellow color of the ferrioxalate complex to the dark blue of the ferric ferrocyanide complex. The demonstration can be done effectively on a large scale by using an appropriately sized cylinder and a rechargeable photo flash unit. By including the fumed silica and surfactant thickening agents and masking portions of the cylinder,
Demonstration of the photochemical reaction and the effect obtained by masking and including the thickening agent. The letters were individually exposed by use of rechargeable photo flash unit. it is porsihle tostencil in persistent patternsor messages of your own that clesrlv demonstrate that it is light that produeep the renrrwn (see figure). There are several alternative ways of performing this demonstration that do not involve the thickening agents. One is to carry out the reaction in a large beaker or crystallizingdish placed on an overhead projector and show the color change on the screen. It is still possible to mask oart of the reaction mixture to illustrate the effect of lieht. ~owevei, the thickening agents should he omitted since they lea2 to light scattering, which causes the projected colors to become indistinct ..... ..
Another approach is to line the inside of a cylinder or benker smoothly with heavy-weighctilterpaper prior toadding thereartion mixture. Hy marking and selwtiwly srpoaing the filter paper, it is again possible to create persistent images. These can he preserved by removing the filter paper and rinsing it free from the reaction solution.
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Fieser. L. F.; Williamson, K. L. "Organic Experiments"; Heath: Lexington. MA. 1983. * Balzani, V.: Carassiti, V. "Photochemistry of Coordination Compounds"; Academic: London, 1970. Vitz, E. W. J. Chem. Educ. 1981, 58.655. Baker, A.D.: Casadavell. A.: Gafnev.H. D.:Gellender. M. J. Chem Educ. 1980, 57,314. Cotton, F. A.: Wilkinson, G. "Advanced InorganicChemistry," 4th ed.; Interscience: New York, 1980. Cab-O-Spersea is a 15% wlw suspension of Cab-O-Silwin water. Cab-O-Sil is fumed silicon dioxide, and both products are trademarks of the Cabot Corporation,P.O. Box 188, Tuscoia, IL 61953. The CabO-Sperse should be well mixed in a blender prior to use. Volume 63 Number 5
May 1986
435
