GEORGEL. GILBERT Denison University Granville, Ohio 43023
Red Cabbage and the Electrolysis of Water Submltted by
be pointed out that the electrode reactions depend on the electrolyte present.
James F. Sklnner
Literature Cited
Williams College Williamstown. MA 01267
(1) Ssliahury,F. B.,and Roaa, C., "Plant Physiaiqy." Wadworth PublishingCompany, Ine., 1969. p. 397. (2) Kasimer, P., J. CHeM. Eouc.,36,A387 (1958).
Checked by
Davld A. Franz Lycoming College Williamspon, PA 17701
Acid-base chemistry and electrochemistry are two integral parts of any introductory college chemistry course. The following demonstration has proven very effective in bringing together concepts from these two areas.
Materials The materials needed are a large flat-bottomed glass dish (e.g., 20-cm crystallizing dish), two inert metal electrodes of substantial surface area (1cm2), some 1M NaN03, an overhead projector, and a 10 VDC power supply. Either uf two indicators has heen found suitahle. The Fisher Universal Indicator Solution (SO-1.601is supplied with a series of small colored transparencies corresponding to the color of the indicator at different pH's. These can be viewed on the projector showing the students the colors expected in strong acid and strong base. Alternatively, a certain amount of intellectual interest can be generated by using the indicator readily isolated from red cabbage. The chopped cabbage is boiled in a minimum amount of water for about 10 min and then filtered to give a deep purple indicator solution. The plant pigments, anthocyanins (I),are red in acidic solution, purple in neutral solution, and green to yellow in basic solution and serve as stable, reversible indicators. Having a cabbage available as part of the demonstration is illustrative. Demonsiration Secure the two electrodes with masking tape to opposite sides of the dish and fill with 1M NaN03 to about 1 cm depth. Add either of the indicators to give a good color intensity. The relevant half-reactions and overall electrolysis reaction
6 HzO
-
0 2
+ 4 H30f + 4e-
4H20+4e--2H2+4OH-
anode cathode
2H20+2H2+02 can he written on a transparent sheet and placed beneath the dish on the projector. When motion in the solution has ceased, apply a potential of 5-10 V. Evidence for the locally high concentrations of Hs0+ and OH- at the anode and cathode, respectively, will come from the indicator colors a t low and hiih pH. (;as bubbles may also he apparent on the screen. Swirlinr the dish, having scopped the electrolysis, results in a solutiin of color close to neutral pH again, consistent with the generation of equal quantities of HsO+ and OH- during the electrolysis. An alternative electrolysis (2) could be performed on the overhead projector. In 1M KI, containing a little phenolphthalein, the anode reaction produces 12, appearing as a yellow-brown solution while the cathode reaction again produces OH- indicated by the pinkphenolphthalein. I t should
A Physical Model to Demonstrate Acid-Base Conjugate Pairs Submitted by
Robert W. Naylor Potomac State College of West Virginia University Keyser, WV 26726 Checked by
Davld Blackman Federal City College Washington. DC 20005
This author has found in over seven years of instruction that, along with the typical problems students have in general chemistry, there is a great deal of confusion over the relationship of the strength of a conjugate acid or base to its respective precursor. Utilizing magnets, an extremely simple method of demonstrating that weak acids give strong conjugate bases and that strong acids give weak conjugate bases can be performed. The same trend for bases also can be easily illustrated. A short piece of iron, such as the size used to join two poles of horseshoe magnets during storage and two horseshoe maenets (one ca~ahleof liftine 25 lb. the other a weak mamet, the-type sold fo; collecting ~ < a i ~ h t ' ~ iare n s )used. At the beeinnine of the demonstration the students are told to imagine That thk magnet is a n anion and that the iron piece is a proton. The magnet-iron bar pair constitute any acid. The students are shown how difficult it is to remove the iron from the strong magnet. In small classes the "acid" can be passed among the students or placed in the laboratory for student investigation. There is something about strong magnets that causes a great deal of excitement and curiosity. The students readily conclude that the horseshoe-metal pair represents a weak acid. Using the same metal piece, it can be shown that the weak magnet is barely capable of holding on to the metal bar. A flicking of the wrist causes the metal to drop from the magnet. A facile loss of the "proton;" therefore, a strong "acid." Next, attention is turned to the "bases" produced by the loss of the "protons." By bringing the magnets in turn over the metal bar, the strong magnet will cause the "proton" to jump several centimeters. The clapping sound can be heard clearly throughout the room. The students are reminded that a property of a Bronsted base is to accept protons. The weak maenet. of course. has difficultv in even liftine- the metal from thetable. The students easily conclude that the loss of the "proton" from the weak "acid" has produced a strong conjugate "base" and the converse for the strong "acid." By starting with just the magnets and describing them as bases, conjugate "acids" can he produced which are opposite in strength to their acid precursors. Volume 58
Number 12 December 1981
1017
