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GEORGEL. GILBERT Denison University Granville. Ohio 43023
Electrochemical Cells Using Sodium Silicate S U B M ~ EBV D
Bernard Rapp, FSC
Lewis University Rorneovllle, IL 60441
Erwin Boschmann
lndianapolls Indlanspollr. IN 46202
IU-PU at
The operation of galvanic cells can be effectively demonstrated in the classroom using a high-impedance digital voltmeter and a 200-mL hydrometer cylinder, the lower part of which contains a gelled half-cell. A wire lead extends from the half-cell electrode to the voltmeter. The addition of a metal ion solution to the uDDer -. -ort ti on of the cvlinder and the insertion of the appropriate metallic electrode clipped to the o o ~ o s i t elead of the voltmeter Droduces an operating electr~>hemicalcell. An acidified solution of sodium silicate is used to prepare the bottom half-cell. When the salicic acid system has solidified the apparatus is ready to use. The two half-cells are joined internally a t the gel interface without appreciable mixing. Because the lower cell is relatively permanent, i t can be used reneatedlv in makine a varietv of combinations with several liiuid phase half-cefs. The gLl surface needs only to be eentlv rinsed with distilled water between uses. The electrochemical cells described here employ the Cu/Cu2+(aq) svstem as the eelled half-cell. Concentrations of the metal ion solutions &e made equal. This permits calculation of voltages using the equation E",
= E0,,JP,>
where the Eo values of the half-cells are reduction Dotentials. For the examples given below, experimental iesults were in aereement with calculated values to within f0.05 V. (See the table.)
Tha Cu2+/C~-sodi~m Silicate half-cell.
Vottaaes 01 HalCcell Comblnatlons with Cu2+lCu. E o = +0.34 V
CBlculated Value Half-cell System Reduction Potential. P Electrode Ag+ Pb2+
---
+ le
+ 2e Cd2+ + 2e Zn2++ 2e
Ag Pb
Cd Zn
+0.80 V -0.13 -0.40 -0.76
cathode anode anode anode
vs. Cu2+/Cu
0.46 V 0.47 0.74 1.10
Materials
one high-impedance voltmeter with easily discernible readout one 200-mL hydrometer cylinder, -3.5 cm i.d. 30 em insulated wire, 18-20 ga. four 250-mL beakers glacial acetic acid sodium silicate (water glass), 4 M 2 ' Be; d = 1.4 g/mL reagent-grade Cu(N0&3H20 100-mLsolutions of Cd(NO& 4Hz0 1.7 g Pb(NOd2 8.3 g AgN03 4.3 g ZnClz 3.4 g electrodes (strip, bar, or wire) of Cu, Cd, Pb, Ag, and Zn The dimensions of the apparatus are not critical to the experimental results and may be altered to suit the classroom situation. The sizes recommended here have been satisfactorily used with groups of 40 to 45 students. Avoltmeter with easilv discernible dieital readout adds to the effectiveness of thk demonstration, although some verbal communication of readings may be necessary. 358
Journal of Chemical Education
Preparation of the Sodlum Silicate CulCu2+ Half-Cell
Cut a thin sheet of comer metal 5 cm X 2.5'cm. Fold the metal widthwise over the kxposed end (-2 em) of a length of insulated wire. Use ~ l i e r to s crim~ the metal securely to the wire at the fold. end the wire-at a right angle where it emerges from the electrode. Lower the electrode into the cylinder so that it is roughly parallel to the base near the 50mL level. The insulated wire will extend upward close to the side of the cvlinder. I t can be bent over the lio and secured with a piecebf masking tape. (See the figure.)' Prepare an approximately 0.25 M solution of copper(I1) ion by dissolving 6.0 g of reagent-grade Cu(N0&3HzO in 45 mL of water. Acidifv the solution bv addine 5 mL of 10 M acetic acid (3 mL acetic acid:i mL H~o).In a second beaker dissolve 8 mL sodium silicate in sufficient water to make 50 mL of solution. With vigorous mechanical stirring add the Cu2+solution to the sodium silicate. Transfer the
A Simple, Vlvld Demonstration of Selective Precipitation
contents to the cylinder. Allow several hours for the system to solidify. Demonstration
Susulm~ sv
Dip each electrode in dil HC1,rinse with water and place in a 250-mL beaker containing the solution of the corresponding ion. Sequentially combine each system with the salicic acid half-cell. Rinse the bottom cell thoroughly with distilled water after each combination. The gel surface will remain intact if the water is noured eentlv down the side of the cylinder. This procedu;e shoul&also be observed when addine the solutions. It is advisable to run the Ae/Ae+ .. ,. half-cell before testing thezinc-zincchloridesystem in order toavoid the formation of AgCl at the gel interface.l
Thomas P. Chirpich
Extensions 1. I t is possible to use the gelled half-cell to demonstrate qualitatively the effect of concentration on cell voltage. One can auicklv make a concentration (entropy) cell by usina distioed water and a second copper electride in the npp& part of the cylinder. Thevoltmeter must be set to read on the millivolt scale. 2. A permanent Daniel1 cell can be prepared after the demonstration. Cut a thin niece of zinc metal to the same dimensions as the copper electrode, and secure it to asecond leneth of wire as nreviouslv described. Mount i t in the cvlinderso that the efectrode i i near the 150-mL level. Preoare 50 mL of sodium silicate solution of the same concentration as before. With rapid stirring add 50 ml, of 1 M acetic arid (3 ml. glacial: 4; mL HdO). Transfer to the cylinder and allow sufficient time for the system to geL2 Voltages very close to the calculated 1.0 V can he observed. Shorting the system for a period of days will produce a dramaticgrowth of copper crystals at the cathode with concurrent lossof bluecolor in thesurroundine medium. Simultaneous corrosion a t the anode is also evident.
Hazards Glacial acetic acid is an eye, skin, and lung irritant. It should be handled carefully in a well-ventilated area.
Solutions of Zn(N0&.6H20 gave consistently low values of 0.95 V. When ZnCI, was used, experimental results matched calculated to within f0.02 V. When zinc chloride is dissolved in water, an oxide1 hydroxide forms that is readily dissolved by adding a trace of HCi. 'Lauren, Paul M. Sci. Teach. 1965, 47(2),32.
Memphls State Unlverslty Memphlf, TN 38152 CHECKED BY
Paul C. Krause UnlverSIt~of Central Arkansas Conray, AR 72032
The second semester of freshman chemistrv olaces h e a w emphasis on calculations invol5,ing equilibria; and studenis often haved~fticultvin thisarea because they fail tovisualize what is happenina. This report describes-a simple, \,ivid demonsiration that is designed to catch the students'attention and to illustrate the principle of selective, or fractional, precipimtion in action. The demonstration consists of three tuhes. One contains 5 mL of 0.1 M Na,CO?, one contains 10 mL oi0.1 M KI, and the third contains twth 5 mL of 0.1 M iYa,CO? and 10 mL c~f 0.1 M KI. Precipitates will form in eac; tube when some 0.1 M P h ( N 0 3 ) ~is added. A fine, white PbC03 precipitate forms when 5 mL of Pb(N03)2 is added to the Na2C03 solution, and bright yellow precipitate forms when 5 mL of Pb(N03)2 is added to the KI solution. Before adding the Pb(NO3I2 to the solution containing both the Na2C03 and the KI, I use the KSp'sof PbC03 (1.5 X 10-13) and of Pb12 (8.7 X 10-9) to show the class that the addition of 4.8 mL of Pb(N0-L to the solution containine both the Na2C03 and the KI s&d produce only the white PbCO? and that the vellow PhIl orecinitate should not aopear s k c e the [Pb2+]:eft after t h k ~ b ~ ~ ~ ~ r e c i p iist atoo tion low for [Pb2+][I-]2to exceed the K, for Pb12. Much to the delight of some students, when the P b ( N 0 3 ) ~ is added to the solution, a vellow turhiditv appears. However, if the tube is capped Gith Parafilm add &erted a few times, it quickly becomes apparent that the vellow color is fading, and complete disappearance of the yellow color will eventually occur if the tube is occasionally mixed by inversion. The tuhes may be conveniently left in view while the rest of the lecture continues to show that all traces of a vellowish tinw will disaooear with time. I also take a few minutes to explain that eqklibrium is most often a state not achieved "instantaneouslv" desoite the first outward impression given in precipkation reactions. (With a more advanced class, one can also discuss local concentrations of ions and the rate of dissolution of precipitates.) All in all, a concrete demonstration, such as this one, of the principles being studied helps the students' understanding in the midst of all the calculations thar thev are learning and can cause some of them to think more deeply about what is actually happening in chemical reactions.
ERIC Clearinghouse tor Sclence, Mathematics, and Environmental Education To Continue at Ohio State University A new, 5-year contract for the ERIC Clearinghouse for Science, Mathematics,and EnvironmentalEducation has been awarded to Ohio State University. ERICISMEAC will continue to process materials for Resources inEducation (RIE) and Current Index to Journals in Education (CIJE), to produce 12 digests and an annual bulletin in 1988, to produce eight monographs, and to provide assistance to users to help them lwate information. People, organizations, and institutions that desire to receive the free annualbulletin can write to: ERICISMEAC, 1200ChambersRoad, Room 310, Columbus,OH 43212 or phone (614) 292-6717.
Volume 65
Number 4
April 1988
359
