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.
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Volume 65
Number 4
April 1988
359
