In the Classroom edited by
Overhead Projector Demonstrations
Doris K. Kolb Bradley University Peoria, IL 61625
Chemical Equilibria Involving Reactions of Silver(I) Ions Roberto Zingales Department of Inorganic and Analytical Chemistry, University of Palermo, Viale delle Scienze – Parco d’Orleans II, I 90128 Palermo, Italy;
[email protected] A series of reactions involving silver(I) ions, proposed earlier by Shakhashiri et al. (1), can easily be demonstrated to a large class, by using an overhead projector. Demonstration I On the stage of an overhead projector arrange five Petri dishes on a clear transparency sheet, so that the dishes can be labelled: (1) HCl, (2) NH3, (3) KBr, (4) Na2S2O3, and (5) KI. Into each of the Petri dishes pour a thin layer of 0.01 M AgNO3. The solution is clear and colorless. Then, to each dish, with gentle swirling, add a few drops of one of the following solutions and observe the following results, 1. 1 M HCl; white solid forms 2. 6 M NH3; solid forms and then disappears 3. 0.1 M KBr; yellowish-white solid forms 4. 0.5 M Na2S2O3; solution remains clear and colorless 5. 1 M KI; yellow solid forms
These solutions can be stored in small dropper bottles. In dishes 1, 3, and 5 the formation of a solid precipitate blocks the light of the overhead projector and causes the image on the screen to appear black. Unfortunately the colors of the opaque precipitates cannot be distinguished. In dish 2 a solid forms initially, but dissolves as more NH3 is added. No precipitate forms in dish 4, so the solution remains clear and colorless. The chemical equations for the reactions occurring in the five dishes are given in Table 1, along with their log Keq values. Demonstration II By carrying out these same reactions sequentially in a single vessel, one can demonstrate the relative stabilities of the products. 1. Into a clean Petri dish pour a thin layer of 0.01 M AgNO3 solution. Then add a couple drops of 1 M HCl solution. Solid AgCl forms and the image on the screen appears black.
2. Next add 6 M NH3 solution dropwise, with gentle swirling. The solid disappears. 3. Then add two or three drops of 0.1 M KBr solution. Solid AgBr precipitates and the image on the screen again appears black. 4. Add 0.5 M sodium thiosulfate solution dropwise, with swirling, until the solid precipitate disappears. 5. Finally, add a few drops of 1 M KI solution and AgI precipitates. Once again the image on the screen appears black.
The equations and log Keq values for the five sequential reactions are given in Table 2. This demonstration shows how the silver halides can be separated from one another using solutions of ammonia and thiosulfate. This demonstration can be used to illustrate how analytical separations can be planned, that is, arranging chemical reactions sequentially in a framework to separate a particular species from a mixture. Hazards Dilution of concentrated hydrochloric acid and concentrated aqueous ammonia should be done carefully in a hood, since both of these liquids are quite volatile as well as irritating. Silver nitrate solid is caustic and can cause burns. When spilled on the skin, it can cause dark stains (which can be removed by treatment with thiosulfate solution). Wear safety glasses when preparing or presenting the demonstrations. Acknowledgment The author is greatly indebted to the Column Editor for her revision of the text. Literature Cited 1. Shakhashiri, B. Z.; Dirreen, G. E.; Juergens, F. J. Chem. Educ. 1980, 57, 813. 2. Brown, T. L.; Le May, H. E., Jr. Qualitative Inorganic Analysis; Prentice Hall: Englewood Cliffs, NJ, 1983. Table 2. Reactions for Demonstration II
Table 1. Reactions for Demonstration I log Keq
Reaction 1. Ag⫹(aq) ⫹ Cl⫺(aq)
AgCl(s)
⫹
⫹
2. Ag (aq) ⫹ 2NH3(aq) 3. Ag⫹(aq) ⫹ Br⫺(aq) ⫹
4. Ag (aq) ⫹ ⫹
[Ag(NH3)2] (aq) AgBr(s)
2S2O32⫺(aq) ⫺
5. Ag (aq) ⫹ I (aq)
AgI(s)
7.23 12.30
3⫺
[Ag(S2O3)2] (aq)
NOTE: The Keq values are from ref 2.
534
9.74
13.46 16.08
log Keq
Reaction 1. Ag⫹(aq) ⫹ Cl⫺(aq)
AgCl(s)
2. AgCl(s) ⫹ 2NH3(aq) 2S2O32⫺(aq) 3⫺
⫺
[Ag(NH3)2] (aq) ⫹ Cl (aq)
3. [Ag(NH3)2]⫹(aq) ⫹ Br⫺(aq) 4. AgBr(s) ⫹
9.74 ⫹
⫺
5. [Ag(S2O3)2] (aq) ⫹ I (aq)
᎑2.51
AgBr(s) ⫹ 2NH3(aq)
5.07
⫺
1.16
3⫺
[Ag(S2O3)2] (aq) ⫹ Br (aq) AgI(s) ⫹
NOTE: The Keq values are from ref 2.
Journal of Chemical Education • Vol. 80 No. 5 May 2003 • JChemEd.chem.wisc.edu
2S2O32⫺(aq)
2.62
