-P
a& 6443
Pacific Southwest Association of Chemistry Teachers
J
J. Rae Schwenck
Sacramento Junior College Sacramento 22, California
The Chemistry of Silver A demonstration sequence
W h y does an "insoluble" substance dissolve in a selmted reagent? Why does the solution have to be acidic? Whv didn't mv sulfide convert to a carbonate? These questions, which involve competing ionic and oxidation-reduction equilibria, constantly challenge the serious student. A useful seriesdemonstration can be presented to provide a foundation for the student's understanding of such problems. The element chosen for this demonstration is silver because of its versatile chemistry. Outlined in the table is a systematic sequence of reactions chosen to illustrate the chemistry of silver. The demonstration proceeds after the oxidation of silver metal to silver ion by nitric acid. The demonstration proceeds by the addition of selected reagents in sequence forming i~isolublesalts and soluble complex ions. Each salt and complex ion in the sequence may be shown to leave a progressively lower and lower concentration of silver ion in the equilibrium mixture. ~~
~
~
Reactions in the silver series Ag Metal
HNOl
OH-
Ag3O Dark brown solid
BrAg(NHJzt Colorless ~olotion Ag(S20&' Colorless solution
CozAg+ -----t Ag2C03 Colorl~ss Creem colored solution solid c1NH1 4 AgCl White solid Sd&AgBr A Pale yellow solid CNAgI 4 Yellow solid A1 A Ag Black In hot NaaC08 Lustrou~ solid solution metal
A
A
I-
s-
Ag(C?i),- -Ag,S Colorless solution
The reagents are first added in sequence as shown in the table to a series of individual test tubes, each containing 5 ml of 0.02 ill .4gN03. This permits the characteristics of each precipitate and solutiou to he observed. Then 100 ml of 0.02 M AgN03 is placed in a tall beaker and to this is added (again in sequence and until a definite change is observed) each of the reagents listed. Silver sulfide, being the least soluble silver salt, forms as the final precipitate. The concluding step of the demonstration shom the formation of silver metal from the sulfide. This demonstration sequence provides a natural
Sunburst o f silver forms on the rheet or aluminum m e t a l toucher the tarnished plote.
and convenient opportunity to explain such concepts as competing reactions, equilibrium constants, methods of dissolving metals and precipitates, coordinate bonding and complex ion formation, effect of concentration on ionic equilibria, and oxidation-reduction potentials. In presenting the demonstration it is most convenient to show the formation of Agf by dissolving a bit of silver metal sheet in nitric acid. A stock solution of 0.02 M AgN03 may then he used for the reactions of the series. A concentration of 1 M for the reagents, i.e., for the ions used to form precipitates, and for the complex-forming molecules or ions, may then be used to produce the sequence of changes. When such an arrangement is used in the order given in the table, a series of various colored precipitates and of colorless solutions will result. The concentration of silver ion, Ag+, progressively decreases in the equilibrium mixture as the sequence is followed. The reduction of silver sulfide is shown in the figure. The sheet of silver metal shown in the back of the beaker first was tarnished heavily by treating the cleaned silver with sodium sulfide solution. The solution in the beaker is hot 2 M Na2C03. The strip of metal touching the silver is aluminum. Tarnish disappears from the poii~tof contact and radiates outward leaving the lustrous silver expmed. This reaction is the last step in the silver series. Repeated roquests have been received for the publication of particulars regarding this extended series of reactions. The series was demonstrated bv the author and Dr. Connell B. Roherts hefore the Pacific Southwest Association of Chemistry Teachers an November 5, 1955. Readers desiring mimeographed sheets giving: (1) tables of applicable mass action constants, (2) suggestions for the classroom use of sequences of reactions and of tables, and (3) a description of some additional and unusual resetions of the silver ion may obtain them by writing to the author.
Volume 36, Number 7, January 1959
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