The Tarnishing Reaction

Wagner (1902) published an analysis of the oxidation of the surface of a metal, based on diffusion reactions, which has remained a classic. This mechanism is called by various names including Tarnishing . The surface of a metal consists of metal atoms bound to the inner structure by a series of hybrid-bonds. If oxygen gas is present (air), the metal, particularly iron, will form an oxide coating, vis  

In several known cases, the oxide-coating protects the surface from further oxide buildup. Yet. there are many cases where the oxide layer continues to grow until the metal is totally consumed. How is this possible  

Wagner hypothesized that both metal and oxide ions diffused through the metal oxide layer so as to build up the layer thickness from both sides, as follows  

In this diagram, oxidation, i.e.- loss of electrons, occurs at the metal surface while reduction, i.e.- acquisition of electrons, occurs at the metal oxide-oxygen gas interface. That is, M, in the metallic state, is oxidized , 

It should be apparent, firom the above diagram, that the thicker x becomes, the slower is the change in the x-dimension, i.e.- 

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We are now ready to consider some soUd state reactions that relate more directly to the real world. These include the tarnishing reaction and Pick s Laws of Diffusion. Both of these scientific areas have been rigorously studied because of their importcmce in revealing how diffusion mechanisms are related to everyday solid state reactions which occur on a daily basis. 

About 1942, Pick formulated laws which described diffusion processes in solids, similar to that already presented for the tarnishing reaction. To do this. Pick hypothesized that atoms (ions) would "hop from site to site in the manner we have already described in 4.2.2. and 4.2.4. 

Actually, we must account for all types of defects, including charged species (see 2.2.1.). To do this, let us reconsider the tarnishing reaction (Section 4.5. given above), using the genered equation ... 

Silver and silver-copper alloys are often even more sensitive to sulfur compounds—the effect is catalyzed in the presence of oxidizing species and high relative humidity [268]. The resulting surface layer—dark, thin and adherent when developed on a polished surface—usually acts as a good physical barrier that slows down the tarnishing reactions. However, since silver objects are expected to be shiny. 

OUR knowledge of reactions in the solid state has been derived mainly from the systematic study of the tarnish reactions and their inverse, dissociation processes. This is due largely to the fact that the product or reactant ionic compounds have been studied by physical methods the nature of the defect, the electronic and ionic processes and the structures are known with some degree of certainty. Moreover, tarnishing reactions are ideal for experimental study, particularly where a coherent film of the product (oxide, halide, etc.) is formed on the metal. 

To do this, let us reconsider the tarnishing reaction (Section D-I), with the general reaction ... 

All of these processes can be described by a diffusion-controlled model originally derived to explain the tarnishing of metals and hence commonly called the tarnishing model. The derivation of this model is based on the assumptions that (a) the reaction site is immobile, (b) the concentration of reaction sites is independent of time and temperature in the absence of the tarnishing reaction, and (c) the reaction rate is very... 

P. A film of cationic solute Af+ in phase 1 (Figure 7.16) deposits on the sheet by diffusion on the outer surfaces and the film is constantly saturated with the electrolyte. Film thickness growth occurs in the xi and X2 directions having a thickness Li in phase 1 and L2 in phase 2, respectively. Thus, the spaces occupied by the d sited film on each side of the sheet are 0 < xi < Li and -L2 < 2 2 < 0. Thus, motion of one or two phases relative to the boundary is caused by the mass transfer diffusion across the interface. This diffusion problem resembles the tarnishing reactions forming an oxide film on a metal surface [25]. 

The kinetics of reactions in which a new phase is formed may be complicated by the interference of that phase with the ease of access of the reactants to each other. This is the situation in corrosion and tarnishing reactions. Thus in the corrosion of a metal by oxygen the increasingly thick coating of oxide that builds up may offer more and more impedance to the reaction. Typical rate expressions are the logarithmic law,... 

Nonstoichiometric oxide phases are of great importance in semiconductor devices, in heterogeneous catalysis and in understanding photoelectric, thermoelectric, magnetic and diffusional properties of solids. They have been used in thermistors, photoelectric cells, rectifiers, transistors, phosphors, luminescent materials and computer components (ferrites, etc.). They are cmcially implicated in reactions at electrode surfaces, the performance of batteries, the tarnishing and corrosion of metals, and many other reactions of significance in catalysis. ... 

The tarnish on silver, Ag2S, can be removed by boiling the silverware in slightly salty water (to improve the water s conductivity) in an aluminum pan. The reaction is an oxidation-reduction reaction that occurs spontaneously, similar to the redox reaction occurring in a voltaic cell. The Ag in Ag2S is reduced back to silver, while the A1 in the pan is oxidized to Al3+. 

Applying Concepts The tarnish on silver is silver sulfide, which is formed when the silver reacts with sulfide compounds in the environment. In this miniLAB, you will use an oxidation-reduction reaction to remove the tarnish from silver or a silver-plated object. 

Write the equation for the reaction of the tarnish (silver sulfide) with the aluminum foil, yielding aluminum sulfide and silver. 

The balanced chemical equation for the tarnishing of silver is the combination of these two half-reactions ... 

Tarnishing reactions. A film of tarnish is formed on the surface of a metal by diffusion of dissolved gas A through the film from an exterior gaseous phase. The reaction rate of gas with metal is assumed to be large,... 

In such reactions as the tarnishing of silver in air, the oxidation of aluminum in air, or attack of lead in sulfate-containing environments, thin, tightly adherent protective films are formed, and the metal surface remains smooth. It should be mentioned that underground corrosion is frequently observed as localized corrosion. Oxidation, sulfidation, carburization, hydrogen effects, and hot corrosion can be considered as types of general corrosion.20... 

A circle of shiny pennies is created by the reaction between the citric acid of the lemon and the tarnish on the surface of the copper. 

Silver is a very inactive metal. It does not react with oxygen in the air under normal circumstances. It does react slowly with sulfur compounds in the air, however. The product of this reaction is silver sulfide (Ag2S), a black compound. The tarnish that develops over time on silverware, jewelry, and other silver-plated objects is silver sulfide. 

Q The tarnish that appears on silver and other metals is actually a solid that forms as a result of chemical reactions that take place when the metal is exposed to traces of sulfur compounds in the air. 

A redox reaction involving silver and environmental sulfides deposited tarnish on the discolored cup. Another redox reaction, essentially the reverse of the first, reduces silver ions in the tarnish to silver atoms that can be rinsed away, leaving the shiny silver cup. 

Primary Step. This step must take place at the interface between the two reacting phases or at the outside surface of a phase undergoing decomposition. A possible exception is that of the dissociation of a lattice point itself, as in photochemical decomposition, in which, say, ions are converted to atoms of a new phase. To illustrate this step, we may consider the simple tarnish reactions, where a metal M combines with a gaseous atom X, to give ultimately an ionic solid MX. 

Wagner (42) discussed tarnish reactions on a quasi-Chemical basis using anions, cations, electrons and positive holes as entities obe dng the mass action laws. As a specific example, we shall consider the formation of a silver halide layer, in which the anions are immobile and form a perfect lattice, while the cation lattice may contain vacancies and interstitial cations. There is a... 

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