Silver-copper reaction, oxidation-reduction

Thus, Experiment 7 involved the same oxidation-reduction reaction but the electron transfer must have occurred locally between individual copper atoms (in the metal) and individual silver ions (in the solution near the metal surface). This local transfer replaces the wire middleman in the cell, which carries electrons from one beaker (where they are released by copper) to the other (where they are accepted by silver ions). 

The action of an active intermediate oxidation product would explain another feature of the reaction. The reduction of silver ions by hydrazine is extremely sensitive to the presence of small amounts of copper. For example, a solution containing a mixture of silver nitrate, sodium sulfite and hydrazine which normally showed no sign of reduced silver for several minutes underwent almost immediate reaction when merely stirred with a clean copper rod. In the presence of gum arabic as stabilizer, streamers of colloidal silver passed out from the copper surface. Similarly, the addition of small amounts of cupric sulfate to a hydrazine solution eliminated the induction period of the reaction with silver chloride. 

Review oxidation-reduction reactions as discussed in Chapter 3. Explain why the copper/silver nitrate reaction in this activity is an oxidation-reduction reaction. 

Many oxidation/reduction reactions can be carried out in either of two ways that are physically quite different. In one, the reaction is performed by bringing the oxidant and the reductant into direct contact in a suitable container. In the second, the reaction is carried out in an electrochemical cell in which the reactants do not come in direct contact with one another. A marvelous example of direct contact is the famous silver tree experiment, in which a piece of copper is immersed in a silver nitrate. solution (Figure 18-1). Silver ions migrate to the metal and are reduced ... 

The basis for such catalysis (e.g., silver ion-catalyzed oxidations by persulfate) is that electron transfer from the reductant to the catalyst, followed by electron transfer from the catalyst to the oxidant, is faster than direct electron transfer from the reductant to the oxidant. A necessary, but insufficient, requirement for such catalysis is the accessibility of two oxidation states of the catalyst, neither of which must be too stable with respect to the other. For reasons that are still not well understood (despite the progress in the understanding of the mechanisms and reactivity patterns of electron transfer reactions), copper and silver salts are especially effective in this type of catalysis. 

An oxidation-reduction reaction is one in which electrons are transferred from one reactant to another. They are often called redox reactions for short. Oxidation is the loss of one or more electrons by a species. The species losing electrons is oxidized. Reduction is the gain of one or more electrons by a species, and that species is reduced. Oxidation and reduction always occur simultaneously. The single-displacement reaction of copper metal with silver nitrate solution is both a single replacement reaction and an oxidation-reduction reaction. 

When elemental copper is placed in a solution of silver nitrate, the following oxidation-reduction reaction takes place, forming elemental silver ... 

The two equations we have written describe what are called half-reactions for the oxidation of copper and the reduction of silver. Neither one can occur on its own because oxidation and reduction must take place in concert with one another. 

Photochromic eyeglasses darken when exposed to ultraviolet light and become transparent again in the absence of ultraviolet light. This process is the result of oxidation-reduction reactions. Silver chloride and copper(l) chloride are embedded in the lenses. The chloride ions absorb photons, and the silver chloride dissociates and forms chlorine atoms and silver atoms. The elemental silver darkens the lenses. Note that the chlorine ions are oxidized and the silver atoms are reduced. Then, the copper(l) ions reduce the chlorine atoms and form copper(ll) ions. In the reverse process, the copper(ll) ions oxidize the silver atoms back to the transparent silver ions. 

For example, if we have a silver anode, the standard oxidation potential is —0.800 V. Adding this value to the copper reduction half-reaction gives —0.458 V. Thus, at a minimum, a potential of 0.458 V must be apphed to get Ag. We can see that the higher up a half-reaction is in Table 9.1, the more it will tend to be in its reduced form. Conversely, the lower it is, the more readily it wiU be oxidized. Thus, Table 9.1 can be quickly scaimed to see what oxidation—reduction reactions will spontaneously occur and provide useful work and what reactions will require the input of work. 

Because silver, gold and copper electrodes are easily activated for SERS by roughening by use of reduction-oxidation cycles, SERS has been widely applied in electrochemistry to monitor the adsorption, orientation, and reactions of molecules at those electrodes in-situ. Special cells for SERS spectroelectrochemistry have been manufactured from chemically resistant materials and with a working electrode accessible to the laser radiation. The versatility of such a cell has been demonstrated in electrochemical reactions of corrosive, moisture-sensitive materials such as oxyhalide electrolytes [4.299]. 

One more example demonstrates how to use standard reduction potentials to determine the standard potential of a cell. Let s say you wanted to construct a cell using silver and zinc. This cell resembles the Daniell cell of the previous example except that a silver electrode is substituted for the copper electrode and a silver nitrate solution is used in place of copper sulfate. From Table 14.2, it is determined that when silver and copper interact silver is reduced and copper oxidized. The two relevant reactions are... 

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