Electrode potentials, Voltaic cells

Even though we used electrode potentials and cell voltage to predict a spontaneous reaction in Example 19-6, we do not have to carry out the reaction in a voltaic cell. This is an important point to keep in mind. Thus, Cu is displaced from aqueous solution simply by adding aluminum metal, as shown in Figure 19-7. Another point, illustrated by Example 19-7, is that qualitative answers to questions concerning redox reactions can be found without going through a complete calculation of Ecell-... 

An interesting application of electrode potentials is to the calculation of the e.m.f. of a voltaic cell. One of the simplest of galvanic cells is the Daniell cell. It consists of a rod of zinc dipping into zinc sulphate solution and a strip of copper in copper sulphate solution the two solutions are generally separated by placing one inside a porous pot and the other in the surrounding vessel. The cell may be represented as ... 

The reaction may be regarded as taking place in a voltaic cell, the two half-cells being a C12,2C1 system and a Fe3+,Fe2+ system. The reaction is allowed to proceed to equilibrium, and the total voltage or e.m.f. of the cell will then be zero, i.e. the potentials of the two electrodes will be equal ... 

Voltaic cells 64. 504 Voltammetry 7, 591 anodic stripping, 621 concentration step, 621 mercury drop electrode, 623 mercury film electrode, 623 peak breadth, 622 peak current, 622 peak potential, 622 purity of reagents, 624 voltammogram, 622 D. of lead in tap water, 625 Volume distribution coefficient 196 Volume of 1 g of water at various temperatures, (T) 87... 

A voltaic cell consists essentially of three parts two electrodes, from which the positive and negative electricity leave the cell, and an electrolyte in which the electrodes are contained. Its form is therefore that of an electrolytic cell, and the difference between the two lies only in the condition that in the former we produce an electric current through the agency of the material changes, whereas in the latter we induce these material changes by a current supplied from an external source the same arrangement may therefore serve as either. The direction in which the current flows through the cell will depend on the potential difference between its terminals. 

Similar considerations apply of course to the opposing electromotive forces of polarisation during electrolysis, when the process is executed reversibly, since an electrolytic cell is, as we early remarked, to be considered as a voltaic cell working in the reverse direction. In this way Helmholtz (ibid.) was able to explain the fluctuations of potential in the electrolysis of water as due to the variations of concentration due to diffusion of the dissolved gases. It must not be forgotten, however, that peculiar phenomena—so-called supertension effects—depending on the nature of the electrodes, make their appearance here, and com-... 

Rabinovich et al. have shown that it is possible to propose an extrather-modynamic definition of single-ion activity, a, as a function of the real potentials of those particles. "" By carrying out the measurements of voltaic cells containing electrodes reversible to the same ionic species in solutions of different concentrations in the same solvent. 

The determination of the real energies of solvation from measurements of the voltaic cells (Section VI) makes it possible to find the absolute electrode potentials in nonaqueous solvents owing to the relation... 

The non situ experiment pioneered by Sass uses a preparation of an electrode in an ultrahigh vacuum through cryogenic coadsorption of known quantities of electrolyte species (i.e., solvent, ions, and neutral molecules) on a metal surface. " Such experiments serve as a simulation, or better, as a synthetic model of electrodes. The use of surface spectroscopic techniques makes it possible to determine the coverage and structure of a synthesized electrolyte. The interfacial potential (i.e., the electrode work function) is measured using the voltaic cell technique. Of course, there are reasonable objections to the UHV technique, such as too little water, too low a temperature, too small interfacial potentials, and lack of control of ionic activities. ... 

This review has been restricted mainly to clarification ofthe fundamentals and to presenting a coherent view ofthe actual state of research on voltaic cells, as well as their applications. Voltaic cells are, or may be, used in various branches of electrochemistry and surface chemistry, both in basic and applied research. They particularly enable interpretations of the potentials of various interphase and electrode boundaries, including those that are employed in galvanic and electroanalytical cells. 

Thus, the Volta potential may be operationally defined as the compensating voltage of the cell of Scheme 16. However, it should be stressed that the compensating voltage of a voltaic cell is not always the direct measure of the Volta potential. The appropriate mutual arrangement of phases, as well as application of reversible electrodes or salt bridges in the systems, allows measurement of not only the Volta potential but also the surface and the Galvani potentials. These possibilities are schematically illustrated by [15]... 

The electrochemical cell with zinc and copper electrodes had an overall potential difference that was positive (+1.10 volts), so the spontaneous chemical reactions produced an electric current. Such a cell is called a voltaic cell. In contrast, electrolytic cells use an externally generated electrical current to produce a chemical reaction that would not otherwise take place. 

Electrodes in a voltaic cell, however, are connected to circuits— paths by which electrons flow. Voltaic cells are sources of electricity, so they can be used to drive electrolytic reactions or perform other activities that require electricity. The term voltaic honors the Italian scientist Alessandro Volta (1745-1827), a pioneer of electrochemistry. A simple voltaic cell can form a battery, invented by Volta in 1800. The unit of electric potential, the volt, also honors Volta. 

In a similar though less diabolical manner, the electrons produced at the anode of a voltaic cell have a natural tendency to flow along the circuit to a location with lower potential the cathode. This potential difference between the two electrodes causes the electromotive force, or EMF, of the cell. EMF is also often referred to as the cell potential and is denoted fj.g,. The cell potential varies with temperature and concentration of products and reactants and is measured in volts (V). The standard cell potential, or E° gn, is the that occurs when concentrations of solutions ire all at 1 M and the cell is at standard temperature and pressure (STP). 

A concentration cell is any voltaic cell in which two half-cells consist of identical electrodes with different solution concentrations. For such a cell, its cell potential under standard conditions, 8°g j, is zero. 

Of interest is the use of this system as both solvent and reactant in a voltaic cell. If two platinum gauze electrodes are immersed in liquid chlorocuprates and a potential is applied, the cell begins charging- At less than 1/of full charge, the potential stabilizes at 0.85 V and remains at that value until the cell is fully charged. The half-reactions for charging are... 

VOLTAIC CELL. Two conductive metals of different potentials, in contact with an electrolyte, which generate an electric current. The original voltaic cell was composed of silver and zinc, with brine-moistened paper as electrolyte Semisolid pastes are now used electrodes may be lead, nickel, zinc, of cadmium. 

A voltaic cell contains one half-cell with a zinc electrode in a Zn2+ (aq) solution and a copper electrode in a Cu2+(aq) solution. At standard condition, E° = 1.10 V. Which condition below would cause the cell potential to be greater than 1.10 V ... 

In a voltaic cell, a zinc electrode is placed in a solution that is 1.0 M for Zn2+, while a copper electrode is placed in a 1.0 M Cu2+ solution. Calculate the cell potential for the voltaic cell. (Assume a salt bridge is in place.)... 

There are several terms you should be familiar with for voltaic cells. First, the voltage that is impressed across the circuit (that is, the difference in electrical potential between the zinc strip and the copper strip) is known as the cell voltage, which is also occasionally called the cell potential or the electromotive force, EMF. The copper electrode, because it becomes negatively charged and attracts cations, is known as the cathode. The zinc electrode becomes positively charged and is known as the anode. You are expected to know which part of the reaction takes place at the cathode and which part takes place at the anode. These can sometimes be difficult to remember, so a simple mnemonic device can help you distinguish between the two. Oxidation occurs at the Anode (note how each term starts with a vowel), and deduction occurs at the Cathode (note how each term starts with a consonant). 

To summarize voltaic cells, let s review the components that create the cell. First, you need two half-cells, each of which contains an electrode immersed in an electrolytic solution (typically containing the cation of the metal in the electrode). A spontaneous reaction must occur between the electrode and the solution. A wire connects the two electrodes and will allow the external flow of electrons from the anode to the cathode. In Figure 18.1, a voltmeter is shown as part of the circuit between the two electrodes. This is not a necessary part of the circuit—it is simply there to measure the voltage across the circuit. The salt bridge completes the electric circuit and allows the flow of cations and anions between the two half-reactions. Sometimes a porous disc is used in place of a salt bridge. The driving force for the current is the difference in potential energies between the two half-cells. 

A voltaic cell is created with one half-cell consisting of a copper electrode immersed in 1.0 M CuS04 solution and the other half-cell consisting of a lead electrode immersed in a 1.0 M Pb(N03)2 solution. Each half-cell is maintained at 25°C. What is the cell potential, in volts ... 

The correct answer is (A). In a voltaic cell, the substance with the most positive reduction potential will be the cathode. Ag+ has a value of 0.80 V, and Ni2+ has a value of -0.25 V. That would make the silver electrode the cathode. In a voltaic cell, the cathode always has a positive charge. 

Potentials in Non-Aqueous Solutions.—Many measurements of varying accuracy have been made of voltaic cells containing solutions in non-aqueous media in the earlier work efforts were made to correlate the results with the potentials of similar electrodes containing aqueous solutions. Any attempt to combine two electrodes each of which contains a different solvent is doomed to failure because of the large and uncertain potentials which exist at the boundary between the two liquids. It has been realized in recent years that the only satisfactory method of dealing with the situation is to consider each solvent as an entirely independent medium, and not to try to relate the results directly to those obtained in aqueous solutions. Since the various equations derived in this and the previous chapter are independent of the nature of the solvent, they may be applied to voltaic cells containing solutions in substances other than water. 

Recall from Chapter 16 that an object s potential energy is due to its position or composition. In electrochemistry, electrical potential energy is a measure of the amount of current that can be generated from a voltaic cell to do work. Electric charge can flow between two points only when a difference in electrical potential energy exists between the two points. In an electrochemical cell, these two points are the two electrodes. The potential difference of a voltaic cell is an indication of the energy that is available to move electrons from the anode to the cathode. 

Over the years, chemists have measured and recorded the standard reduction potentials, abbreviated of many different half-cells. Table 21-1 lists some common half-cell reactions in order of increasing reduction potential. The values in the table are based on using the half-cell reaction that is being measured as the cathode and the standard hydrogen electrode as the anode. All of the half-reactions in Table 21-1 are written as reductions. However, in any voltaic cell, which always contains two halfreactions, the half-reaction with the lower reduction potential will proceed in the opposite direction and will be an oxidation reaction. In other words, the half-reaction that is more positive will proceed as a reduction and the half-reaction that is more negative will proceed as an oxidation. 

The basic components of a voltaic cell are a wire, two electrodes and two partially-separated solutions. When the electrodes are placed in their respective solutions and the wire is used to connect them, a spontaneous flow of electrons occurs in the wire from one electrode to the other. The impetus for current flow comes from the difference between the oxidation potentials of the electrodes and the solutions, or between the electrodes themselves or between the two solutions in which the electrodes are immersed. A chemical redox reaction occurs between these separated species such that the oxidation half of the reaction occurs in one solution and the reduction half occurs in the other. The partial separation of the solution can be accomplished by a membrane or a salt bridge, which allows an electrolytic connection but does not allow a general mixing of the two solutions. Within the cell, electrical current moves in the form of free electrons in the wire and as ions in the electrolyte. 

Fig. 3-1. An example of a voltaic cell spontaneously generating current in a wire. The impetus for electron movement in the wire comes from the difference in oxidation potential between Zn,s) and Cu. The reducing agent, Zn(s), gives up electrons at the anode to become Zn ". The oxidising agent, Cu acquires electrons at the cathode and plates-out on the copper electrode as Cu(s). The semi-permeable membrane allows ions to move between the two solutions preventing charge imbalances and completing the electrical circuit (from Hamilton, 1998).

Figure 3-1 shows an example of a voltaic cell. A zinc electrode is immersed in a solution of NaCl and a copper electrode in a solution of CuCl2,with a semi-permeable membrane separating the two solutions. If a wire connects the two electrodes, electrons flow spontaneously from the zinc electrode to the copper electrode because is a stronger oxidising agent than Zn(s). At the copper cathode, Cu in the solution is reduced to CU(s) by electrons that are the product of the simultaneous oxidation of Zn(s) to Zn at the zinc anode. The difference in oxidation potential of the two metals results in a differential of approximately 1.10 volts between the two electrodes (assuming equal concentrations of Cu and Zn ). Across the membrane, Cf ions must move toward or... 

Fig. 3-2. Differences in oxidation potential around the electrodes in an electrolytic cell (requires external power) and a voltaic cell (spontaneous) (from Hamilton, 1998).

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