Galvanic cells concentration dependence

This effect appears to be of importance in the case of normal galvanic cells, the electromotive forces of which depend on the concentration of solutions in equilibrium with depolarising solids such as calomel or mercurous sulphate. The exact relationships are, unfortunately, not yet wholly elucidated. 

A voltmeter joined between the two electrodes of a galvanic cell shows a characteristic voltage, which depends on the concentration and nature of participating reactants. For example, in the Cu-Zn cell, if Cu2+ and Zn2+ are at 1 mol dm-3 (1 M) concentrations and the temperature is 298 K, the voltage measured would be 1.10 V. This voltage is characteristic of the reaction as shown below ... 

The electrode in the half-cell in which oxidation is occurring is said to be the anode (here, the zinc metal), whereas the other is the cathode (here, the platinum). In principle, we could connect any pair of feasible half-cells to form a galvanic cell the identity of the half-cells will determine which electrode will act as the anode, and which the cathode. The electromotive force (EMF, in volts) of the cell will depend on the identity of the half cells, the temperature and pressure, the activities of the reacting species, and the current drawn. An EMF will also be generated by a cell in which the two half cells are the chemically identical except for a difference in reactant activities (concentrations) this is called a concentration cell. 

Because cell potentials depend on concentration, we can construct galvanic cells in which both compartments contain the same components but at different concentrations. For example, in the cell in Fig. 11.11 both compartments contain aqueous AgNC>3, but with different molarities. Let s consider the potential of this cell and the direction of electron flow. The half-reaction relevant to both compartments of this cell is... 

The standard state cell potential is simply the sum of the standard state potentials of the corresponding half reactions. The cell potential for a galvanic cell is always positive a galvanic cell always has chemical energy that can be converted to work. The real cell potential depends upon the half reactions, the concentrations of the reactants and products, and the temperature. 

Galvanic Cells. In the initial experiments with electric cells, the measured values depend on the concentration and electrode distances and are therefore not reproducible. It is therefore advisable to measure the electrical voltage between two standardized half-cells (see Fig. 8.4). In using solutions of salts which are 1 molar in concentration of the metal ions, one speaks of standard conditions in the related example (see Fig. 8.4) the standard voltage of 1.1 V (see E8.7) can be measured. 

So far we have only looked at galvanic cells under standard conditions. Nevertheless cell potentials depend on the concentration of the ions that are in the half cells. E.g. the following overall cell reaction ... 

Further we looked at galvanic cells where it was possible to extract electrical energy from chemical reactions. We looked into cell potentials and standard reduction potentials which are both central and necessary for the electrochemical calculations. We also looked at concentration dependence of cell potentials and introduced the Nemst-equation stating the combination of the reaction fraction and cell potentials. The use of the Nemst equation was presented through examples where er also saw how the equation may be used to determine equilibrium constants. 

During his Leipzig period, Nernst performed a series of electrochemical studies from which, at the age of twenty-five, he arrived at his well-known equations. These equations described the concentration dependence of the potential difference of galvanic cells, such as batteries, and were of both great theoretical and practical importance. Nernst started with the investigation of the diffusion of electrolytes in one solution. Then he turned to the diffusion at the boundary between two solutions with different electrolyte concentrations he determined that the osmotic pressure difference would result in an electric potential difference or electromotive force (emf). Next he divided both solutions into two concentration half-cells, connected to each other by a liquid junction, and measured the emf via electrodes dipped into both solutions. The data supported his first equation where the... 

When predicting the products of galvanic cells, Sanger Greenbowe (1997) reported several conceptions about concentration of the cells, for instance, the idea that the direction of electron flow is not dependent on the relative concentration of the ions. When predicting potential differences of these cells, many students held the conception of independency of the value of the potential difference on the relative concentration of the ions. 

Oxygen control. In order to make measurements of oxygen activity in sodium the electrochemical control technique based on galvanic cell has been mastered with sodium flowing over electrolytic pellet of thorium and yttrium sealed into the metal tube. Reference electrode is located inside the tube. E.m. F. generated depends on the temperature and oxygen concentration e.m. F. = f (T, Co2). Service life of such device is over 10" hours. 

Electromotive force (emf) measurements are frequently used to determine activity coefficients of electrolyte solutions. Equation (136a) relates the emf to the activities of the reacting cell components. From concentration-dependent measurements the standard potential E° of the cell reaction and the activity coefficients can be obtained. As an example, according to Eq. (136a), the emf of the Galvanic cell... 

Because electrode potential depends on ion concentrations, it is possible to constmct a galvanic cell from two half-cells composed of the same material but differing in ion concentrations. Such a cell is called a concentration cell. 

The potential difference between the two electrodes in a galvanic cell is called the electromotive force, or emf, of the cell. The emf of the cell in Fig. 16.1 depends on the temperature and the concentrations of the ZnS04 and CUSO4 solutions. Let us assume that the emf is 1.10 V. This means that when 1 mole of zinc and 1 mole of Cu are consumed, and 2 faradays flow, through the circuit, the work this electricity will do is... 

From the equation it is apparent that when Vq < Fg, / will reverse its sign, i.e. cell E will operate as a galvanic and cell G as an electrolytic cell. The dependence of cell voltage on concentration, i.e. the conditions under which Vo < Ve, are given by the Nernst equation, which will be dealt with later in this chapter. 

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