Galvanic cells, activity chemical reaction

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

Potentiometry is used in the determination of various physicochemical quantities and for quantitative analysis based on measurements of the EMF of galvanic cells. By means of the potentiometric method it is possible to determine activity coefficients, pH values, dissociation constants and solubility products, the standard affinities of chemical reactions, in simple cases transport numbers, etc. In analytical chemistry, potentiometry is used for titrations or for direct determination of ion activities. 

Galvanic Cells. When you place a piece of zinc metal into a solution of CuSO, you expect a chemical reaction because the more active zinc displaces the less active copper from its compound. This is a redox reaction, involving transfer of electrons from zinc to copper. 

One or more electrochemical cells connected in series constitute an electrical battery . Primary electrochemical (galvanic) cells are ready to produce current immediately and do not need to be charged in the way secondary cells (described below) do. In disposable cells, the chemical half reactions are not easily reversible, so the cells cannot be reliably recharged. Common disposable cells include the zinc-carbon cells and the alkaline cells. Secondary electrochemical cells contain the active materials in the disclWged state, so they must be charged before use. The oldest form of rechargeable cell is the lead-acid battery. 

The general conclusion to be drawn from this specific examples is that solid state galvanic cells with solid electrolytes can be used primarily to measure free energies of reactions. From this, it is often possible to deduce the difference in chemical potentials (or the ratios of activities) of components of the participating phases. 

Suppose we have a galvanic cell in a parricular zero-current equilibrium state. Each phase of the cell has the same temperature and pressure and a well-defined chemical composition. The activity of each reactant and product of the cell reaction therefore has a definite value in this state. 

In principle the activity coefficients yb of solute substances B in a solution can be directly determined from the results of measurements at ven temperature of the pressure and the compositions of the liquid (or solid) solution and of the coexisting gas phase. In practice, this method fails unless the solutes have volatilities comparable with that of the solvent. The method therefore usually fails for electrolyte solutions, for which measurements of ye in practice, much more important than for nonelectrolyte solutions. Three practical methods are available. If the osmotic coefficient of the solvent has been measured over a sufficient range of molalities, the activity coefficients /b can be calculated the method is outlined below under the sub-heading Solvent. The ratio yj/ys of the activity coefficients of a solute B in two solutions, each saturated with respect to solid B in the same solvent but with different molalities of other solutes, is equal to the ratio m lm of the molalities (solubilities expressed as molalities) of B in the saturated solutions. If a justifiable extrapolation to Ssms 0 can be made, then the separate ys s can be found. The method is especially useful when B is a sparingly soluble salt and the solubility is measured in the presence of varying molalities of other more soluble salts. Finally, the activity coefficient of an electrolyte can sometimes be obtained from e.m.f. measurements on galvanic cells. The measurement of activity coefficients and analysis of the results both for solutions of a single electrolyte and for solutions of two or more electrolytes will be dealt with in a subsequent volume. Unfortunately, few activity coefficients have been measured in the usually multi-solute solutions relevant to chemical reactions in solution. 

Fuel Cell A galvanic cell in which the active materials are continuously supplied from a source external to the cell and the reaction products continuously removed converting chemical energy to electrical energy. 

It is characteristic of the electrochemical cell that passage of electric current in the cell is coupled with the occurrence of a chemical reaction. During such reaction, an electrochemical cell can have a passive function as an electrolytic cell or an active function as a galvanic cell the significance of this distinction is as follows ... 

Batteries are galvanic cells whereby the chemical energy of the components of the cathode (positive terminal) and the anode (negative terminal) is converted to electrical energy via the cell reaction. Fuel cells are galvanic cells in which the active components of the two electrodes are continuously replenished and the products of the cell reaction are continuously removed. 

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