The Effect of Concentration on Cell Emf

Check The lai]ge positive value of AG° teUs us that the reaction favors the reactants at equilibrium. The result is consistent with the fact that E° for the galvanic cell is negative. 

So far we have focused on redox reactions in which reactants and products are in their standard states, but standard-state conditions are often difficult, and sometimes impossible, to maintain. Nevertheless, there is a mathematical relationship between the emf of a galvanic cell and the concentration of reactants and products in a redox reaction under nonstandard-state conditions. This eqnation is derived next. 

During the operation of a galvanic cell, electrons flow from the anode to the cathode, resulting in product formation and a decrease in reactant concentration. Thus, Q increases, which means that E decreases. Eventually, the cell reaches equilibrium. At equilibrium, there is no net transfer of electrons, so E = 0 and Q = K, where K is the equilibrium constant. 

Remember that concentrations of pure solids (and pure liquids) do not appear in the expression for Q (see p. SOS). 

Predict whether the following reaction would proceed spontaneously as written at 298 K  

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The effect of concentration on cell emf can be obtained from the effect of concentration on free-energy change. (Section 19.7) Recall that the free-energy change for any chemical reaction, A G, is related to the standard free-energy change for the reaction, A G° ... 

Having introduced matters pertaining to the electrochemical series earlier, it is only relevant that an appraisal is given on some of its applications. The coverage hereunder describes different examples which include aspects of spontaneity of a galvanic cell reaction, feasibility of different species for reaction, criterion of choice of electrodes to form galvanic cells, sacrificial protection, cementation, concentration and tempera lure effects on emf of electrochemical cells, clues on chemical reaction, caution notes on the use of electrochemical series, and finally determination of equilibrium constants and solubility products. 

Taking another example to illustrate the dependency of emf on concentration (temperature effect is not considered, as in the previous example), reference may be drawn to the familiar Cu-Zn cell a shown in standard notation as ... 

If the Nernst equation is applied to more concentrated solutions, the terms in the reaction quotient Q must be expressed in effective concentrations or activities of the electroactive ionic species. The activity coefficient y (gamma) relates the concentration of an ion to its activity a in a given solution through the relation a = yc Since electrode potentials measure activities directly, activity coefficients can be determined by carrying out appropriate EMF measurements on cells in which the concentration of the ion of interest is known. The resulting Es can then be used to convert concentrations into activities for use in other calculations involving equilibrium constants. 

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