Cell Potential on Concentration

So far we have described cells under standard conditions. In this section we consider the dependence of the cell potential on concentration. Under standard conditions (all concentrations 1 M), the cell with the reaction 

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Because cell potentials depend on concentration, we can construct galvanic cells where both compartments contain the same components but at different concentrations. For example, in the cell in Fig. 18.9, both compartments contain aqueous AgNOj, 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 

Flowever, in the cell described here, the concentrations of Ag in the two compartments are 1 Mand 0.1 M. Because the concentrations of Ag are unequal, the half-cell potentials will not be identical, and the cell will exhibit a positive voltage. In which direction will the electrons flow in this cell The best way to think about this question is to recognize that nature will try to equalize the concentrations of Ag in the two compartments. This can be done by transferring electrons from the compartment containing 0.1 MAg to the one containing 1 MAg (left to right in Fig. 18.9). This electron transfer will produce more Ag in the left compartment and consume Ag (to form Ag) in the right compartment. 

A cell in which both compartments have the same components but at different concentrations is called a concentration cell. The difference in concentration is the only factor that produces a cell potential in this case, and the voltages are typically small. 

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The Nemst equation describes the dependence of the half-cell potential on concentration ... 

The dependence of the cell potential on concentration results directly from the dependence of free energy on concentration. Recall from Chapter 10 that the equation... 

Thinking it Through The Nemst equation expresses the dependence of cell potential on concentration of the cell components. The equation itself may be given in the problem, or may be found in a general table of useful equations in other ACS exams. There may also be questions where you are expected to know the relationship, or derive it from changes in free energy. The value of Q is defined in the same manner as an equilibrium constant, but remember that these cell concentrations are not standard state concentrations.. For this problem, Q is evaluated by this expression. 

Dependence of Cell Potential on Concentration Concentration Cells... 

The standard values of free energy and also the redox potentials refer to standard concentrations of oxidized and reduced reactants, including the concentration (more precisely, the activity) of H = 1, pH = 0. The dependency of half-cell potentials on concentration is given by the following expression ... 

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