Halfcells with solutions

Instead of pure solids or liquids, one or more of the constituents may be in solution with a different substance. This nonstandard condition can prevail in one or both half-cells. Dilution of an reactive component reduces its activity and consequently affects the cell EMF. To illustrate this effect consider the cell of (10.4) modified to permit species X to be dissolved in a solvent metal Z. This cell is described by:

Using Eq (10.7) with the activities of all species except X equal to unity, Eq (10.6) gives:

AG = ^YO X XO y = gYO + (gX + RTlnaX) - gXo - gY = AGo + RTlnaX (10.10)

where AGo is the standard state free energy change of the overall cell reaction and aX is the activity of component X in the X-Z alloy. Substituting Eq (10.10) into Eq (10.5a) gives the cell EMF:

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