A The Effect of Electrolytes on Chemical Equilibria

The concentration-based equilibrium constant embodied in Equation 9-7 on page 234 provides only an approximation to real laboratory measurements. In this chapter, we show how the approximate form of the equilibrium constant often leads to significant error. We explore the difference between the activity of a solute and its concentration, calculate activity coefficients, and use them to modify the approximate expression to compute species concentrations that more closely match real laboratory systems at chemical equilibrium. 

Experimentally, we find that the position of most solution equilibria depends on the electrolyte concentration of the medium, even when the added electrolyte contains no ion in common with those involved in the equilibrium. For example, consider again the oxidation of iodide ion by arsenic acid that we described in Section 9B-1  

If an electrolyte, such as barium nitrate, potassium sulfate, or sodium perchlorate, is added to this solution, the color of the triiodide ion becomes less intense. This 

Curve C is a plot of K (XIO ), the concentration quotient for the equilibrium involving the dissociation of acetic acid, as a function of electrolyte concentration. Here again, the ordinate function approaches a limiting value A, which is the thermodynamic acid dissociation constant for acetic acid. 

we consider how we can take the electrolyte effect into account when we wish to make more accurate equilibrium calculations. 

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