Common-Ion Effect in Acid-Base Equilibria

This behaviour is very similar to the common ion effect in acid-base equilibria where the addition of a strong acid to a weak acid will suppress the dissociation of the weak acid. Here addition of a common ion will suppress the solubility of a sparingly soluble salt. 

Words that can be used as topics in essays 5% rale buffer common ion effect equilibrium expression equivalence point Henderson-Hasselbalch equation heterogeneous equilibria homogeneous equilibria indicator ion product, P Ka Kb Kc Keq KP Ksp Kw law of mass action Le Chatelier s principle limiting reactant method of successive approximation net ionic equation percent dissociation pH P Ka P Kb pOH reaction quotient, Q reciprocal rule rule of multiple equilibria solubility spectator ions strong acid strong base van t Hoff equation weak acid weak base... 

The common-ion effect is an application of Le Chatelicr s principle to equilibrium systems of slightly soluble salts. A buffer is a solution that resists a change in pH if we add an acid or base. We can calculate the pH of a buffer using the Henderson-Hasselbalch equation. We use titrations to determine the concentration of an acid or base solution. We can represent solubility equilibria by the solubility product constant expression, Ksp. We can use the concepts associated with weak acids and bases to calculate the pH at any point during a titration. 

Weak acid and its salt, or weak base and its salt (see p 356). Here the common-ion effect is involved the solutions are buffers. The equilibria involved are the same as in type 4 or 5 above, except that for tljff j-idfg Juljgp... 

You will see this topic appear twice, once in this chapter and once the next chapter. For now, you will see how this phenomenon affects acid-base equilibria. In the next chapter, you will see its effects on solubility equilibria. The common-ion effect is not too different from what its name suggests. If you have an equilibrium system and add a solute to it that contains one of the ions in the equilibrium, it will cause the equilibrium to shift. That is the common-ion effect (common because the solute has an ion in common with the equilibrium system). From a conceptual standpoint, this can be addressed using Le Chatelier s Principle. For example, consider our favorite equilibrium system below ... 

SECTION 17.1 In this chapter we have considered several types of important equilibria that occur in aqueous solution. Our primary emphasis has been on acid base equilibria in solutions containing two or more solutes and on solubility equilibria. The dissociation of a weak acid or weak base is repressed by the presence of a strong electrolyte that provides an ion common to the equilibrium. This phenomenon is called the common-ion effect. 

The solubility may differ considerably from the real value when it is calculated as we ve shown through the solubility products. This is the case when supplementary equilibria involving the ionic species figuring in the solubility product are effective. These equilibria may be of several origins. For example, they may be acid-base, redox, or other precipitation equilibria. (In the enumeration of these supplementary equilibria, we have followed the systematic presentation of this book.) Common ion effects may also add themselves to the equilibrium under study. Of course, ionic strength effects may also occur. All these effects are controllable (see the following chapters). 

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