Ksp(AgCl) = [Ag+] [Cl-]
The solubility product of AgCl is the concentration of soluble silver ions at equilibrium multiplied by the concentration of soluble chloride ions at equilibrium. Students should note the difference between solubility product and ionic product. Ionic product is simply the product of concentration of ions at a given time, i.e the concentration of ions may not be at equilibrium.
Note that the solubility product concept applies to only sparingly soluble ionic compounds, i.e. it cannot be used for soluble compunds like NaCl. The solubility product is a modified equilibrium constant hence it is affected by temperature.
Common ion effect
The solubility of a sparing soluble ionic compound AB can be reduced by the presence of A+ or B- from a second source. For example we can compare the solubility of AgCl in water and in NaCl solution. The common ion is the chloride ion. Intuitively we know that AgCl is more soluble in water than in NaCl solution. Why is this so? Consider the solubility equilibria of AgCl:
In a solution of pure water, there are no chloride ions, so AgCl dissolves until the ionic product equals to Ksp. In a solution of NaCl, less AgCl can dissolve, as the ionic product will reach Ksp sooner as there are chloride ions already present in solution.
Its quite hard to explain in words but in the A level examinations, students will be asked to do some calculations. Based on the calculations, the effect of the common ion will be more apparent.
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