The solubility product constant is the equilibrium constant for the dissolution of a solid substance into an aqueous solution. It is denoted by the symbol Ksp.
The solubility product is a kind of equilibrium constant and its value depends on temperature. Ksp usually increases with an increase in temperature due to increased solubility.
Solubility is defined as a property of a substance called solute to get dissolved in a solvent in order to form a solution. The solubility of ionic compounds (which disassociate to form cations and anions) in water varies to a great deal. Some compounds are highly soluble and may even absorb moisture from the atmosphere whereas others are highly insoluble.
Solubility depends on a number of parameters amongst which lattice enthalpy of salt and solvation enthalpy of ions in the solution are of most importance.
Salts are classified on the basis of their solubility in the following table:
Suppose barium sulphate along with its saturated aqueous solution is taken.
The following equation represents the equilibrium set up between the undissolved solids and ions:
The equilibrium constant in the above case is:
In case of pure solid substances the concentration remains constant, and so we can say:
Here Ksp is known as the solubility product constant. This further tells us that solid barium sulphate when in equilibrium with its saturated solution, the product of concentrations of ions of both barium and sulphate is equal to the solubility product constant.
Some important factors that have an impact on the solubility product constant are:
The common ion effect is an effect that suppresses the ionization of an electrolyte when another electrolyte (which contains an ion which is also present in the first electrolyte, i.e. a common ion) is added. It is considered to be a consequence of Le Chatlier’s principle (or the Equilibrium Law).
The statement of the common ion effect can be written as follows – in a solution wherein there are several species associating with each other via a chemical equilibrium process, an increase in the concentration of one of the ions dissociated in the solution by the addition of another species containing the same ion will lead to an increase in the degree of association of ions.
An example of the common ion effect can be observed when gaseous hydrogen chloride is passed through a sodium chloride solution, leading to the precipitation of the NaCl due to the excess of chloride ions in the solution (brought on by the dissociation of HCl).
This effect cannot be observed in the compounds of transition metals. This is because the d-block elements have a tendency to form complex ions. This can be observed in the compound cuprous chloride, which is insoluble in water. This compound can be dissolved in water by the addition of chloride ions leading to the formation of the CuCl2– complex ion, which is soluble in water.
The way in which the solubility of a salt in a solution is affected by the addition of a common ion is discussed in this subsection.
However, it can be noted that water containing a respectable amount of Na+ ions, such as seawater and brackish water, can hinder the action of soaps by reducing their solubility and therefore their effectiveness.
When the conjugate ion of a buffer solution (solution containing a base and its conjugate acid, or acid and its conjugate base) is added to it, the pH of the buffer solution changes due to the common ion effect.
Thus, the common ion effect, its effect on the solubility of a salt in a solution, and its effect on the pH of a solution.
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1. What is solubility equilibrium? |
2. How does temperature affect the solubility equilibrium of sparingly soluble salts? |
3. What factors determine the solubility of sparingly soluble salts? |
4. How can we calculate the solubility product constant (Ksp) for sparingly soluble salts? |
5. What are some common applications of solubility equilibria for sparingly soluble salts? |
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