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The solubility product constant (Ksp) is a simplified equilibrium constant that is specified for the equilibrium between a solid and its respective ions in a given solution. It is symbolised by the letters Ksp. Its value indicates the extent to which a chemical can dissociate when exposed to water. The bigger the value of the solubility product constant, the more soluble the substance is considered. It is the product of the concentrations of the ions that determines the Ksp expression for a given salt. To obtain the solubility equilibrium, each concentration is raised to a power that is equal to the coefficient of that ion in a balanced equation. Because of their low solubility, the solubility product constants of ionic compounds are employed to describe the saturated solutions of these compounds. An undissolved solid is present in a saturated solution, and the dissolved and dissociated ionic compounds are in a dynamic equilibrium condition with the undissolved solid.
Solubility Product Constant, Ksp
Solubility product constant (Ksp) is the equilibrium constant for a solid material dissolving in an aqueous solution when it is dissolved in water. It reflects the concentration at which a solute dissolves in a liquid solution. The higher the Ksp value of a compound, the more soluble the substance is in water.
The following is an example of a generic dissolving reaction (in aqueous solutions):
aA(s)⇌cC(aq)+dD(aq)
In order to find the Ksp, it is essential to multiply the molarities or concentrations of the products (cC and dD) by a constant. The product must be raised to the coefficient power if any of the products have coefficients in front of them; otherwise, the product will fail (and also multiply the concentration by that coefficient). This is illustrated in the following way:
Ksp=[C]c[D]d
It is important to note that the reactant, aA, is not included in the Ksp formula. It is not necessary to add solids when calculating equilibrium constant expressions since their concentrations have no effect on the expression; any change in their concentrations is negligible, and as a result, they are not included. As a result, Ksp denotes the maximum extent to which a solid can be dissolved in solution.
Applications of Solubility product-
I) Salting out of soap –
The salting out of soap is one of the most important applications for the Solubility product. As the name implies, soap is the sodium salt of a very fatty acidic compound. Adding concentrated solution of NaCl (sodium chloride) causes it to precipitate out of the solution.
C15H31COONa → C15H31COO – + Na+
sodium palmitate(soap)
NaCl → Na+ + Cl-
NaCl is a very strong electrolyte; it ionises entirely in solution, resulting in an increase in the concentration of sodium ions (Na+).
The ionic product of [C15H31COO-] [Na+] is greater than the Ksp of [C15H31COO-] [Na+] (soap)
The soap precipitates out of the solution as a result.
ii) Purification of common salt-
Natural common salt comprises contaminants that are both soluble and insoluble. The insoluble impurities are filtered out of a saturated solution of common salt after it has been prepared and filtered.
A stream of HCl gas is fed through a saturated solution of NaCl.
NaCl → Na+ + Cl-
HCl → H+ + Cl-
Both result in the formation of chloride ions ( Cl-) in solution, resulting in an increase in the concentration of Cl-. Hence,
The ionic product of [Na+] [Cl-] is greater than Ksp of NaCl.
As a result, pure sodium chloride precipitates out of the solution.
iii) Precipitation of IIIrd group metal hydroxides –
In the presence of ammonium chloride, the IIIrd group metal hydroxides [Al(OH)3, Fe(OH)3 , and Cr(OH)3] precipitate with ammonium hydroxide (NH4OH) and ammonium chloride (NH4Cl).
NH4Cl NH4+ + Cl-
NH4OH NH4+ + OH-.
This is because, in the presence of NH4Cl (which is a strong electrolyte and produces NH4+), dissociation of the weak electrolyte NH4OH diminishes due to the common ion effect, which is due to the presence of a strong electrolyte. As a result, the concentration of [OH-] must be decreased. Thus
IIIrd group metal hydroxides have a higher Ksp than the ionic product of [M+++] [OH-]3.
As a result, only metal hydroxides of the IIIrd group precipitate in the IIIrd group.
Solubility Product Formula
It is necessary to utilize the solubility product constant to characterize saturated solutions of ionic substances that have a poor solubility. Described as being in a condition of dynamic equilibrium between the ionic chemical and the undissolved solid, the saturated solution has reached its maximum concentration. It is possible to express the Ksp formula in terms of the following equation:
MxAy(s)→xMy++ (aq) + yAx- (aq)
The general equilibrium constant,Kc, can be expressed as [My+]x[Ax-]y
Significance of the Solubility Product
Many factors influence solubility, including the lattice enthalpy of salt and the solvation enthalpy of ions in the solution, which are both measured experimentally. The importance of these two aspects cannot be overstated. Take a closer look at the solubility product to understand its significance in greater detail.
When a salt is dissolved in a solvent, the strong forces of attraction of the solute, which are represented by the lattice enthalpy of the salt’s ions, must be overcome by the interactions between the ions and the solvent in order for the salt to be dissolved.
This means that energy is released during the process, as long as the solvation enthalpy of ions remains negative at all times.
Because of the nature of the solvent, the amount of energy released during the solvation process, also known as solvation enthalpy, can be determined.
When it comes to nonpolar solvents, the solvation enthalpy is modest, which indicates that this energy is insufficient to overcome the lattice enthalpy of the solvation process.
As a result, the salts do not dissolve in the non-polar solvents used. Consequently, for salt to dissolve in a solvent, the solvation enthalpy of the salt must be larger than the lattice enthalpy of the solvent.
The solubility of a salt is determined by its temperature, and it varies depending on the salt.
Conclusion
When investigating the solubility of distinct solutes, the solubility product constant, also known as Ksp, is an important component of chemistry to consider. The chemistry Ksp value reflects how much of a solute will dissolve in solution, and the more soluble a material is, the higher the chemical Ksp value for that substance. The salting out of soap is one of the most important applications for the Solubility product. As the name implies, soap is the sodium salt of a very fatty acidic compound. Adding concentrated solution of NaCl (sodium chloride) causes it to precipitate out of the solution.
