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The equilibrium constant for the maximum quantity of solid which can be dissolved in aqueous solution is called the solubility product.
The equilibrium constant for a solid which dissolves in aqueous solution is known as the solubility product constant (Ksp). All of the rules for calculating equilibrium constants are still in effect. Once a reaction has reached equilibrium, an equilibrium constant is defined as the ratio of a concentration of the products divided by that of the concentration of the reactants. Take the following example:
AgCl(s) → Ag+(aq)+Cl–(aq)
The equilibrium expression for the reaction is as follow:
Keq = [Ag+][Cl+]/[AgCl]
Because AgCl is now a solid, its concentration is the same and during the reaction. As a result, the equilibrium equation may be written as Ksp = [Ag+][Cl-]
Thus stoichiometric coefficients of a reaction now become exponents for ions in the solubility-product for substances where the ions weren’t in a 1:1 ratio.
Molar Solubility
The ratio of equilibrium concentrations is known as the solubility product (Ksp) of a chemical. The number of moles of a solute that may be dissolved per litre of solution before the solution becomes saturated is known as molar solubility, which is directly connected to the solubility product. Any additional solute precipitates out of a solution once it becomes saturated. Molarity (M) and mole liter-1 (mol/L) are the units of measurement.
Relative Solubility
It is possible to segregate substances with differing solubilities, or relative solubilities. In the extraction process, for example, scientists take something dissolved in one liquid and compel it to dissolve in another. To be used in beverages like soda, caffeine must first be extracted from coffee beans. Caffeine is usually dissolved in liquid carbon dioxide that has been heated to above 300 K and compressed to 73 atm. The temperature is then decreased (reducing the caffeine’s solubility in carbon dioxide) and water is added. The system is allowed to get to a state of equilibrium. Caffeine is more soluble in water than in carbon dioxide, hence the majority of it is absorbed by the water.
The relative solubilities of substances are also used in a process known as paper chromatography. A tiny amount of the mixture is placed on the paper, about 1 cm from the edge, for paper chromatography. The paper is then placed in a tight container and suspended in a little amount of the solvent. The components in the mixture separate based on relative solubility as the solvent rises up the paper. A mark is placed on the paper to record the level when the solvent approaches the top, then the paper is removed and dried. Some components are coloured and visible to the naked eye, while others require staining or UV exposure. As long as the temperature remains constant, the solute will always travel at the same proportion of the distance as the solvent. The solute’s travel distance in a given solvent can be utilized to determine the compound’s identity.
Relating Solubility and Ksp
When it comes to characterising the solubility of mildly ionic substances, the relationship between solubility and Ksp is crucial. I bring up ionic compounds because unlike molecular molecules, most ionic compounds are soluble in water or other solvents.
In this case, though, we’re talking about ionic compounds that are difficult to dissolve and are referred to as mildly soluble or almost insoluble. As a result, those solutes are assigned solubility product constants (Ksp), and we may calculate the molar solubility of the molecules that make up the solute using these constants. We can also use this relationship to calculate the Ksp of a slightly soluble solute using its solubility.
Solubility Product Constant, Ksp
The equilibrium constant for a solid material dissolving in an aqueous solution is known as the solubility product constant, or Ksp. It’s the point at which a solute dissolves in water. A substance’s Ksp value increases as it becomes more soluble.
Consider the following (in aqueous solutions) dissolving reaction:
aA(s)↽−−⇀cC(aq)+dD(aq)
To find the Ksp, multiply the molarities (cC and dD) of the products. If any of the products have coefficients before them, they must be raised to that coefficient power (and also multiply that concentration by that coefficient). Here’s an example:
Ksp=[C]c[D]d
It’s worth noting that the Ksp equation does not contain the reactant, aA. Solids are removed from equilibrium constant statements because their concentrations do not modify the equation; any change in their concentrations is therefore inconsequential. As a result, Ksp stands for the maximum extent to which a solid can dissolve in water.
Factors Affecting Solubility
Solubility and Temperature
Solubility of solids, liquids, and gases is affected differentially by temperature variations. But only for solids and gases are these effects finitely specified.
Gas Solubility is Affected by Pressure
Only the solubility of gases in liquids is affected by pressure.
Solids and Liquids: Pressure fluctuations have a modest influence on the solubility of solids and liquids.
Gases: Using a combination of Henry’s law and the Le Chatelier principle, the effects of pressure mostly on solubility of gases in liquids can be best characterised. When temperature remains constant, Henry’s law states that a gas’s solubility corresponds to its partial pressure.
Conclusion
Instead of concentrations reported in marginally soluble solutions, ionic activities must be established for highly soluble ionic substances.
Common Ion Effect: The common ion reduces the reaction’s solubility. A reaction with such a common ion present has a lower Ksp for a given equilibrium, while a reaction without the ion has a higher Ksp.
Differential ion impact (salt effect): Uncommon ions have the opposite effect on the Ksp value as common ions do. Uncommon ions raise the Ksp value. Ions that aren’t in equilibrium are known as uncommon ions.
Due to the ions involved in pairing, the Ksp value computed for an ionic pair (a cation and an anion) is less than the experimental value.
