Ksp stands for solubility product constant while Keq stands for equilibrium constant. Ksp is also a type of equilibrium constant, but it deals only with the solubility of solid substances. Keq is a more general term we can use to determine the properties of any type of equilibrium.
What is Ksp
Ksp stands for solubility product constant. It is applicable to the dissolution of solid substances in aqueous solutions. This constant describes the level at which a chemical substance dissolves in the aqueous solution. Higher the solubility, the higher the value of Ksp. For a general solubility reaction, we can give the equation for Ksp as follows:
Therefore, the solubility product constant comes from the multiplication of the molar concentrations of products we get from the dissolution of the solid substance. However, if there is a stoichiometric relationship between reactants and product, we must include the stoichiometric coefficient in our equation. It is necessary to raise the concentration of the product to the coefficient power.
Common Ion Effect:
We should always remember that the solubility of the equilibrium reaction is reduced by the common ion effect. For example, if there is a common ion in the solution and the solid compound we are going to dissolve in that solution, we can observe a lower Ksp than that of expected. Without that ion, the Ksp would be a large value.
The presence of uncommon ions in the solution can also affect the Ksp of an equilibrium. For example, if there is a salt ion in the solution in addition to the ions present in the solid, then we call it an uncommon ion, and it can increase the value of Ksp.
What is Keq?
Keq stands for equilibrium constant. The equilibrium constant is the ratio between the concentrations of products and the concentrations of reactants at equilibrium. This term is only applicable to reactions that are in equilibrium. The reaction quotient and equilibrium constant are the same for reactions that are in equilibrium.
We can give the equilibrium constant as the concentrations raised to the power of stoichiometric coefficients. The equilibrium constant is dependent on the temperature of the system considered since the temperature affects the solubility of components and the volume expansion. However, the equation for the equilibrium constant does not include any details about solids that are among the reactants or the products. Only substances in the liquid phase and gaseous phase are considered.
For example, let us consider the equilibrium between carbonic acid and bicarbonate ion.
H2CO3 (aq) ↔ HCO3–(aq) + H+ (aq)
The equilibrium constant for the above reaction is given below.
Equilibrium Constant (K) = [HCO3–(aq)] [H+ (aq)] / [H2CO3 (aq)]
What is the Difference Between Ksp and Keq?
Ksp is a type of Keq. Ksp stands for solubility product constant while Keq stands for equilibrium constant. The key difference between Ksp and Keq is that the term Ksp describes the solubility of a substance, whereas the term Keq describes the equilibrium state of a particular reaction. When considering the equations, Ksp only has the products we get after the dissolution of the solid while Keq contains both products and reactants which are in an aqueous state. So, this is another significant difference between Ksp and Keq.
Summary – Ksp vs Keq
Ksp is a type of Keq. Ksp stands for solubility product constant while Keq stands for equilibrium constant. The key difference between Ksp and Keq is that the term Ksp describes the solubility of a substance, whereas the term Keq describes the equilibrium state of a particular reaction.
1. “Solubility Product Constant, Ksp.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.