Question

What is the difference between Ksp(solubility porduct) and Keq(equilibrium constant)? What are the definitions for both?

Answer 1

Solubility Product Constant, Ksp

The solubility product constant, Ksp?, is the equilibrium constant for a solid substance dissolving in an aqueous solution. It represents the level at which a solute dissolves in solution. The more soluble a substance is, the higher the Ksp value it has.

Consider the general dissolution reaction below:

aA(s)?cC(aq)+dD(aq)

To solve for the Ksp it is necessary to take the molarities or concentrations of the products (cC and dD) and multiply them. If there are coefficients in front of any of the products, it is necessary to raise the product to that coefficient power(and also multiply the concentration by that coefficient). This is shown below:

Ksp=[C]c[D]d

Note that the reactant, aA, is not included in the Ksp equation. Solids are not included when calculating equilibrium constant expressions, because their concentrations do not change the expression; any change in their concentrations are insignificant, and therefore omitted.

Hence, Ksp represents the maximum amount of solid that can be dissolved in the aqueous solution. The answer will have the units of molarity, mol L-1, a measure of concentration.

equilibrium constant

A reaction's equilibrium constant, K_eq, measures the extent to which reactants are converted to products by the reaction. Equilibrium constants are found by multiplying the concentrations of the reaction's products raised to the power of their stoichiometric coefficients divided by the multiplication product of the concentrations of its reactants raised to the power of their stoichiometric coefficients.