In chemistry, the dissociation constant, often denoted as Kw or Kdiss, is a fundamental concept used to describe the extent of dissociation of a chemical compound into its constituent ions in a solution. It is a specific type of equilibrium constant that applies to reactions involving the dissociation of an acid or a base in aqueous solutions.
Dissociation of Acids: The dissociation constant for an acid, known as the acid dissociation constant (Ka), quantifies the degree to which an acid donates a proton (H+) in a solution. The general dissociation reaction for an acid, HA, is as follows:
HA (acid) ⇌ H+ (hydrogen ion) + A- (conjugate base)
The expression for the acid dissociation constant is defined as:
Ka = [H+][A-] / [HA]
Where [H+] represents the concentration of hydrogen ions, [A-] represents the concentration of the conjugate base, and [HA] represents the concentration of the undissociated acid. A larger Ka value indicates a stronger acid that dissociates more in the solution.
Dissociation of Bases: The dissociation constant for a base, known as the base dissociation constant (Kb), quantifies the degree to which a base accepts a proton (H+) in a solution. The general dissociation reaction for a base, B, is as follows:
B (base) + H2O ⇌ BH+ (conjugate acid) + OH- (hydroxide ion)
The expression for the base dissociation constant is defined as:
Kb = [BH+][OH-] / [B]
Where [BH+] represents the concentration of the conjugate acid, [OH-] represents the concentration of hydroxide ions, and [B] represents the concentration of the undissociated base. A larger Kb value indicates a stronger base that dissociates more in the solution.
Dissociation of Water: Water itself undergoes dissociation to form hydrogen ions (H+) and hydroxide ions (OH-):
H2O (water) ⇌ H+ (hydrogen ion) + OH- (hydroxide ion)
The equilibrium constant for this dissociation reaction is known as the dissociation constant of water (Kw). At room temperature, the value of Kw