Dissociation (chemistry)

Dissociation in chemistry and biochemistry is a general process in which ionic compounds (complexes, molecules, or salts) separate or split into smaller molecules, ions, or radicals, usually in a reversible manner. When a Bronsted-Lowry acid is put in water, a covalent bond between an electronegative atom and a hydrogen atom is broken by heterolytic fission, which gives a proton and a negative ion. Dissociation is the opposite of association and recombination. The process is frequently confused with ionization.

Dissociation constant
For reversible dissociations in a chemical equilibrium
 * $$ AB \leftrightarrow a + b $$

the dissociation constant Kd is the ratio of dissociated to undissociated compound
 * $$K_d = \frac{[a][b]}{[ab]}$$

Salts
The dissociation of salts by solvation in a solution like water means the separation of the anions and cations. The salt can be recovered by evaporation of the solvent. See also: Solubility equilibrium

Acids
The dissociation of acids in a solution means the split-off of a proton H+, see Acid-base reaction theories. This is an equilibrium process, meaning that dissociation and recombination takes place at the same time. The acid dissociation constant Ka is an indicator of the acid strength: stronger acids have a higher Ka value (and a lower pKa value).

Fragmentation
Fragmentation of a molecule can take place by a process of heterolysis or homolysis

Receptors
Receptors are proteins that bind small ligands. The dissociation constant Kd is used as indicator of the affinity of the ligand to the receptor. The higher the affinity of the ligand for the receptor the lower the Kd value (and the higher the pKd value).