Dissociation (chemistry)

For other uses, see dissociation (psychology) and dissociation (neuropsychology).

Dissociation in chemistry and biochemistry is a general process in which ionic compounds (complexes, or salts) separate or split into smaller particles, ions, or radicals, usually in a reversible manner. For instance, 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 A + B

the dissociation constant K_d is the ratio of dissociated to undissociated compound

$K_d = \mathrm{\frac{[A][B]}{[AB]}}$

where the brackets denote the equilibrium concentrations of the species.

Dissociation degree

The dissociation degree is the fraction of original solute molecules that have dissociated. It is usually indicated by the Greek symbol α. There is a simple relationship between this parameter and the van 't Hoff factor i. If the solute substance dissociates into n ions, then

i = 1 + α(n − 1)

For instance, for the following dissociation

KCl K⁺ + Cl^-

As n = 2, we would have that i = 1 + α

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

An electrolyte refers to a substance that contains free ions and can be used as an electrically conductive medium. Most of the solute does not dissociate in a weak electrolyte whereas in a strong electrolyte a higher ratio of solute dissociates to form free ions.

A weak electrolyte is a substance whose solute exists in solution mostly in the form of molecules, with only a small fraction in the form of ions. Simply because a substance does not readily dissolve does not make it a weak electrolyte. Acetic acid (CH₃COOH) and ammonium (NH₄⁺) are good examples. Acetic acid is extremely soluble in water, but most of the compound dissolves into molecules, rendering it a weak electrolyte. Weak bases and weak acids are generally weak electrolytes. In an aqueous solution there will be some CH₃COOH and some CH₃COO^- and H⁺.

A Strong electrolyte is a solute that exists in solution completely or nearly completely as ions. Again, the strength of an electrolyte is defined as the percentage of solute that is ions, rather than molecules. The higher the percentage, the stronger the electrolyte. Thus, even if a substance is not very soluble, but what does dissolve dissociates completely into ions, the substance is defined as a strong electrolyte. Similar logic applies to a weak electrolyte. Strong acids and bases are good examples such as HCl, H₂SO₄, NaCl. These will all exist as ions in an aqueous media.

Gases

The degree of dissociation in gases is denoted by the symbol α where α refers to the percentage of gas molecules which dissociate. Various relationships between K_p and α exist depending on the stoichiometry of the equation. The example of dinitrogen tetroxide (N₂O₄) dissociating to nitrogen dioxide (NO₂) will be taken.

N₂O₄ 2NO₂

If the initial concentration of dinitrogen tetroxide is 1 mole per litre, this will decrease by α at equilibrium giving, by stoichiometry, 2α moles of NO₂. The equilibrium constant (in terms of pressure) is given by the equation;

$K_p = \mathrm{\frac{p(NO_2)^2}{p(N_2O_4)}}$

Where p represents the partial pressure. Hence, through the definition of partial pressure and using p_T to represent the total pressure and x to represent the mole fraction;

$K_p =\mathrm{ \frac{(p_T)^2(x(NO_2))^2}{(p_T)(x(N_2O_4))} = \frac{(p_T)(x(NO_2))^2}{(x(N_2O_4))}}$

The total number of moles at equilibrium is (1-α)+(2α) which is equivalent to 1+α. Thus, substituting the mole fractions with actual values in term of alpha and simplifying;

$K_p = \frac{(\mathrm{p_T})(4\alpha^2)}{(1+\alpha)(1-\alpha)} = \frac{(\mathrm{p_T})(4\alpha^2)}{(1-\alpha^2)}$

This equation is in accordance with Le Chatelier's Principle. K_p will remain constant with temperature. The addition of pressure to the system will increase the value of p_T so α must decrease to keep K_p constant. In fact, increasing the pressure of the equilibrium favours a shift to the left favouring the formation of dinitrogen tetroxide (as on this side of the equilibrium there is less pressure since pressure is proportional to number of moles) hence decreasing the extent of dissociation α.

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 K_a is an indicator of the acid strength: stronger acids have a higher K_a value (and a lower pK_a 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 K_d 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 K_d value (and the higher the pK_d value).

See also

• Ionization
• Acid
• Base (chemistry)
• Thermal decomposition