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Dissociation inchemistry is a general process in which molecules (or ionic compounds such assalts, orcomplexes) separate or split into other things such as atoms, ions, orradicals, usually in a reversible manner. For instance, when anacid dissolves in water, acovalent bond between anelectronegative atom and a hydrogen atom is broken byheterolytic fission, which gives a proton (H+) and a negativeion. Dissociation is the opposite ofassociation orrecombination.
For reversible dissociations in achemical equilibrium
thedissociation constantKd is the ratio of dissociated to undissociated compound
where the brackets denote the equilibrium concentrations of the species.[1]
The dissociation degree is the fraction of original solute molecules that have dissociated. It is usually indicated by the Greek symbol α. More accurately, degree of dissociation refers to the amount of solute dissociated into ions or radicals per mole. In case of very strong acids and bases, degree of dissociation will be close to 1. Less powerful acids and bases will have lesser degree of dissociation. There is a simple relationship between this parameter and thevan 't Hoff factor. If the solute substance dissociates into ions, then
For instance, for the following dissociation
As, we would have that.
The dissociation of salts bysolvation in asolution, such aswater, means the separation of theanions andcations. The salt can be recovered byevaporation of the solvent.
Anelectrolyte refers to a substance that contains free ions and can be used as anelectrically 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 (which are said to be "undissociated"), 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) andammonium (NH+4) 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 someCH3COOH and someCH3COO− andH+.
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 does dissociate 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 andH2SO4. These will all exist as ions in an aqueous medium.
The degree of dissociation ingases is denoted by the symbolα, whereα refers to the percentage of gas molecules which dissociate. Various relationships betweenKp andα exist depending on thestoichiometry of the equation. The example ofdinitrogen tetroxide (N2O4) dissociating tonitrogen dioxide (NO2) will be taken.
If the initial concentration of dinitrogen tetroxide is 1 mole perlitre, this will decrease byα at equilibrium giving, by stoichiometry,α moles ofNO2. Theequilibrium constant (in terms of pressure) is given by the equation
wherep represents thepartial pressure. Hence, through the definition of partial pressure and usingpT to represent the total pressure andx to represent themole fraction;
The total number of moles at equilibrium is(1 −α) + 2α, which is equivalent to1 +α. Thus, substituting the mole fractions with actual values in term ofα and simplifying;
This equation is in accordance withLe Chatelier's principle.Kp will remain constant with temperature. The addition of pressure to the system will increase the value ofpT, soα must decrease to keepKp 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 dissociationα.
The reaction of an acid in water solvent is often described as a dissociation
where HA is a proton acid such as acetic acid, CH3COOH. The double arrow means that this is an equilibrium process, with dissociation and recombination occurring at the same time. This implies that theacid dissociation constant
However a more explicit description is provided by theBrønsted–Lowry acid–base theory, which specifies that theproton H+ does not exist as such in solution but is insteadaccepted by (bonded to) a water molecule to form thehydronium ion H3O+.
The reaction can therefore be written as
and better described as anionization or formation of ions (for the case when HA has no net charge). The equilibrium constant is then
where is not included because in dilute solution the solvent is essentially a pure liquid with athermodynamic activity of one.[2]: 668
Ka is variously named adissociation constant,[3] anacid ionization constant,[2]: 668 anacidity constant[1] or anionization constant.[2]: 708 It serves as an indicator of the acid strength: stronger acids have a higherKa value (and a lower pKa value).
Fragmentation of a molecule can take place by a process ofheterolysis orhomolysis.
Receptors areproteins that bind smallligands. The dissociation constantKd is used as indicator of theaffinity of the ligand to the receptor. The higher the affinity of the ligand for the receptor the lower theKd value (and the higher the pKd value).
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