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Dissociation in
chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...
is a general process in which molecules (or ionic compounds such as salts, or complexes) separate or split into other things such as atoms, ions, or
radicals Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
, usually in a reversible manner. For instance, when an acid dissolves in water, a
covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between ato ...
between an
electronegative Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the ...
atom and a hydrogen atom is broken by
heterolytic fission In chemistry, heterolysis or heterolytic fission () is the process of cleaving/breaking a covalent bond where one previously bonded species takes both original bonding electrons from the other species. During heterolytic bond cleavage of a neutr ...
, which gives a proton (H+) and a negative ion. Dissociation is the opposite of association or recombination.


Dissociation constant

For reversible dissociations in a
chemical equilibrium In a chemical reaction, chemical equilibrium is the state in which both the reactants and products are present in concentrations which have no further tendency to change with time, so that there is no observable change in the properties of the ...
:AB <=> A + B the dissociation constant ''K''d is the ratio of dissociated to undissociated compound :K_d = \mathrm where the brackets denote the equilibrium concentrations of the species.


Dissociation degree

The dissociation degree \alpha 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 the van 't Hoff factor i. If the solute substance dissociates into n ions, then :i = 1 + \alpha (n - 1) For instance, for the following dissociation :KCl <=> K+ + Cl- As n = 2, we would have that i = 1 + \alpha.


Salts

The dissociation of salts by
solvation Solvation (or dissolution) describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of t ...
in a
solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Solutio ...
, such as
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
, means the separation of the
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s and cations. The salt can be recovered by
evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...
of the solvent. 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 (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 () and ammonium () 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 and some and . 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 and . These will all exist as ions in an aqueous medium.


Gases

The degree of dissociation in gases is denoted by the symbol , where refers to the percentage of gas molecules which dissociate. Various relationships between and exist depending on the
stoichiometry Stoichiometry refers to the relationship between the quantities of reactants and products before, during, and following chemical reactions. Stoichiometry is founded on the law of conservation of mass where the total mass of the reactants equ ...
of the equation. The example of
dinitrogen tetroxide Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russia rocket engineers) as amyl, is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium ...
() dissociating to nitrogen dioxide () will be taken. :N2O4 <=> 2NO2 If the initial concentration of dinitrogen tetroxide is 1  mole per
litre The litre (international spelling) or liter (American English spelling) (SI symbols L and l, other symbol used: ℓ) is a metric unit of volume. It is equal to 1 cubic decimetre (dm3), 1000 cubic centimetres (cm3) or 0.001 cubic metre (m3 ...
, this will decrease by at equilibrium giving, by stoichiometry, moles of . The
equilibrium constant The equilibrium constant of a chemical reaction is the value of its reaction quotient at chemical equilibrium, a state approached by a dynamic chemical system after sufficient time has elapsed at which its composition has no measurable tendency ...
(in terms of pressure) is given by the equation :K_p = \frac\ce\ce where represents the
partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...
. Hence, through the definition of partial pressure and using to represent the total pressure and to represent the mole fraction; :K_p = \frac\ce\ce = \frac\ce\ce The total number of moles at equilibrium is which is equivalent to Thus, substituting the mole fractions with actual values in term of and simplifying; :K_p = \frac\ce = \frac\ce This equation is in accordance with
Le Chatelier's Principle Le Chatelier's principle (pronounced or ), also called Chatelier's principle (or the Equilibrium Law), is a principle of chemistry used to predict the effect of a change in conditions on chemical equilibria. The principle is named after French ...
. will remain constant with temperature. The addition of pressure to the system will increase the value of so must decrease to keep 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


Acids in aqueous solution

The reaction of an acid in water solvent is often described as a dissociation :HA <=> H+ + A- 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 the acid dissociation constant :K_ = \ce However a more accurate description is provided by the
Brønsted–Lowry acid–base theory The Brønsted–Lowry theory (also called proton theory of acids and bases) is an acid–base reaction theory which was proposed independently by Johannes Nicolaus Brønsted and Thomas Martin Lowry in 1923. The fundamental concept of this theory ...
, which specifies that the ''proton'' H+ does not exist as such in solution but is instead ''accepted'' by (bonded to) a water molecule to form the
hydronium In chemistry, hydronium (hydroxonium in traditional British English) is the common name for the aqueous cation , the type of oxonium ion produced by protonation of water. It is often viewed as the positive ion present when an Arrhenius acid ...
ion H3O+. The reaction is therefore more correctly written as :HA + H2O <=> H3O+ + A- and better described as an ''ionization'' or formation of ions (for the case when HA has no net charge). The equilibrium constant is then :K_ = \ce where
_2O 2O or 2-O may refer to: *2o Sector, see Secondary sector of the economy *2'-O-methylation *2O, IATA code for Island Air Service in Alaska; see Essential Air Service Essential Air Service (EAS) is a U.S. government program enacted to guarantee t ...
/chem> is not included because in dilute solution the solvent is essentially a pure liquid with a
thermodynamic activity In chemical thermodynamics, activity (symbol ) is a measure of the "effective concentration" of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depe ...
of one. Ka is variously named a ''dissociation constant'', Laidler K.J. ''Physical Chemistry with Biological Applications'' (Benjamin/Cummings) 1978, p.307 an ''acid ionization constant'', an ''acidity constant''Atkins P. and de Paula J. ''Physical Chemistry'' (8th ed. W.H.Freeman 2006) p.763 or an ''ionization constant''. It serves as an indicator of the acid strength: stronger acids have a higher ''K''a value (and a lower p''K''a value).


Fragmentation

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


Receptors

Receptors are
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s that bind small
ligand In coordination chemistry, a ligand is an ion or molecule (functional group) that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's elect ...
s. 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 p''K''d value).


See also

* Bond-dissociation energy * Photodissociation, dissociation of molecules by photons (light, gamma rays, x-rays) * Radiolysis, dissociation of molecules by ionizing radiation * Thermal decomposition


References

{{Authority control Chemical processes Equilibrium chemistry