Dissociation (chemistry)

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In chemistry, dissociation (from the Latin dissociare "to separate") is understood as the stimulated or automatic process of dividing a chemical compound into two or more molecules , atoms or ions . The degree of dissociation or the dissociation constant is used as a measure of the dissociation . The degree of dissociation indicates the ratio of the dissociated particles to the formal initial concentration of the undissociated chemical compound. The dissociation energy is the energy that is necessary to break a chemical bond .

Dissociation of gases and thermal dissociation

The first methods of determining molar mass were based on vapor density measurements. However, there were occasional deviations, which led to conceptual conclusions that molecules in the gas phase must be dissociated.

The term dissociation was coined in 1857 by Henri Étienne Sainte-Claire Deville . When determining the vapor densities of inorganic and organic compounds, Cannizzaro, Kopp and Kekulé found deviations in the molar masses in the gas phase. The gas density was often lower than expected. Sainte-Claire Deville was able to determine a lower density of phosphorus pentachloride in the gas phase, at the same time he observed a greenish color and concluded from this that phosphorus pentachloride must have broken down into chlorine and phosphorus trichloride. When heating ammonium chloride in a thin tube, Pebal and Skraup were able to demonstrate with litmus that the ammonium chloride dissociates in the gas phase into ammonia and hydrochloric acid through the different gas velocities (glass tube with constriction for measuring the effusion, see Thomas Graham (chemist) ).

Thermal dissociations are usually much slower than electrolytic dissociations. An example of thermal dissociation is provided by nitrous oxide , which is present in the form of colorless crystals at −10 ° C. When heated, the molecule dissociates into the intensely brown-red colored nitrogen dioxide :


This reaction is reversible. On cooling, the sample becomes discolored again due to the recombination to dinitrogen tetroxide. Dissociations occur especially with macromolecules at relatively low temperatures.

When heating peroxides or azo compounds, the bonds of which dissociate thermally at around 150 ° C, radicals are formed. Radicals can be determined with electron spin resonance spectroscopy ( ESR spectroscopy ).

Photochemical Dissociation

If molecules are dissociated after they have passed into an electronically excited state due to absorption of light, one speaks of photochemical dissociation or also of photolysis or photodissociation. Photolysis is an important aspect of atmospheric chemistry and has preparative and industrial significance.

Electrolytic dissociation

Electrolytic dissociation is the reversible breakdown of a chemical compound into anions and cations in a solvent (e.g. salts in water). The ions are then surrounded ( solvated ) by solvent and thus freely movable, which results in electrical conductivity . Such solutions are called electrolytes .

With the so-called real or permanent electrolytes , the ions are already present in the solid (→ ion lattice ). In the case of solid table salt, Na + and Cl - ions are already present in the lattice . When the salt is dissolved in water, freely moving ions are now formed in the water. When salts dissociate into ions, the very high lattice energy of the crystal is used up by hydration energy during the dissolution process.

In the so-called potential electrolytes, there are no ionic bonds in the pure substances. As a pure substance, they are non-conductors . When introducing these pure substances (AB) in a solvent, the formation of ions is carried out by a chemical reaction between solute and solvent: . A prerequisite for such a reaction is a polar bond between parts A and B of the compound (AB) and a polar solvent. For example, if pure acetic acid is added to water, cations and anions are formed

If the gas hydrogen chloride (HCl) is introduced into water, an electrolytic solution is formed, which is called hydrochloric acid :


If the gas ammonia (NH 3 ) is introduced into water, cations and anions are formed :

The equilibrium reactions in these examples are also called protolysis . This behavior leads to the acidic character of the acids (e.g. acetic acid ) and to the basic character of the bases (e.g. ammonia ). The electrical conductivity of these solutions is the experimental proof of the formation of freely moving anions and cations.

Dissociation in Organic Chemistry

Knowledge of dissociation is also of great importance in organic chemistry.

Many organic reactions are only possible if carboxylic acids and hydroxyl groups are present as anions so that material conversions such as alkylations can be carried out. Hammet studied the dissociation of organic bases and carboxylic acids in water and various solvents. The first investigations into the formation of organometallic compounds were carried out by Conant and Wheland. In 1965 Cram then set up an acidity scale (MSAD scale) for various hydrocarbon molecules, and in 1963 EM Arnett determined the dissociation constants for esters, amides, thiols, amines and phenols. On the basis of these acidity scales, organic chemists can more easily estimate which base is necessary for a substance conversion.

See also

Individual evidence

  1. Walther Nernst: Theoretical chemistry from the standpoint of Avogadro's rule and thermodynamics , 5th edition, published by Ferdinand Enke 1907, pp. 346-347.
  2. Sur la dissociation ou la decomposition spontanee des corps sons l'influence de la chaleur, Compt. rend. 45 , 857 (1857).
  3. Liebigs Ann. 128, 199 (1862).
  4. Exner's Repert. d. Phys. 21 , 501 (1884).
  5. LP Hammet: Physikalische Organic Chemie, Verlag Chemie 1973, chap. 9.
  6. LP Hammett, AJ Deyrup, J. Am. Chem. Soc. 54, 272 (1932).
  7. JB Conant, GW Wheland, J. Am. Chem. Soc. 54, 1212 (1932).
  8. K. Ziegler and H. Wollschitt, Ann. 479, 123 (1930).