Where does photochemical dissociation of water occur

This side is one Disambiguation to distinguish several terms with the same word.

Under dissociation In chemistry, one understands the excited or the automatic process of breaking down a molecule into two or more simpler 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 proportion of the original particles that is dissociated. Dissociation energy is the energy that is necessary to separate a chemical bond.

Electrolytic dissociation

The electrolytic dissociation is the reversible decomposition of a compound into anions and cations in a solvent. Such solutions are called electrolytes. Such a dissociation takes place when salts are dissolved in water. In this polar solvent, what is dissolved is in the form of freely moving ions. These freely moving anions and cations lead to an electrical conductivity of the water.

With the so-called real or permanent electrolytes the ions are already present in the solid (→ ion lattice). So with solid table salt there are already Na in the grid+ and Cl--Ions before. When the salt is dissolved in water, freely moving ions are now formed in the water. When salts are dissociated into ions, the very high lattice energy of the crystal is applied by hydration energy during the dissolution process.

With the so-called potential electrolytes there are no ionic bonds in the pure substances. As a pure substance, they are non-conductors. When these pure substances (AB) are introduced into a solvent, ions are formed through a chemical reaction between the dissolved and the 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, H cations are formed3O + and the anions H3C - COO

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


If the gas is ammonia (NH3) introduced into water, forms as cations and the anions OH:

The equilibrium reactions of these examples are also called protolysis and are described in more detail there. This behavior turns acetic acid and hydrochloric acid into acids. The behavior of ammonia makes ammonia a base. The electrical conductivity of these solutions is the experimental proof of the formation of freely moving anions and cations.

see also:

Thermal dissociation

In the thermal dissociation Molecules disintegrate into fragments due to their own thermal movement (vibrations, rotations, etc.). Thermal dissociation is usually much slower than electrolytic dissociation. An example of one thermal dissociation offers dinitrogen tetroxide, 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 form dinitrogen tetroxide. Dissociations occur especially with macromolecules at relatively low temperatures.

Photochemical Dissociation

Photochemical reactions are reactions that take place through the absorption of UV radiation, visible or infrared radiation. Addition, substitution, dissociation and other reactions can take place. It is interesting that dissociation reactions also take place at other binding sites than, for example, thermal dissociation. They also offer us the opportunity to capture direct and reflected light photographically. Light quanta can also trigger chain reactions that spread because they themselves emit light of the corresponding wavelength.

In photochemical dissociation, for example, a silver halide breaks down into elemental silver and a residue. The ionic character of silver ions is not very pronounced, and there are ions that can migrate in the lattice.

The energy required for the reaction comes from a light quantum with one wavelength

Ag+ + e- → Ag (finely divided, black)

Homolytic bond cleavage

Another important type of dissociation is the homolytic cleavage of a covaltent bond in which half of all binding electrons are released to the two binding partners. See main article.

Categories: Chemical Reaction | Chemical bond | Acid-base reaction