Molecular autoionization
Encyclopedia
Molecular autoionization is a reaction between molecule
s of the same substance to produce ion
s and is created by semipolar bonds. The oxidation number
on all atoms in such a reaction remains unchanged. Such autoionization can be protic (H+ transfer), or non-protic. Some examples of each are:
Protic solvents
:
2 H2O H3O+ + OH-
2 NH3 NH4+ + NH2-
2 H2SO4 H3SO4+ + HSO4-
3 HF H2F+ + HF2-
Non-protic solvents:
2 PF5 PF6- + PF4+
2 SO2 SO32- + SO2+
4 AlCl3 3 AlCl4- + Al3+
This type of autoionization can occur in gas
es or solid
s, but occurs more readily in liquid
s. In gases, the molecules are so far apart that ion formation is very unfavorable, and in solids, there is not enough molecular movement. As long as the phase remains constant, autoionization normally increases substantially with increasing pressure
.
The self-ionization of water
is particularly well studied, due to its implications for acid-base chemistry of aqueous solution
s.
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s of the same substance to produce ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s and is created by semipolar bonds. The oxidation number
Oxidation number
In coordination chemistry, the oxidation number of a central atom in a coordination compound is the charge that it would have if all the ligands were removed along with the electron pairs that were shared with the central atom. Oxidation numbers are often confused with oxidation states.The...
on all atoms in such a reaction remains unchanged. Such autoionization can be protic (H+ transfer), or non-protic. Some examples of each are:
Protic solvents
Autoprotolysis
In autoprotolysis a proton is transferred between two identical molecules, one of which acts as a Brønsted acid, releasing a proton which is accepted by the other molecule acting as a Brønsted base. For example water undergoes autoprotolysis in the self-ionization of water reaction.Every solvent...
:
2 H2O H3O+ + OH-
2 NH3 NH4+ + NH2-
2 H2SO4 H3SO4+ + HSO4-
3 HF H2F+ + HF2-
Non-protic solvents:
2 PF5 PF6- + PF4+
2 SO2 SO32- + SO2+
4 AlCl3 3 AlCl4- + Al3+
This type of autoionization can occur in gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
es or solid
Solid
Solid is one of the three classical states of matter . It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a...
s, but occurs more readily in liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
s. In gases, the molecules are so far apart that ion formation is very unfavorable, and in solids, there is not enough molecular movement. As long as the phase remains constant, autoionization normally increases substantially with increasing pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
.
The self-ionization of water
Self-ionization of water
The self-ionization of water is the chemical reaction in which a proton is transferred from one water molecule to another, in pure water or an aqueous solution, to create the two ions, hydronium, H3O+ and hydroxide, OH−...
is particularly well studied, due to its implications for acid-base chemistry of aqueous solution
Aqueous solution
An aqueous solution is a solution in which the solvent is water. It is usually shown in chemical equations by appending aq to the relevant formula, such as NaCl. The word aqueous means pertaining to, related to, similar to, or dissolved in water...
s.