An acid is a chemical Composé generally defined by its reactions with another type of complementary chemical compound, the bases.
DefinitionsAcidity was initially defined by Svante August Arrhenius at the end of the XIXe century: an acid is a chemical compound being able to release from the Proton S (H+ ions) in aqueous solution and a base a chemical compound being able to release from the ions hydroxides (OH-) in aqueous solution. But this definition was not general enough and did not explain the alkalinity of some chemical compounds not releasing from OH- in aqueous solution.
Acid of Bronstedt
The Théorie of Bronsted-Lowry (1923) says to us that an acid releases protons and a base collects protons:
By this definition, an acid is a chemical compound which tends to give a electron to a complementary entity, the base. The reactions which will take place between an acid and a base are named: reactions acido-basic, or reactions acid-bases. Such an acid is called acid of Bronsted.
Thus, in water an acid couple/base reacts with another couple by exchanging ions, here the acido-basic couples are AH/A- AND BH/B-.
Acid of Lewis
See also: Acid of Lewis
The definition of Lewis (1923) is broadest by far: an acid of Lewis is acceptor of Doublet S, having thus an orbital vacuum, and a base of Lewis is a donor of doublet, having a free doublet. By this definition, an acid is a chemical compound which can, during a reaction, to accept a pair of electrons (a doublet). It is thus a chemical compound électrophile, which has an electronic gap in its structure.
One can easily recognize an acid thanks to tests of pH. An acid solution has a pH lower than 7, with 25°C. The lower the pH is, the more acidity is strong. To measure the pH, one can carry out various tests: papers pH, indicators, use of a PH-meter, inter alia.
The acid species within the meaning of Bronsted can release one or more protons one then speaks respectively the mono one and about Polyacide S
Acid in waterFor the reactions in water, one uses usually the definition of Bronsted. An acid can be represented by generic formula AH
One establishes a distinction between the weak acid and the strong acid. The latter are characterized by the fact that when they are placed in water, entity AH does not exist any more in solution because the reaction of dissociation is total.
Among the strong acids, one finds hydracids (HCl, HBr, HI) and the Oxacides (molecules acid having a central Atome with a top Step oxidation surrounded by atoms of Oxygène, for example: Acid nitric, Acid sulphuric, Perchloric acid, permanganic acid).
One classifies the weak acids (formic acid, acetic acid “vinegar”) according to their Constante of acidity.
In water, acidity is measured using the scale of the pH. Let us note that water is at the same time a weak acid and a weak base (it is a Amphotère or ampholytic).
One does not speak any more acid/Extremely bases “(E) S” but of acid or bases “completely dissociated”.
There is a levelling effect of water (therefore the scale of pH in water goes from 0 to 14) To compare the forces of the acids/bases completely dissociated, one will use another Solvant (ex: ethanol…).
In water, the strongest acid is H3O+ (H (aqueous) + solvent) and the strongest base is HO-. It is advisable to keep in memory that these notations (H3O+ and HO-) constitute only one diagrammatic simplification of the system. In reality, H+ and HO- are both surrounded by a Sphère of solvation (several molecules of Eau, polar, establishing connections of the electrostatic type with the Ion S). A more rigorous notation should thus be H (H2O) n+ and HO (H2O) n-. However, this more rigorous notation does not bring anything to comprehension acido-basic phenomena.
Acids in other solvents
The generalization of the concept of acidity made it possible to extend the study to other solvents.
Solvents proticThey are solvents able to yield and collect a hydrogen atom. For example NH3 (NH2- and NH4+ gives), HF, CH3COOH, H2SO4,… In these solvent the concept of couple acid-bases remainder centered on the exchange of proton and the couples in these solvents remain the same ones as in water. The interest of this type of solution comes from the reaction of Autoprotolyse of the solvent which limits the force of the acids (and the bases) being able to exist in solution. One will thus use a solvent basic than water like NH3 to work with very strong bases like H-CC: - and a more acid solvent like H2SO4 to work with strong acids.
Solvents nonproticThe particle transfer can be made with another thing that a proton. In a solvent like NO2, one with the reaction 2 NO2 - > NO3- + NO+ which can be seen as the transfer of an ion O2- it is the definition of the acids of Lux-Flood. One can then define a base as a donor of ion O2- (rich in electron) and an acid like an acceptor of ion O2-. In other cases it is a transfer of ion F-. This type of design of the reaction in solution plays a big role in Métallurgie and in the study of the rocks (see below).
Acids in geologyIn Geology, one says that a stone is acid if it contains more than silica 65%. This is a rule which translates the definition of the acids of Lux-Flood: the Silice which is an acceptor of ion oxidizes O2- is acid.
- SiO2 + H2O = H2SiO3
For this last application, one often estimates acidity by the acid value , which is the number of oxygen atoms divided by the number of others atomes1; for example, SiO2 has an acid value of 2, CaCO3 has an acid value of 1,5.
TasteThe acid taste is recognized thanks to particular receivers PKD2L1, which are probably sensitive to the presence of the H+ ions, or with certain cyclic nucleotides. In spite of certain studies one knows now that its recognition is independent of the recognition of the salted taste. These receivers play also an important physiological part in the remainder of the organization and are expressed on the surface of certain cells to control the quantity of CO2 in the blood or the quantity of liquid cérébrospinal.
Acid dissolutionOne of the principal properties of the acid solutions is to be able to dissolve a great number of Matériau X. The capacity of dissolution depends on the concentration of the acid and the chemical nature of material and the acid.
The metals, when the Potential redox of the couple metal/drifting cation of this metal is lower than 0, are not stable in the acid solutions, they are thus oxidized (i.e. ionized by loss of one or more electron S); the metal Ion can then remain in dissolved form (solvatée), or combine with one or more ions Oxygène and form an oxide. Acidity is one of the important parameters of the aqueous Corrosion. The dissolution of metals is used in engraving of art, it is the technique of the Eau-forte; it is also used in Métallographie to reveal defects (for example grain boundary).
The concentrated acids can cause Brûlure S on the skin and the mucous membranes (eyes, nose, stops). In the event of burn by acid, it is necessary:
- to protect: while protecting itself (Glove S…), to close the container and to prevent that the acid is not spread (use of absorbing paper), to open the windows to avoid the vapor releases;
- to withdraw soaked clothing (by paying attention not to contaminate other healthy people or parts of the victim);
- to make stream water to wash the part reached, while avoiding contaminating the healthy parts;
- to prevent the helps (“112” in the European Union, “15” in France) by mentioning the product in question.
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