Acid

Definitions

Acidity 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:

$\ begin {matrix} \ mbox {AH} &+& \ mbox {B} ^ - &=& \ mbox {has} ^ - &+& \ mbox {BH} \ \ \ mbox {acide1} && \ mbox {base2} && \ mbox {base1} && \ mbox {acide2} \ end {matrix}$

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

Acid in water

For 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).

Note:

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 protic

They 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 nonprotic

The 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 O^2- it is the definition of the acids of Lux-Flood. One can then define a base as a donor of ion O^2- (rich in electron) and an acid like an acceptor of ion O^2-. 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 geology

In 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 O^2- is acid.

SiO₂ + H₂O = H₂SiO₃

Thus the rocks rich in silica are acid, the rocks rich in limestone are basic. That has an importance to include the behavior of the magma S (in particular in the Volcan S), and when one wants to dissolve a rock to analyze it, for example dissolution in an acid for analysis by ICP or dissolution in glass for analysis by Spectrométrie of x-ray fluorescence (technical known as of the “molten Perle”).

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.

Taste

The 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 dissolution

One 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 Calcaire dissolves easily with weak acids; one cleans the valves and fittings with juice of Citron (Acid citric) or Vinaigre (Acid acetic).

The Verre S can be dissolved by hydrofluoric Acid , but the handling of this acid is extremely dangerous, because of presence of ions Fluorure S.

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.

The use of concentrated acids must always be done by people trained and equipped (blouse, gloves, glasses), under hood.

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