Salt (chemistry)

As salts of chemical compounds are referred to, the positively charged ions (cations) and negative ions (anions) are constructed. Are ionic bonds between these ions. For inorganic salts the cations often of metals and the anions are frequently formed from non-metals or their oxides. As solids, they form an ionic lattice. Suitable organic salts are referred to all compounds in which at least one anion or cation is an organic compound.

• 2.1 Examples of organic cations and anions

• 3.1 Reactions of acids and bases
• 3.2 From other salts
• 3.3 Reaction of oxides
• 3.4 Other reactions

Inorganic salts

In the narrowest sense is meant by salt, sodium chloride ( NaCl, table salt). In a broad sense refers to all compounds which are composed of anions and cations, such as NaCl - for example calcium chloride ( CaCl2) - the form of salts. Sodium chloride and anions Cl - up of the cations Na. The salt is calcium chloride and Cl- formed by Ca2. The formulas NaCl and CaCl2 are the ratio formulas of the compounds ( Na: Cl = 1:1, and Ca: Cl = 1:2). Ions can be mono-or multi-valued, so wear one or more positive or negative charges.

The empirical formula of a salt is determined by the charge number of the ions, since positive and negative charges must compensate. Ratio formulas of salts are in clear contrast to formulas of compounds such as water ( H2O) and methane (CH4 ), which are molecular compounds.

When inorganic salts are ionic bonds between the ions before. A very high number of ions formed in compliance with the respective ratio formula ionic lattice with a specific crystal structure. The figure at right shows a small section of the structure of a sodium chloride crystal lattice. Since quite a lot of different cations and anions exist, even a large number of different salts are known. Some of the ions are listed in the tables below.

Ion salts, may exist in more than one atom. They are called complex ions. Examples of complex anions are nitrate anion ( NO3- ) and the sulfate anion ( SO42 - ). In complexes of an atom as the central atom is present at the other atoms (or groups of atoms ) are attached and are called ligands. In the two examples N and S are the respective central atom ligands are in both cases oxygen atoms ( oxo complexes ). The central atoms and the ligands are linked together by covalent bonds. Ionic bonds are only between the anions and cations. From nitrates, for example, the salt is sodium nitrate ( NaNO 3 ), among the sulfates of sodium sulfate ( Na2SO4 ) is known.

Cations are usually made ​​of metals and their salts mentioned metal salts. For non-metals, there is a complex cations ammonium (NH4 ) with nitrogen as the central atom and hydrogen as a ligand. Ammonium ion, for example, form the salt ammonium sulphate ( (NH4) 2SO4 ). The ammonium compounds exist analog organic compounds ( quaternary ammonium compounds), which are further described below.

In polyvalent oxo complexes also contain OH groups can act as ligands, such as the salt sodium bisulfate ( NaHSO 4 ). Analog salts are known under the phosphates: In addition, there are sodium phosphate and disodium hydrogen phosphate and sodium dihydrogen phosphate salts. From the usual formula notation ( formula unit ) for these compounds, the OH groups can not recognize immediately as a ligand. The formula units of such salts are derived from the traditional spelling of acids such as sulfuric acid (H2SO4 ) and phosphoric acid ( H3PO4) from.

Transition metals can form not only cations but also anions as oxo complexes. Thus, chromium chromates ( [ CrO4 ] 2 -), the anion in potassium chromate ( K 2 [ CrO4 ] ) and manganese the Permanganate ( [ MnO 4 ] - ), the anion in potassium (K [ MnO4 ] ) form.

Complex anions can have as central atom and metals. With potassium hexacyanoferrate (II ) (K4 [Fe (CN ) 6] ) is the ferric ion Fe 2 together with six groups cyanide (CN- ), together with a stable anion four negative charges. Salt are ionic bonds between potassium ion and the hexacyanoferrate (II) anion. Analogously, the ferric ion Fe3 is potassium hexacyanoferrate ( III) ( K 3 [ Fe (CN ) 6] ), a complex salt. With K3 [Fe (CN ) 6], the ferric ion Fe 3 together with a six cyanide groups (CN- ) forms, together with a stable anion three negative charges.

Examples of cations and anions

Properties of salts

• Many salts are solids with relatively high melting points at room temperature. Some salts are quite hard and brittle and have smooth edges break under mechanical processing. These characteristics are quite typical for solids, which are built up by the ion lattice and thus form crystals. But not everyone crystalline substance is a salt. So sugar forms (sucrose) crystals also, but has no ionic lattice and not one of the salts.

• Many salts are soluble in water and insoluble in most organic solvents. For water- soluble salts of the water overcomes the lattice energy of the ionic lattice by hydration. If the hydration energy of similar size or larger than the lattice energy, the salt is moderately or highly soluble. In solutions, the individual ions from water molecules are quite closely and intensively covered. This is often represented in the chemical reaction as: The ( s ) indicates a solid and (aq ) indicates that the ion is present hydrated.

• Dry salt crystals are electrical insulators. Molten salts and aqueous solutions, however, conduct electricity because of their free-moving ions as charge carriers; they are electrolytes.

• The dissolution of salts in water can change the pH value of the solution. Influences the salt not the value, it is called neutral salts. The neutral salts also include the sodium chloride. Other salts raise or lower the pH. One speaks of basic or acidic salts. As a specific salt reacts can be difficult to assess from the composition of the compound. However, a general rule, the anions ( acid residues) of strong acids react mostly neutral. Acid residues of weak acids react most basic. Exemplary of salts of polybasic acids which are known, the behavior of the phosphates. Dissolving salts in aqueous solutions of organic molecules, such as biomolecules can lead to denaturation of the biomolecules or cause the precipitation of the macromolecules. This effect of salts is characterized by the so-called Hofmeister series.

Other cations and anions

• Metal oxides make up a large part of the Earth's crust and may be considered as salts. The anion O2 - (oxide -ion) occurs as such only in the solid or molten state, in aqueous solutions, it is not known. The oxygen in the oxide ion has the oxidation number -2. The oxidation number of metals thus determines the ratio formula of each compound: MI2O, milo, MIII2O3. Is an oxide water-soluble, a specific chemical reaction takes place, for example: Sodium oxide reacts with water to form hydroxide ions to form sodium hydroxide solution. Similar to calcium oxide reacts (CaO), also known as burnt lime to slaked lime (Ca (OH) 2). Many oxides react with water. The iron ( III) oxide (Fe2O3 ) is not a water-soluble compound.
• Sulfides: minerals are commonly found in nature as sulfides (S2 - ), such as pyrite and chalcocite. Also sulfides may be regarded as salts. Sodium sulfide ( Na2S ) is a soluble salt, most of sulfides such as zinc sulfide (ZnS ) and copper ( II ) sulphide ( CuS ), are virtually insoluble in water. In analytical chemistry, the various (poor ) Solubility of various metal sulfides is used to separate the elements ( in the separation process, the hydrogen sulfide group).

Water of crystallization

Many salts contain not only the ions in certain amounts and water molecules, the so-called water of crystallization. It is stated in the ratio formula, as here in the example of sodium sulfate decahydrate: Na2SO4 · 10 H2O.

Double salts

Addition salts with only one type of cation (M) and salts having two different cations are known. These are called double salts salts such as alums with the general composition Mimiii (SO4 ) 2 Example: aluminum potassium sulfate dodecahydrate ( KAl ( SO4) 2 · 12 H2O).

Boundaries of the concept salts

• Materials are salts, if ionic bonds between the particles of the compound present. Whether this binding type is present, but can not easily be derived. While when calcium oxide ( CaO) act ionic bonds, there are the case of chromium (VI ) oxide ( CrO3 ) only covalent bonds between the chromium and oxygen atoms before; it is thus no salt. Therefore, it is better to talk in these cases instead of salts of metal oxides.
• Salts are historically regarded generally as chemical compounds, since they have a defined composition of various chemical elements. However, there are mixed crystals of two salts, are known which are not of stoichiometric composition: To potassium forms (K [ MnO4 ] ) with barium sulfate (Ba [ SO4 ] ) in almost any proportion mixed crystals (although only up to a certain maximum amount of barium sulfate ) because the components have similar crystal structures and lattice spacings. A chemical similarity of the compounds involved, or an equal value is not necessary for the formation of mixed crystals.

Organic salts

Besides the inorganic salts described above, there are also numerous salts of organic compounds. The anions of these salts are derived from the organic acids. Important are the salts of carboxylic acids, such as acetic acid, many of the salts, which are known so-called acetate ( CH3COO - ). Thus, Na form with the salt sodium acetate or with Cu2 , the copper acetate. Acetic acid is a monocarboxylic acid ( having only one-COOH group), and forms only monovalent anions. Citric acid is a tricarboxylic acid ( three -COOH groups ), and capable of forming trivalent anion; their salts called citrates. Known examples are the salts of sodium citrate and calcium citrate. Many acetates and citrates form crystals, but this is not the real reason is to call salts. The real and only reason is the presence of ionic bonds between anions and cations. Within the ion of organic compounds are covalent bonds.

Practical significance of the salts of carboxylic acids, which are among the fatty acids. The sodium or potassium salts of fatty acids known as soaps. In soaps are mixtures of various fatty acid salts. They find practical use as a washing soap or soap. As a concrete example is the palmitic acid salts, which are called Palmitate. Salts based on such a large organic molecules are non-crystalline in the rule.

Analogous to the inorganic sulfates ( SO42 - ), there are also organic sulfates (RO - SO3 - ) as sodium lauryl sulfate, which are used as surfactants in shampoos and shower gels use. Also alcohols, salts, alkoxides, are known. Alcohols are very weak acids and therefore are almost never called that. Under aggressive reaction conditions, compounds of the form RO -M ( M = metal) can be obtained. In analogy to many inorganic oxides ( MO) alkoxides react on contact with water, hydrolysis and form the corresponding alcohols.

Among the organic cations to the ammonium cation ( NH4 ) have analog connections importance. They are called general quaternary ammonium compounds. In these compounds, the nitrogen atom bears usually four alkyl groups ( R ) and a positive charge. The alkylammonium compound cetyl trimethyl ammonium bromide, for example, an organic ammonium compound, in which a bromine atom is present as an anion. Practical importance ammonium compounds with three short and one long alkyl group, as these cations in aqueous solution show the property of surfactants. Compounds of this type play an important role in the metabolism of living organisms, such as choline.

In principle, any organic amine by acceptance of a proton (H ) are added to a cation. Analogous to the reaction of ammonia (NH3) to ammonium ion ( NH4 ) reacts, for example, a primary amine (R- NH2, R = organic radical ) to the cation R -NH3 . Since such compounds usually polar and therefore more soluble in water than the original substances are, for example, nitrogen-containing drugs ( active pharmaceutical ingredients ) by the addition of hydrochloric acid to form salts, transferred to the hydrochlorides so-called. This facilitates their absorption into the body. Hydrochloride can be easily purified by recrystallization in contrast to the amines. Analog form amines with hydrogen bromide hydrobromide and hydrogen fluoride hydrochloride fluorides.

Next molecules that carry a positive or negative charge, also exist molecules that have a negative and positive charge. They are called Internal salts or zwitterions. The class of Betaine is one of the inner salts whose simplest connection is the betaine.

The amino acids have a carboxyl group (- COOH) and an amino group (-NH2) and can react so sour and basic. In an inner neutralizing an anionic form ( -COO- ) and a cationic ( -NH3 ) group and thus a zwitterion. The simplest amino acid is readily soluble in water glycine. Zwitterion show, in contrast to other ions dissolved in water is poor ( no ) electric conductivity. ( Ampholytes )

Examples of organic cations and anions

Preparation of inorganic salts

Reaction of acids and bases

Salts formed in the reaction of acids with bases (Greek basis; Arrhenius: Bases are the basis for salts). Here, the oxonium ion of the acid forms with the hydroxide ion of the base water ( neutralization). Some salts are sparingly soluble in water and form directly to the solid. In general, the salt is present in solution and can be recovered by evaporation of the water as a solid.

For other salts

Some salts can be obtained from two other salts. On mixing aqueous solutions of two salts, a third salt can be formed as a solid. This is only possible if the third salt in contrast to the other two is less soluble.

Reaction of oxides

As described above, many metal oxides tend to form hydroxides with water. Under acidic conditions, metal oxides, which in pure water "insoluble" respond ( = stable) are. In this way can be many salts, such as copper sulfate win.

Other reactions

The ions in the reactions described above are not only formed, but they exist prior to the formation of a new salt. In reactions to form a new salt is not present or not all of the ions with the necessary charge, redox reactions are taking place. So can be obtained from elemental metals and non-metals salts. Reactions of this type are described in detail under salt-forming reaction.