Acid–base reaction

To the concepts of acid and base have evolved in chemistry several concepts that are based on different definitions. The drive of this development is due in part to the search for a possible comprehensive and universal definition, on the other hand to a particular application of a concept.

The case now commonly as the basis used acid-base definition in the strict sense is the by Brønsted and Lowry, although, as her previous definition according to Arrhenius, of proton transfer reactions runs out of water, but even those without the presence of water discussed. While this represents a more specific definition, it is still widely used in chemistry. In a broader sense, however, is now working mostly with the definition according to Lewis, where one usually explicitly speaks in this case of Lewis acids or Lewis bases.

• 2.1 Examples of acid -base reactions according to Brønsted

Definition according to Arrhenius

The acid-base concept by Svante Arrhenius in 1887 set up and is based on the ionic theory, which contain experimentally determinable electrolytic conductivity of aqueous solutions containing salts, acids or bases. In addition to the salts which are referred to as real electrolytes, acids, electrolytes are potential because they do not conduct electricity as pure substances. However, they are subject to electrolytic dissociation in water. The conductivity is based on the formation of positively charged floating particles, the cations and negatively charged particles, the anions.

Acids

The characteristic feature of an acid is the dissociation into positively charged hydrogen ions ( H ions ), and negatively charged anion in an aqueous solution. The anion of an acid is called acid residue.

As examples of the reactions of the acids hydrochloric acid, acetic acid and hydrogen cyanide when administered in water:

These potential electrolytes subject to a dissociation equilibrium ( an equilibrium response ). The acids can be qualitatively classified into strong, medium strong and weak electrolytes. While the comparison of each molar solutions, strong acids such as hydrochloric largely dissociated weak acids such as hydrogen cyanide, lead only to a small degree of dissociation and are weak electrolytes.

Bases

Bases are compounds in water to hydroxide ions (OH - ions) and dissociate into cations. The cations of bases are called base residues. Play an important role in metal hydroxides; their cations are metal ions.

Neutralization and salt formation

The reaction of a strong or moderately strong acid, with an equivalent amount of a strong or moderately strong base is known as neutralization. It usually forms a solution of a salt.

And OH - - Here, put H ions to form water around, with a neutral solution (pH = 7) is:

Salts are compounds which dissociate in water or in the melt in Basenrestion and acid radical ion. The reaction product of the above reaction corresponds to a solution that is formed by the introduction of sodium chloride in water.

Salts are also formed by other chemical reaction, see salt formation reaction.

Hydrolysis by dissolving salts

The reverse reaction towards a neutralization is by Arrhenius as hydrolysis: (also referred to " Salzhydrolyse "). Hydrolyzing a salt occurs when at least the acid moiety or the base moiety of a weak acid or a weak base is derived. By solving such salts are alkaline (pH> 7) or acidic (pH < 7) solutions form. Accordingly, we call these salts basic or acidic salts.

Upon release of sodium cyanide is formed from the very weak acid radical CN- molecular hydrogen cyanide. In this case, sodium hydroxide in solution, which leads to an alkaline solution is formed.

By analogy, upon release of ammonium chloride and ammonia dissociated hydrogen chloride, resulting in an acidic solution.

From the hydrolysis of a salt can be a hydrolysis constant KHydr. derived. In the case of a residue of a weak acid is considered:

In the case of a residue of a weak base where:

In these cases, there is an equality with the definition in the Salzhydrolyse Bronsted -Lowry base to the constant K, the acidity constant KS and the ion product, which are shown in the equations in parentheses.

Limits the definition

In this definition, acids and bases is limited to water as the solvent. Acids in the release of H ions is postulated to be hydrated. On closer inspection, however, form oxonium ions ( H3O ), since the solvent is water important reaction partner.

Basic reactions of substances which no OH - ions can write, can not be described with this model. Thus the basic reaction of ammonia or of element-organic compounds in water can not be described with this definition of a base. The focus of the reaction of an acid with a base is the neutralization reaction and the formation of salts, while according to the model of Brønsted, which has largely replaced the model according to Arrhenius, the distinction of acidic, basic and neutral solutions is secondary and buffer systems describe better leave. Between salts whose ions in solution and molecules is distinguished from the perspective of their conductivity. Modern considerations organize these substances on their chemical reactivity with non-reactive particles can be ignored.

Definition according to Brønsted and Lowry

John Nicholas Brønsted- Lowry described Thomas 1923 independently an acid as a particle, the protons (H ions) can be transferred to a second reaction partner, so-called base. However, in contrast to Arrhenius bases and acids with them no more certain classes of substances, but particles that exhibit certain properties in a reaction with H ions:

• Particles can release protons are accordingly referred to as proton donors or acids.
• Particles, however, can accommodate protons are accordingly referred to as proton acceptors or bases.

Acid-base reactions in which in the above. Way protons are transferred, are also called proteolysis. Free protons (H ), however, there exist no time: Each reaction of a partner as an acid is mandatory in the presence of a second partner as base ahead of the acid can transfer their protons:

Systems of this kind are also called conjugate or conjugate acid -base pairs, between which there is always a chemical equilibrium after a certain time. In the above equation are the HX and HY acids, Y and X, however, the base - which both pairs differs is only leave or resume their ability protons. As can also be seen, is formed in a protolytic reaction of an acid ( HX here ) always absorbed her. conjugate or conjugate base ( here X ), from a base ( here Y) always absorbed her. conjugated or corresponding acid (in this case HY ) and vice versa.

Can a chemical both protons give and take, so he act both as an acid as base, it is called to be amphoteric from a ampholytes or property. Ampholyte is the most well known water, which allows for both the formation of OH - and H3O .

The practically significant proteolytic reactions are accordingly reactions with water:

The equilibrium of this reaction is determined by the acid strength of HX, described numerically by the acidity constant.

Examples of acid -base reactions according to Brønsted

• Reaction of hydrogen chloride (HCl ) and water, forming hydrochloric acid [ HCl (aq )]:

• Reaction of sulfuric acid and water:

• Reaction of ammonia and water:

• The Brønsted- Lowry acid concept explained - in contrast to Arrhenius - but also the acid -base reaction of hydrogen chloride and ammonia gas to form ammonium chloride ( NH4Cl), despite the absence of water:

Definition according to Lewis

Gilbert Newton Lewis published 1938/1939 a treatise on its acid - base theory. Thus, a Lewis acid is an electron pair and electrophilic a Lewis base electron pair donor a.

Among the Lewis acids include:

• Compounds with incomplete electron octet as: B ( CH3 ) 3, BF3, AlCl3, FeCl2
• Metal cations as central atoms in chemical complexes
• Molecules with polarized double bonds, such as CO2
• Halides with unsaturated coordination, such as SiCl4 or PF5

All bases according to the Brønsted -Lowry bases are also according to Lewis.

Example of a Lewis acid-base reaction

Definition according to Lux and Flood

Established in the center of 1939 by Hermann Lux and Håkon Flood 1947 extended concept are protons instead of the oxide ions in the foreground. This was set up in order to describe the acid -base reactions in aprotic systems, as it occurs in the inorganic melt.

After Lux and Flood acids oxide ion acceptors, bases are oxide ion donors. One considers this non-metal oxides (eg, SO2, CO2) as acid anhydrides, since they are acidic in aqueous solution are in accordance with metal oxides (eg, MgO, Fe2O3) Basenanhydride because they form hydroxide ions in aqueous solution.

Definition according Usanowitsch

In 1939, the Russian scientist Mikhail Usanowitsch following definition of the acid-base concept at:

" An acid is any chemical compound that will react with bases, gives the cations or anions or receiving electrons. Accordingly, a base any compound which reacts with acids, anions or electrons to or combined with cations. "

This definition of violence includes the reactions after the Lewis concept, extended selbiges the fact that the recording or release of electrons is not limited to common pairs, and includes all redox reactions, in which a complete electron transfer is involved.

One criticism of this little common theory is that it is too universal, and the term acid-base reaction to almost any kind of reaction is applicable.

Concept of hard and soft acids and bases by Pearson

Ralph G. Pearson developed in 1963 the concept of hard and soft acids and bases ( Hard and Soft Acids and Bases, HSAB concept). It reads:

"Hard acids combine preferentially with hard bases and soft acids with soft bases combine preferred. "

Here, considered is the electronegativity and the polarizability of the particle under consideration:

The concept are tendencies again, there are few absolute hard or soft particles. However, it can help in the estimation of the stability of the compounds. So, for example, comes before the softer Fe2 in nature as sulphide, while the harder Fe3 is present as the hydroxide or oxide.