Bases (chemistry)

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As bases (for ancient Greek βάσις basis , German , basis' ) are in the chemistry referred narrow definition compounds which in aqueous solution are capable of hydroxide ions (OH - form and thus to) pH increase of a solution. Hydroxide ions are chemical compounds that protons can take over from an acid to form a water molecule. A base is therefore the counterpart to an acid and is able to neutralize it .

In addition, there are further definitions of the term “ bases” of various acid-base concepts for a much broader range of chemical reactions that can extend beyond those of hydroxide ions in water. The concepts according to Lewis ( Lewis base and Lewis acid ) and that of Pearson, who speaks of hard and soft acids and bases ( HSAB concept ), are particularly important .

Historical development of the term "base"

In alchemy , some alkalis were known , such as lime ( CaCO 3 , CaO and Ca (OH) 2 ), soda , soda , potash and ammonia . Until the beginning of the 18th century, however, no precise distinction was made between soda and potash. The term “alkalis” was rarely used and no precise connection between these substances was recognized. The base (alkali) as the opposite pole of the acid was postulated in chemiatry , a medical-theoretical teaching structure by Otto Tachenius in the 17th century.

Until the 18th century there was a close connection between alkalis and fire or “fire matter”, also because of the well-known exothermic reactions. The term "base" was introduced in the 17th century by chemists such as Georg Ernst Stahl , Robert Boyle and Guillaume François Rouelle because "basic" substances formed the non-volatile basis for fixing volatile acids and can neutralize the (caustic) effect of acids. Antoine Laurent de Lavoisier took fundamental steps in chemistry . He thought that acids were always made from non-metal oxides and water, and bases were made from metal oxides and water. Sir Humphry Davy and Justus von Liebig viewed acids as hydrogen compounds that can be converted into salts by metals . In 1887, Svante Arrhenius defined bases as substances that dissociate when they dissolve in water, releasing hydroxide ions, and acids as substances that dissociate, releasing protons. Acids and bases neutralize each other. The theory was still insufficient, however, as compounds without oxygen were not included: ammonia also neutralizes an acid.

In 1923 Johannes Nicolaus Brønsted presented his model. It has become widely accepted and has proven very effective, especially in analytical chemistry . According to his theory, the base and acid interact in a proton transfer reaction. Bases take up protons from acids. The model, also presented by Gilbert Newton Lewis in 1923, is helpful when considering reaction processes in organic chemistry and complex chemistry and goes beyond the usual definitions. Therefore one speaks preferentially of Lewis base and Lewis acid. Many compounds normally referred to as acids are not acids according to this model. The concept of hard and soft acids and bases ( HSAB concept ) was developed by Ralph G. Pearson in 1963 and thus expanded the way of looking at reactions in organic and complex chemistry.

What are bases?

The presence and certain properties of the water are usually closely related to bases, and often without explicit mention . Pure water is subject to what is known as autoprotolysis , in which oxonium ions (H 3 O + ) and hydroxide ions (OH - ) are formed in very low and equal concentrations :

In this reaction equation for water, the property of a base is shown by the formation of OH - ions in water. At the same time, H 3 O + ions are formed in water, a property that characterizes an acid. However, water is neither called a base nor an acid and its behavior is called neutral. This relates to the pH value , which indicates the concentration of H 3 O + ions in water. Pure water has a pH of 7, a very small concentration. This reaction is - like all the reactions described in this section - an equilibrium reaction: The formation of the ions and their combination to form water takes place continuously and with the same frequency.

Many compounds called bases have hydroxide ions (OH - ) and dissociate in water into metal and hydroxide ions. The solution is often referred to as an alkaline solution or lye . The solid sodium hydroxide (NaOH) in water forms the so-called sodium hydroxide solution and potassium hydroxide (KOH) forms the potassium hydroxide solution .

Other compounds do not have any OH - ions themselves , but form them in a reaction with water. They react alkaline by taking over a proton H + from an H 2 O molecule and thus leave an OH - ion behind. For example, the salt trisodium phosphate (Na 3 PO 4 ) or the salt sodium carbonate (Na 2 CO 3 ) forms hydroxide ions in aqueous solution. Organic compounds such as salts of carboxylic acids and amines, derived from ammonia , also react in the same way . The caustic effect of all these bases is essentially due to the formation of OH - ions.

Basic reactions

Acetate ion Acetic acid skeleton

In the case of weak and moderately strong bases, all components involved in the reaction are in the solution in the equilibrium reactions. Two of the reactants differ only in the presence or absence of a proton (H + ). They form a corresponding acid-base pair. Particles that have a suitable proton are called proton donors ; Particles that have the ability to accept a proton are called proton acceptors . The entire reaction is called protolysis . The strength of a base is described by the position of the equilibrium (the base constant ).

With strong and very strong bases, the equilibrium reactions are completely on the side of the OH - ions. This is the case, for example, with the reaction of alkali hydroxides with water:

The Na + cation does not play a role. The hydroxide ion is the actual base and water is the proton donor:

Because of this equilibrium, strong and very strong bases (such as sodium ethoxide and other superbases ) can no longer be distinguished in aqueous solution by their alkalinity . Here one speaks of the leveling effect (from French niveler , 'equalize' ) of the water. In order to be able to distinguish even very strong bases with regard to strength, equilibrium constants are determined in non-aqueous solutions and these are approximately transferred to the solvent water.

Water plays an important role in acid-base reactions. In addition to the protolysis described above, water is also capable of what is known as autoprotolysis . It can give up protons and form OH - , or take up protons and form H 3 O + . One is a reaction as a base and the other is a reaction as an acid. That is why water is called an ampholyte .


Certain compounds are called bases because of their special chemical properties. The wide range of these chemicals can be classified into groups according to various characteristics. Bases can be divided into neutral, anionic or cationic bases according to their ionic charge . Ammonia (NH 3 ) has no ionic charge and is therefore a neutral base. Sodium hydrogen carbonate can be designated as an anionic base , since the anion HCO 3 - is present in solution . The hydroxide anion (OH - ) itself can also be referred to as an anionic base.

Another way of classification is to subdivide into monovalent or divalent bases. In solution, sodium hydroxide (NaOH) forms one OH - and is monovalent, calcium hydroxide (Ca (OH) 2 ) forms two OH - per Ca and is therefore bivalent.

As Basenbildner can be described compounds in which before the basic reaction yet another chemical reaction is upstream. The metal oxides which form the corresponding hydroxides when dissolved in water can be described as base formers . Calcium oxide (CaO) forms the base Ca (OH) 2 with water . Base metals such as alkali metals can be oxidized beforehand by the action of water. In the vigorous reaction of sodium , hydrogen is also developed in addition to the sodium hydroxide solution . Also amphoteric oxides may represent Basenbildner. Electron donors of the base formers are in the left part of the periodic table .

Types example reaction
neutral bases Ammonia (NH 3 )
anionic bases Sodium hydrogen carbonate (NaHCO 3 )
cationic bases [Al 3+ (OH) - (H 2 O) 5 ] in aqueous solution
monovalent bases Sodium hydroxide (NaOH)
Potassium hydroxide (KOH)

divalent bases Calcium hydroxide (Ca (OH) 2 )
bases formers base metals, such as the alkali metals
Calcium Oxide (CaO)
Barium Oxide (BaO)

Properties of bases

Contact of ammonia water and hydrochloric acid: Here the gases hydrogen chloride react as acid and ammonia as base to form ammonium chloride smoke - a neutralization
  • Many bases are soluble in water (e.g. sodium hydroxide , ammonia ), but not all (e.g. aluminum hydroxide )
  • They are corrosive and have a destructive effect on organic substances.
  • They form soaps and glycerine from oils and fats .
  • There are strong and weak bases .
  • Bases can be diluted with water; depending on the dilution, their effect is significantly weaker.
  • The alkaline solutions lead to a reddening of phenolphthalein and color red litmus paper blue.
  • The "opponents" of the bases (base solution = lye ) are the acids (see illustration). You can neutralize bases. Acids are also corrosive and attack many other substances that do not necessarily react with bases.
  • Clothing, skin and eyes are at risk on contact. Care must be taken to wear protective goggles, as chemical burns can always occur.


The basis of neutralization is based on the fact that when mixed with a base, the effects of an acid do not add but cancel each other out. Thus, a base can be neutralized with an appropriate amount of an acid . Bases and acids react to form water .

Reaction of sodium hydroxide in and with water to form sodium hydroxide solution :

Reaction of hydrogen chloride in and with water to form hydrochloric acid :

Reaction of a sodium hydroxide solution with hydrochloric acid (neutralization):

Sodium hydroxide + hydrochloric acid reacts to form dissolved sodium chloride and water .

The crucial process is the reaction between the hydroxide and oxonium ions :

Acid-base balance

The strength of a base is known as its basicity and is described by the base constant. The base constant ( K b ) describes the position of the equilibrium in the reaction between an acid-base pair in aqueous solutions. The negative decadic logarithm of K b , the so-called p K b value, is often given.

In the reaction

The base constant K b is defined as follows:

, with c (X) = concentration of X

The p K b value is accordingly:


Acid-base reactions without water

Analogous to the acid-base reactions that take place in aqueous solutions and with the participation of water, there are reactions in other media. In anhydrous ethanol , a reaction takes place with hydrogen chloride , in which ethanol takes on the role of a base:

In the gas phase, the gases ammonia and hydrogen chloride react to form the salt ammonium chloride .

In addition to water, other sufficiently polar solvents can also act as reactants in acid-base reactions. A good example is the autoprotolysis of liquid ammonia:

Web links

Commons : Basen  - collection of images, videos and audio files

Individual evidence

  1. Springer Fachmedien Wiesbaden GmbH: Chemistry: Basics, technical applications, raw materials, analytics and experiments . 11th, revised and expanded edition. Wiesbaden, ISBN 978-3-658-27502-0 .