What is Lewis base and Broensted acid

The terms acid and base have been different in chemistry Concepts based on different definitions of terms. The drive for this development is based on the one hand on the search for a definition that is as comprehensive and generally applicable as possible, and on the other hand on a specific area of ​​application of a concept. The definition according to Brønsted and Lowry deals with proton transfer reactions (also) in water. It is more of a more specific definition, but it has a very broad application in chemistry and is the most common basis of the acid-base definition.

Definition according to Arrhenius

In 1887, S. Arrhenius saw the characteristic feature of an acid as being in aqueous solution to freely moving, positively charged hydrogen ions (H+Ions) and freely moving negatively charged anions dissociated. In contrast, an Arrhenius base dissociated into cations and OH-Ions. In this definition, acids and bases are limited to water as a solvent. In contrast to later model ideas, Arrhenius said nothing about the interactions of the hydrogen ions with water. Acid or basic reactions of substances that are neither H.+ still OHIons can be given off are not explained with this model.

  • Example of an acid according to Arrhenius:
  • Example of a base according to Arrhenius:

Definition according to Brønsted and Lowry

Johannes Nicolaus Brønsted and Thomas Lowry independently described one in 1923 acid as a particle containing protons (H.+Ions) to a reaction partner, the so-called base can transfer. In contrast to Arrhenius, bases and acids are no longer certain classes of substances, but rather defined particles that have certain properties in a reaction with H.+-Ions show. A Acid-base reaction is called Protolysis.

  • One denotes the particles that the protons can give off, thus as Acids or proton donor.
  • Particle, Which Can absorb protons, will be as Bases and consequently referred to as proton acceptors.

Free protons (H.+) do not exist at any point in time. Every acid reaction is related to a base to which the acid has a proton transfer can. Such systems are called Acid-base pairs between which a chemical equilibrium is established.

In this reaction equation, HX and HY are+ Acids, Y and X Bases. Acids and bases differ in their ability to accept or release protons. The most important reactions in practice are reactions with water:

The position of equilibrium of this reaction is determined by the Acid starch determined and is described by the acid constant of the acid HX.

If a chemical substance can both absorb and release protons, one speaks of an ampholyte or the property of being amphoteric. The best-known ampholyte is water, which causes both the formation of OH as well as H3O+ allowed.

In contrast to Arrhenius, Brønsted and Lowry's concept of acid also explains the acid-base reaction of hydrogen chloride (HCl) and ammonia (NH3) Gas to ammonium chloride (NH4Cl) despite the absence of water.

Examples of the Brønsted reaction:

Lewis definition

Gilbert Newton Lewis published a treatise on his acid-base theory in 1938/1939. Accordingly, a Lewis acid is an electrophilic electron pair acceptor and a Lewis base is an electron pair donor.

The Lewis acids include:

All Brønstedt and Lowry bases are also Lewis bases.

Example of a Lewis acid-base reaction

The Lewis acid AlCl3 reacts with the Lewis base Cl- forming the Lewis acid-base adduct AlCl4-.

Definition according to lux and flood

The focus of the concept established by Lux in 1939 and expanded by Flood in 1947 is the oxide ions instead of protons. This was set up in order to be able to describe acid-base reactions also in proton-free systems, as occurs in inorganic melts.

According to Lux and Flood, acids are oxide ion acceptors and bases are oxide ion donors. One considers non-metal oxides (e.g. SO2, CO2) as acid anhydrides, since they react acidic in aqueous solution, correspondingly are metal oxides (e.g. MgO, Fe2O3) Base anhydrides, as they form hydroxide ions in aqueous solution.

Definition according to Usanovich

In 1939, the Russian scientist Usanovich came up with the following definition of the acid-base term:

An acid is any chemical compound that reacts with bases, releases cations or accepts anions or electrons. Correspondingly, a base is any compound that reacts with acids, releases anions or electrons or combines with cations.

This definition of terms includes the reactions according to the Lewis concept, extends the same by the fact that the uptake or release of electrons is not limited to common pairs, and includes all redox reactions in which a more complete Electron transfer is involved.

One point of criticism of this uncommon theory is that it is too general, and the term Acid-base reaction is applicable to almost any type of response.

Pearson's concept of hard and soft acids and bases

Ralph G. Pearson developed the concept of hard and soft acids and bases in 1963 (Hard and S.often A.cids and B.ases, HSAB concept). It is said:

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

The electronegativity and the polarizability of the observed particle are considered:

Hard acids to have: Hard bases to have: Soft acids to have: Soft bases to have:
low electronegativity
low polarizability
high electronegativity
low polarizability
high electronegativity
high polarizability
low electronegativity
high polarizability
z. E.g .: H.+, N / A+, K+ z. E.g .: OH, F, SO42− z. E.g .: Cu+, Ag+, I.2 z. B .: I., SCN, R2S.

The concept reflects tendencies, there are few absolutely hard or soft particles. However, it helps in estimating the stability of connections. So z. B. the softer Fe2+ in nature as a sulphide, while the harder Fe3+ is present as a hydroxide or oxide.

Category: acid-base reaction