What is hard acid base
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Area of Expertise - General Chemi
The HSAB concept (also HSAB principle; engl.: "hard and soft acids and bases", German translation: concept of hard and soft acids and bases) was developed in 1963 by R. G. Pearson. It is a classification principle for Lewis acids and bases with special consideration of size, charge, electronegativity and polarizability:
- Hard acids:
- Lewis acids with low polarizability, i.e. small and highly charged cations as well as molecules in which an electronegative binding partner induces a high positive charge on the central atom (electrophilic center), e.g.,,.
- Soft acids:
- Lewis acids with high polarizability, i.e. cations with low charge and large radius or atoms and molecules with occupied outer orbitals (easily removable valence electrons), e.g.,.
- Hard bases:
- Lewis bases with high electronegativity and low polarizability, e.g.,,.
- Soft bases:
- Lewis bases with low electronegativity and greater polarizability, e.g.,.
According to Pearson, the following applies to the course of an acid-base reaction: hard acids react preferentially with hard bases, soft acids react more with soft bases.
The HSAB concept is only of qualitative character, but allows far-reaching assessments of the course of reactions, the position of equilibria and the stability of reaction products.
See also: Lewis acid, Lewis base
Learning units in which the term is dealt with
Acid-base theories40 min.
chemistryGeneral Chemistryacids and bases
This learning unit deals with the different acid-base theories.
Reactivity: hardness, polarizability, HSAB concept30 min.
chemistryTheoretical chemistryChemical reactivity
In this learning unit, HOMO and LUMO energies are presented as a quantitative measure of the reactivity of inorganic nucleophiles and electrophiles.
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