Soft and hard acids and bases

A+ A-

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Soft and hard acids and bases (SHAB) principles are very helpful for study the stability of the complex between acid and bases and it is very useful for study online college courses. This principle was proposed in 1963 by Ralph G. Pearson.

A + :B → A : B

According to the SHAB principle, the complex most stable when participating acid and base are either both soft or both hard and least stable when one of the reactants (namely acids and base) is very hard and the other one is very soft.

To study the donner properties of different bases and preferences of a particular base to bind acid. Hydrogen ion and methyl mercury (II) ion is used for this comparison study.

Proton and methyl mercury cation have only one coordination position to accommodate only one coordinate bond but the two cations vary widely in their preferences to bases.

This preference was estimated from the experimental determination of equilibrium constants for the exchange reactions.

BH⁺ + [CH₃Hg(H₂O)]⁺ ⇄ [CH₃HgB]⁺ + H₃O⁺

Experimental results indicate that bases in which nitrogen be a donor atom, oxygen or fluorine prefer to coordinate with the hydrogen ion. Bases which has donor atom phosphorus, sulfur, iodine, bromine, chlorine or carbon prefers to coordinate with mercury.

The neutralization reaction of acids and bases

Lewis defined, an acid-base neutralization reaction involves an interaction of a vacant orbital of an acid and a filled or unshared orbital of a base.

A + : B A: B
Lewis acid Lewis base
Adduct

The species A is called Lewis acid or a generalized acid and B is called Lewis base or a generalized base. Strong acid and a strong base will form a stable complex.

What are soft and hard bases?

The donor atoms of the second category are of low electronegativity, high polarizability, and are easy to oxidize. Such donors have been called ‘soft bases' by Pearson since they are holding on to their valence electrons rather loosely.

The donor atoms in the first group have high electronegativity, low polarisability and hard to oxidize. Such donors have been named ‘hard bases' by Pearson since they hold on to their electrons strongly.

Question
Classify the following as soft and hard acids and bases.

  1. Hydride ion.
  2. Nickel (IV) ion.
  3. Iodine (+1) ion.
  4. Hydride ion.

Answer
  1. The hydride ion has a negative charge and too large in size compared to the hydrogen atom. Electronegativity of hydride ion quite low. Due to high polarizability and large size, the valence electron is loosely bound in hydride ion and it is a soft base.
  2. The nickel (IV) has quite a high positive charge and small size compared to the nickel (II). Electronegativity of nickel (IV) will be very high and polarisability will low. Hence it is hard acid.
  3. Mono-positive iodine has a low positive charge and has a large size. It has a low electronegativity and high polarizability.
  4. Hydrogen ion has the smallest size with a high positive charge density and absence of unshared pair of electrons in its valence shell. All these will give a high electronegativity and very low polarizability. Hence hydrogen ion is a hard acid.

Study the properties of soft and hard bases

In simple terms, hardness associated with a tightly held electron shell with little tendency to polarize. On the other hand, softness associated with a loosely bound polarizable electron shell.

It will be seen that within a group of the periodic table softness of the Lewis bases increases with the increase in the size of the donor atoms. Among the halide ions, softness increases in the order.
F⁻ㄑCl⁻ㄑBr⁻ㄑI⁻
Online college courses for soft and hard acids bases
Soft and hard acids and bases
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What are soft and hard acids?

After having gone through a classification of bases, a classification of Lewis acids is necessary. The preferences of a given Lewis acid towards ligands of different donor atoms are usually determined from the stability constant values of the respective complexes or from some other useful equilibrium constant measurements.

When this is done, metal complexes with different donor atoms can be classified into two sets based on the sequences of their stability.

Hard acids have small acceptor atoms, are of high positive charge and do not contain unshared pair of electrons in their valence shell, although all these properties may not appear in one and the same acid.
These properties lead to high electronegativity and low polarizability. In keeping with the naming of the bases, such acids are termed as 'hard acids'.

N≫P; O≫S; F〉Cl〉Br〉I

Soft acids have large acceptor atoms, are of low positive charge and contain ushered pairs of electrons in their valence shell. These properties lead to high polarizability and low electronegative. Again in keeping with the naming of the bases, such acids are termed 'Soft acids'.

N≫P; O≫S; F〉Cl〉Br〉I
Hard AcidsBorderline acidsSoft Acids
H⁺, Li⁺, Na⁺
K⁺, Be⁺², Mg⁺², Ca⁺², Sr⁺², Mn⁺², Al⁺³, Ga⁺³, In⁺³, La⁺³, Lu⁺³, Cr⁺³, Co⁺³ Fe⁺³, As⁺³, Si⁺⁴, Ti⁺⁴, U⁺⁴, Ce⁺³, Sn⁺⁴, VO⁺², UO₂⁺², MoO⁺³
Fe⁺, Co⁺², Ni⁺², Cu⁺², Zn⁺², Pb⁺², Sn⁺², Sb⁺², Bi⁺², Rh⁺², B(CH₃)₃, SO₂, NO⁺, GaH₃Cu⁺, Ag⁺, Au⁺, Tl⁺, Hg⁺, Pd⁺, Cd⁺, Pt⁺, CH₃Hg⁺, Tl⁺³, BH₃, GaCl₃, InCl₃, I⁺, Br⁺, I₂, Br₂, and zerovalent metal atom

SHAB principle for soft and hard acids and bases

This principle also means that if there is a choice of the chemical reaction between an acid and two bases and two acids and a base, A hard acid will prefer to combine with a hard base and a soft acid will prefer to combine with soft base and thus a more stable product will be obtained.

The hard acid - hard base may interact with strong ionic forces. Hard acids have small acceptor atoms and positive charge while the hard bases have small-donor atoms but often with a negative charge. Hence a strong ionic interaction will lead to the hard acid-base combination.

On the other hand, a soft acid - soft base combination mainly a covalent interaction. Soft acids have large acceptor atoms, are of low positive charge and contain ushered pairs of electrons in their valence shell.

Application of SHAB principle in acid chemistry

The SHAB concept is extremely useful in elucidating many properties of chemical elements in acid chemistry and will be often referred to at appropriate places.
  1. Boron trifluoride and boron trihydride the behavior of boron is different in these two compounds.
    The presence of hard fluoride ions in boron trifluoride easy to add hard bases.
    The presence of soft hydride ions in boron trihydride easy to add soft bases.
  2. [CoF₆]⁻³ more stable than [CoI₆]⁻³. It will be seen that cobalt (III) ion is a hard acid, fluoride ion is a hard base and iodide ion is a soft base.
    In [CoF₆]⁻³, hard acid, and hard base form a stable complex than the [CoI₆]⁻³ form by a hard acid and soft base.
  3. The existence of certain ores can also be rationalized by applying the SHAB principle. Thus hard acids such as magnesium, calcium, and aluminum occur in nature as magnesium carbonate, calcium carbonate, and aluminum oxide but not as magnesium calcium and aluminum sulfides
    Since the anion CO₃⁻² and O⁻² are hard bases and S⁻² is a soft base. Soft acids such as Cu⁺, Ag⁺ and Hg⁺², on the other hand, occur in nature as sulfides.
  4. The borderline acids such as Ni⁺², Cu⁺², and Pb⁺² occur in nature both as carbonates and sulfides. The combination of hard acids and hard bases occurs mainly through ionic bonding as in Mg(OH)₂ and that of soft acids and soft bases occurs mainly by covalent bonding as in HgI₂.
Question
Why AgI2- stable, but AgF2- does not exist

Answer
We know that mono positive silver ion is a soft acid, fluoride ion is hard to base and iodide ion is the soft base.
Hence AgI₂⁻ (soft acid + soft base) is a stable complex and AgF₂⁻ (soft acid + hard base) does not exist.

Question
Mercury hydroxide dissolved readily in acidic solution but mercury sulfide does not?

Answer
In the case of mercury hydroxide and mercury sulfide, mercury is a soft acid and hydroxide and sulfide are a hard base and soft base respectively.

Mercury sulfide formed by soft acid and soft base will be more stable than mercury hydroxide formed by soft acid and hard base.

More stability of mercury sulfide than that of mercury hydroxide explains why mercury hydroxide dissolved readily in acidic solution but mercury sulfide does not dissolve readily in acidic solution.

Online college courses for soft and hard acids bases, Neutralization of acids and bases, a study and application of SHAB principle in chemistry

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