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Hard - Soft Acids and Bases

Introduction

The most common theory of acids and bases are the "Lewis theory" and "Lowry-Bronstead Concept"

An acid is a proton donor and a base is a proton acceptor according to Lowry-Bronstead theory. Weak acids in solution exists as an equilibrium mixture of the undissociated acid and the ions formed by dissociation.

In acid base reaction, the equilibrium is always in favor of the weak acid and weak base. In the above equilibrium, it would be in favor of acetic acid and water because acetate and hydronium ion are stronger base and acid.

However the theory cannot explain features of certain reactions

HgCl2   +   H2S   --->   2HCl   +   HgS

The above reaction takes place spontaneously and the reaction is almost complete. This means it is in favor of the product side which include a strong acid and a strong base. This can be explained by using the Hard and Soft Acid Base theory.

According to this theory a soft acid will react with a soft base more easily. Similarly a hard acid will react with a hard base.

Hg2+ is a soft acid and S2- is a soft base. So interaction between these two takes place very easily.

Lets examine what these terms mean.

  • This concept is classified into hard, soft and borderline acids bases.
  • Hard acids and bases have a tendency to form ionic bonds while soft ones form covalent bonds.
  • Interactions between soft acid and soft base or hard acid and hard base are more effective.

Characteristics:

Soft acids:

The acceptor atoms are larger, have lower positive charge, have unshared pair of electrons in their valence shell. Also they have higher polarisability and lower electronegativity.

Soft bases:

The donor atoms are smaller, have higher positive charge, donot have unshared pair of electrons in their valence shell. Also they have higher polarisability and lower electronegativity. They are easily oxidised and the valence electrons are loosely held.

Hard acids:

The acceptor atoms are smaller in size, have higher positive charge, no unshared pair of electrons in their valence shell. Also they have lower polarisability and higher electronegativity.

Hard bases:

The donor atoms have higher electronegativity, lower polarisability, difficult to oxidise. The valence electrons are held tightly.

Absolute Hardness:

The average of the difference between the ionisation energy and electron affinity is termed as Absolute Hardness (η)

η   =   (I-A)  /  2

The reciprocal of η is termed as Absolute Softness (σ)

Some examples:

Hard Bases: OH-, F- , NH3

Soft Bases: I-, R3P, CN-, CO

Borderline Acids: Zn2+, Cu2+, Fe2+

Borderline Bases: Br-, N3-, NO2-, ArNH2

Hard Acids: H+, Li+, Mg+, BF3, AlCl3

Soft Acids: Cu+, Ag+, Hg2+, I2