Question

RD AND SOFT ACIDS AND BASES (HSAB)
(a) What are the characteristics of a hard acid and a hard base ?
(b) Why is pyridine a border line base while ammonia is a hard base ?
(c) What are the limitations of HSAB principle ?
(a) What are the characteristics of a soft acid and a soft base ?
(b) Classify the following into hard, soft and border line acids and bases
I, K+, HCI, CO, Ni2+, CO2, Ag+, NHz, Cut, 17+
(a) Hard-hard interaction is the major driving force for a reaction to pro
How does HSAB principle explain the validity of the following reactions
(1) Lil + CSF
LiF + CSI
(ii) Cul2 + 2CuF2
CuF, + 2Cul.
Fynlain the following​

Answer 1

a) Hard acids consist of small highly charged cations and molecules in which a high positive charge can be induced on the central atom. Hard bases are highly electronegative and of low polarizability. Hard bases react more readily to form stable compounds and complexes with hard acids.

b) The conjugate acid of pyridine (pyr. H+ (protonated on nitrogen)) is stablised by the electron releasing effect of the alkyl groups, but cannot form hydrogen bonds. Overall, this results in it being a weaker base.

c) Since hydrogen ion, H+ is a hard acid and hydride ion, H- is a soft base, according to HSAB principle the interactions between them must be polar covalent and H2 must be unstable. Indeed H2 is a stable molecule with pure covalent nature.

Answer 2

Concept

The HSAB concept is an initialism for "hard and soft (Lewis) acids and basses" and it is also known as Pearson acid-base concept

Given

RD AND SOFT ACIDS AND BASES (HSAB)

Find

1. The characteristics of a hard acid and a hard base
2. The limitations of HSAB principle
3. The HSAB principle

Solution

1. The characteristics of a hard acid and a hard base

• "Hard" acids and bases have a high charge to ionic radius ratio (positive for acids, negative for bases) as well as greater oxidation states.
• Hard acids have large charge densities and are not highly polarizable.

2. Limitations of HSAB principle

• Hard acids prefer to bind with hard base
• Soft acids prefer to bind with soft base
• Hard metal ion prefer to bind with hard ligands
• Soft metal ion prefer to bind with soft ligands

3. The HSAB principle

• HSAB is widely used in chemistry for explaining stability of compound, reaction mechanism and pathway
• It assigns the terms hard or soft and acid or base to chemical species
• Hard applies to species which are small and have high charge states
• Soft applies to species which are big and have low charge states and strongly polarizable

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