Study Palace: Factors that are responsible for the strength of acids and bases

(1) Size of metal ions:

Base	Size of metal ion (Å)	Charge of ion	Qualitative strength
LiOH	0.60	+1	Weak base
NaOH	0.95	+1	Strong base
KOH	1.33	+1	Very strong base
CsOH	1.69	+1	Strongest base
Mg(OH)₂	0.65	+2	Weaker base
Al(OH)₃	0.50	+3	Weaker base (amphoteric)

(2) Ionization constant: The smaller the value of ionization constant (K), the less is the degree of ionization and hence less strong the acids (or bases).

H₃PO₄ « H⁺ + H₂PO₄^-¹

(3) Oxidation number: The oxidation number of the central atom determines the strengths of the acids.

(4) Ionic potential: The ratio between the charge and the radius of an ion is called ionic potential.

(5) Electro negativity: The decreasing electro-negativity gives rise to decreasing strengths of acids in the order.

F > O > N , HF > H₂O > NH₃

Decreasing the atomic size of halogens decreasing strengths of halogens acids (ii). Increasing bond length, decreasing bond energy, easily gives up proton. Bond distance of HI (1.7Å) is greater than HF(1.0Å).

HI > HBr > HCl > HF

(6) Number of oxygen and hydrogen atoms: The greater the difference between the number of oxygen and number of hydrogen atoms, the stronger the acid.

H₂SO₄ > H₂SO₃, HNO₃ > HNO₂

(7) Number of non-hydrogenated oxygen molecule: The greater the number of non-hydrogenated O atoms per molecule, the greater is the strength of acid.

Hard and Soft Acids and Bases (HSAB). HSAB is an extremely useful qualitative theory that enables predictions of what adducts will form in a complex mixture of potential Lewis acids and bases.
Hard acids (HA) are characterized by (s,f blocks, left side of d block in higher OS's)

Low electronegativity values in the range 0.7-1.6 of the metal atom;
Relatively small size;
Relatively high charge (> 3+).

High charge often results in small size, because the remaining electrons are contracted toward the nucleus by the substantial excess positive charge. Specific examples of hard acids are the metal cations from the s and f blocks, and the higher-charged ions from the left side of the d block. Na⁺, Mg²⁺, Fe³⁺, and Al³⁺ are examples of hard acids. Exceptions- H⁺, B⁺³, C⁺⁴ are hard acids.
Hard bases (HB) are characterized by

Very high electronegativity values in the range 3.4-4 of the donor atom;
Relatively small size.

The combination of high electronegativity and small size results in a nonpolarizable electron cloud surrounding the donor atom. The only 2 donor atoms with electronegativities in the specified range are oxygen and fluorine. So the hard bases are those in which the donor atom is either O or F. Specific examples are O^2-, F^-, SO₄^2-, CO₃^2-, and PO₄^3-.
Soft acids (SA) are characterized by an acceptor atom of

intermediate to high electronegativity (1.9-2.5);
large size;
low charge (1+, 2+)

Species of large size generally have many electrons, some of which can be quite far from the nucleus. The low charge of the species results in a polarizable (distortable) electron cloud. Specific examples of soft acids include Cu⁺, Hg²⁺, Au₊, Ag⁺, and Pb²⁺. Note that these metals are all clustered in the same region of the periodic table.
Soft bases (SB) are characterized by donor atom of

intermediate to high electronegativity (2.1-3.0)
large size, leading to polarizability

Specific examples of soft bases are S^2-, PEt₃, RSe-, I-, and Br-. Note that these fall in groups 15-17 in periods with n > 3.

Applications of the HSAB Principle.

1) Predicting favorable equilibria.
2) Geochemistry of the elements.
3) Toxicology, Medicinal Chemistry. Ions of many so-called heavy metals, such as Hg²⁺ and Pb²⁺, are highly toxic. Why? Heavy metal ions are soft acids, and therefore have high affinity for S^2-, a soft base. S occurs in the side chains of two amino acids, methionine and cystine, and is important in maintaining tertiary structure of proteins and enzymes upon which life depends. Ingested heavy metal ion seeks out and coordinates with amino acid sulfur, disrupting protein structure and deactivating the protein. Eventual death is the usual result of prolonged exposure to heavy metal ions.
Exercise: Why are HCN, CO, H₂S, H₂Se, and PH₃ poisons?

4) Ligand selections in metalloproteins and enzymes. Ions of many of the 3d transition metals are essential for life in trace amounts. A number of proteins and enzymes incorporate these metal ions specifically into their structures, forming adducts with the metal ion using donor atoms on the side chains of their amino acids. Side chains containing oxygen, nitrogen, and sulfur donors are usually involved in adduct formation.

Exercise: What donor atoms might be appropriate for binding Cu⁺ in an enzyme or protein? Na⁺? K⁺? Ca²⁺?

5) Reduction Potentials. The electron has been termed the "ultimate soft base". Viewing the electron in these terms, standard reduction potential can be understood in terms of HSAB theory. Several standard reduction potentials are given below.

Fe³⁺(aq) + 3e-   → Fe(s),     e^o = -.036
Cu⁺(aq) + e-     → Cu(s),     e^o = .522
Na⁺(aq) + e-     → Na(s),     e^o = -2.711
La³⁺(aq) + 3e- → La(s),      e^o = -2.37

Recall that the more positive the value of e^o, the more spontaneous the reduction. The data reveal that the e^o values parallel the hardness/softness of the metal cation (acid). Thus the soft acid, Cu⁺, interacts quite spontaneously with the soft electron. The hard acids, Fe³⁺, Na⁺, and La³⁺ are very difficult to reduce because their interaction with the soft electron is not favorable.

Basic Principles of the Various Theories of Acids & Bases
Traditional approach	Substance that have certain properties Acid: sour taste, turns litmus red; Base: bitter taste, turns litmus to blue
Arrhenius	Acid : give H⁺ in aqueous solution Base : give OH^- in aqueous solution At neutrality: [H⁺] = [OH^-]
Bronsted-Lowry	Acid : H⁺ donor Base : H⁺ acceptor Conjugate acid-base pairs No concept of neutrality
Lewis	Acid : a potential electron-pair acceptor Base : a potential electron-pair donor
Usanovich	Acid: a substance that donates a cation, or accepts an anion or an electron Base: a substance that donates an anion, or accepts a cation.