Physical Inorganic Chemistry : THE STRENGTH OF ACIDS & BASES 4/10/2019

1. The Bronsted-Lowry Concept Acid Strength is a measure for the tendency of an acid to lose proton : An acid is called to be stronger as the tendency increases. Base Strength is a measure for the tendency of an acid to gain proton : A base is called to be stronger as the tendency increases. 4/10/2019

HF HCl HBr HI CH4 NH3 H2O HF Acid strength increases 4/10/2019

The concept is essentially based on electrostatic interaction between the corresponding acids and bases 4/10/2019

HA + H2O H3O+ + A- [H3O+][A-] Ka = [HA] - ∆Ho/RT Ka = e 4/10/2019

Acid Conjugate Base K H2SO3 HSO3- 1.72 x 10-2 H3PO3 H2PO3- 1.60 x 10-2 HNO2 NO2- 4.00 x 10-4 H2CO3 HCO3- 4.30 x 10-7 SO3= 6.24 x 10-8 HPO4= 6.20 x 10-8 H2S HS- 5.70 x 10-8 HCN CN- 7.20 x 10-10 NH4+ NH3 5.50 x 10-10 CO3= 5.60 x 10-11 PO43- 4.80 x 10-13 S= 1.20 x 10-15 4/10/2019

2. The Lewis Concept Acid Strength is a measure for the tendency of an acid to accept an electron pair to form chemical bond : An acid is called to be stronger as the tendency increases. Base Strength is a measure for the tendency of a base to donate an electron pair to form chemical bond : A base is called to be stronger as the tendency increases. 4/10/2019

The Strength of Bases Donor of Electron Pair 4/10/2019

The ability of a base to furnish a pair of electrons to an acceptor depends on : δ- δ+ The more negatively charged the donor atom, the more effective the chemical bond formed THE NEGATIVE PARTIAL CHARGE OF THE DONOR ATOM 4/10/2019

δ- δ+ Involvement of the Donor in Bond Multiplicity Capacity of the Groups Attached to the Donor THE NEGATIVE PARTIAL CHARGE OF THE DONOR ATOM Polarizability of the Donor Concentration of the Donor Electron Pair 4/10/2019

Acceptor of Electron Pair The Strength of Acids Empty Orbital Acceptor of Electron Pair 4/10/2019

The ability of an acid to gain a pair of electrons from a donor depends on : The more positively charged the acceptor atom, the more effective the chemical bond formed THE POSITIF PARTIAL CHARGE OF THE ACCEPTOR ATOM 4/10/2019

The more empty orbitals available in the acceptor atom, the more effective the chemical bond formed THE EMPTY ORBITALS AVAILABLE IN THE ACCEPTOR ATOM 4/10/2019

The concept is essentially based on a covalent interaction between the corresponding acids and bases 4/10/2019

THE CONCEPT OF HSAB HARD AND SOFT ACIDS AND BASES 4/10/2019

? The Goldschmidt (1935) : Geochemical Ore Classification Nickel (Ni), zinc (Zn), and copper (Cu) ores are exclusively found as sulfides, whilst aluminum (Al) and calcium (Ca) are usually found as oxide and carbonate, respectively. ? 4/10/2019

An acid = an electron pair acceptor A base = an electron pair donor The Lewis concept of acids and bases involves covalent interaction to form a covalent (coordination) bond : An acid = an electron pair acceptor A base = an electron pair donor 4/10/2019

Polarizability The capacity of a group of atoms in a molecule and/or an ion to polarize its electron. δ- δ+ 4/10/2019

The Elements Li Be B C N O F Na Mg Al Si P S Cl K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi - 4/10/2019

The Elements Elements tend to form bases Elements tend to form acids Li Be B C N O F Na Mg Al Si P S Cl K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi - Elements tend to form acids 4/10/2019

Elements tend to form bases Harder Base Electronegativity factors Softer Base N O F P S Cl As Se Br Sb Te I Size factor 4/10/2019

Elements tend to form acids Harder Acid Electronegativity factors Li Be B C Na Mg Al Si K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Softer Acid Size factor 4/10/2019

Hard & Soft Acids Li Be B C Na Mg Al Si K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Hard Acid Soft Acid Borderline 4/10/2019

The HSAB Concept : HSAB classification of metal ions (acids) : Perason, R.G., Journal of American Chemical Society, 1963, 85, 3533–3539. HSAB classification of metal ions (acids) : class (a) metal ions : class (b) metal ions Alkali : H+, Li+, Na+, K+, Rb+, and Cs+, Heavier transition metals of lower oxidation states : alkaline earth: Be2+, Mg2+, Ca2+, Sr2+, and Ba2+, Cu+, Ag+, Hg+, Hg2+, Pd2+, and Pt2+. Lighter transition metals of higher oxidation states : Ti4+, Cr3+, Fe3+, and Co2+. 4/10/2019

HSAB classification of bases (ligands) : Tendency to complex with class (a) metal ions : Tendency to complex with class (b) metal ions N >> P > As > Sb N << P < As < Sb O >> S > Se > Te O << S < Se < Te F > Cl > Br > I F < Cl < Br < I 4/10/2019

class (a) metal ions : class (b) metal ions Tendency to complex with class (a) metal ions : Tendency to complex with class (b) metal ions N >> P > As > Sb N << P < As < Sb O >> S > Se > Te O << S < Se < Te F > Cl > Br > I F < Cl < Br < I class (a) metal ions : class (b) metal ions H+, Li+, Na+, K+, Rb+, Cs+, Be2+, Mg2+, Ca2+, Sr2+, Ba2+, Ti4+, Cr3+, Fe3+, and Co2+. Cu+, Ag+, Hg+, Hg2+, Pd2+, and Pt2+. 4/10/2019

Hard Acids Hard Bases Soft Acids Soft Bases 4/10/2019

The HSAB Concept : Hard Acids prefer to form complex with Hard Bases and Soft Acids prefer to form complex with Soft Bases 4/10/2019

HSAB Classification of Lewis Acids and Bases Hard Borderline Soft ACIDS H+, Li+, Na+, K+, Rb+, Cs+, Be2+, Mg2+, Ca2+, Ba2+, Cr3+, SO3, BF3. Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, SO2, BBr3. Cu+, Ag+, Au+, Tl+, Hg+, Pd2+, Cd2+, Pt2+, Hg2+, BH3. BASES F-, OH-, H2O, NH3, CO32-, NO3-, O2-, SO42-, PO43-, ClO4-. NO2-, SO32-, N3- Cl-, C6H5N, SCN- H-, R-, CN-, CO, I-, R3P, C6H6, R2S 4/10/2019

Thank you 4/10/2019