Chapter 6 Lecture 1 Acid-Base Concepts
I.
Unifying Concepts
A.
The Acid-Base Concept
1) There are many acid-base definitions, each at times useful
2) Acid-Base concepts are not facts or even theories, but are useful
generalizations for classification, and organization
3) Acid-Base concepts are powerful ways to explain data and predict trends
B.
Arrhenius Concept
1) An acid forms H+ in water; a base forms OH- in water
2) Applicable to aqueous solutions only
3) HCl + NaOH
H+ + OH- + Na+ + Cl-
C.
Bronsted-Lowery Concept
1) Acid is a proton donor; Base is a proton acceptor
2) Conjugate acid/base pairs differ only by a proton
3) Reactions proceed to produce the weakest acid and base
4)
5)
D.
H3O+ + NO2H2O + HNO2
Includes non-aqueous systems
NH4+ + NH22 NH3
Solvent System Concept
1) Useful for aprotic, non-aqueous systems
2) Applies to any solvent that can dissociate to cation (acid) and anion (base)
3) For water: 2 H2O
H3O+ + OHa) Any solute increasing [H3O+] is an acid
HCl + H2O
H3O+ + Clb) Any solute increasing [OH-] is a base
NH3 + H2O
NH4+ + OH4)
5)
Aprotic, non-aqueous example: 2 BrF3
BrF2+ + BrF4a) Acid: SbF5 + BrF3
BrF2+ + SbF6b) Base: F- + BrF3
BrF4The acid-base reaction: acid + base = solvent (reverse of the ionization)
a) Arrhenius: acid + base = salt + water
b) Bronsted: acid1 + base2 = base1 + acid2
6)
E.
pKion = -log[acid][base]
a) pKW = -log[H3O+][OH-] = -log[10-7][10-7] = 14
b) pKH2SO4 = -log[H3SO4+][HSO4-] = 3.4
c) The smaller the number, the more dissociation has occurred
Lewis Concept
1) Acid = e- pair acceptor; Base = e- pair donor
2) Includes metal ions and non-aqueous systems; encompasses other concepts
3) We will use this concept throughout the rest of the chapter and course
4)
Metal ion Example: Ag+ + 2 NH3
[Ag(NH3)2]+
a) Acid-Base product is called an adduct
b) If the acid is a metal ion, it is also called a coordination compound or a
coordination complex or a complex ion
Chapter 6 Lecture 2 Hard-Soft Acid-Base Concepts
I.
Hard and Soft Acids and Bases
A.
Factors other than acid/base strength determine acid/base reactivity
1) Silver Halide solubility
a) AgX(s) + H2O
Ag+(aq) + X-(aq)
b) Ksp’s: AgF = 205, AgCl = 1.8 x 10-10, AgBr = 5.2 x 10-13, AgI = 8.3 x 10-17
2) Explanation of Solubility
a) Solvation: F- much better solvated (small, high charge)
b) Degree of Ag—X Interaction must also play a role
3) HSAB Theory can help explain this data
a) Hard acids/bases are small and nonpolarizable
b) Soft acids/bases are large and polarizable
c) Hard/Hard and Soft/Soft interactions are the most favorable
d) Polarizable = easily distorted by other charged ions
4) AgX data and HSAB
a) Ag+ is large and polarizable = Soft
b) Softness of Halides: I- > Br- > Cl- > Fc) AgI has the strongest interaction, thus the lowest solubility
d) Softness is also associated with covalent bonds, not ionic bonds
5)
Coordination of Thiocyanate (SCN-) to metal ions
a) SCN- binds to large, polarizable metals, through S: Hg2+----SCN
b) SCN- binds to smaller, less polarizable metals through N:
Zn2+----NCS
c) Explanation: Hard/Hard and Soft/Soft interactions are favored
6)
Exchange Reactions of [CH3Hg(H2O)]+
a) [CH3Hg(H2O)]+ + HCl
CH3HgCl + H3O+ K = 1.8 x 1012
b) [CH3Hg(H2O)]+ + HF
CH3HgF + H3O+ K = 4.5 x 10-2
c) Explanation: Hard/Hard and Soft/Soft interactions are favored
7)
B.
LiX solubility: LiBr > LiCl > LiI > LiF
a) Li+ is a hard ion
b) LiF would be expected to be very ionic and soluble
c) Very favorable hard-hard LiF interaction even overcomes solubility
d) LiBr, LiCl are more soluble because of less favored interactions
e) LiI is out of order because of poor I- solvation
Pearson’s Hard and Soft Acids and Bases (1963)
1) Most metal ions are hard acids (class a), some borderline depending on charge
2) Large polarizable metal ions are soft (class b)
3) Lewis bases can also be categorized as hard or soft
4) Reactions favor hardness matches
5) Hard/hard more energetically favored than soft/soft interactions
Soft and borderline
soft metal ions
6)
Polarizability = degree to which an atom’s electron cloud is distorted by
interactions with other ions
a) Hard = small, compact charge, nonpolarizable = M3+, O2b) Soft = large, polarizable = M0, S2c) Comparison is easiest within a column of the periodic table