Acid-Base Chemistry
Big-picture perspective:
Acid-base chemistry is highly diverse, encompassing not only the traditional H+/OH– chemistry
that characterizes aqueous solutions but also proton transfer in non-aqueous systems, and aprotic
solvent systems and metal cations. In this unit, we dive into acid-base chemistry, emphasizing the
connections especially to inorganic chemistry.
Learning goals:
• Understand the Bronsted and Lewis definitions of acids and bases.
• Understand the periodic trends of acidic, basic, and amphoteric compounds
• Predict, describe, and rationalize acid/base chemistry in non-aqueous systems, including acidic
and basic solvents, aprotic solvents, and molten salts.
• Apply the principles of acid-base chemistry to the design of molecules and Lewis acids with
target functions.
• Understand the connection between acid-base chemistry and the stabilization of oxidation
states.
• Predict favorable and stable compounds using hard-soft acid-base (HSAB) theory.
Theories of acid-base chemistry
Arrhenius concept: Acids (bases) release H+ (OH–) in water
(Limited to aqueous systems)
Brønsted concept: Acids (bases) lose (gain) a proton
(Focus on proton transfer and expand to non-aqueous systems)
Upon proton transfer, HCl becomes “deprotonated” to form Cl– and H2O
becomes “protonated” to form H3O+
Strong acid: fully deprotonated
Weak acid: partially deprotonated
(dynamic equilibrium)
Lewis concept: Bases donate an electron pair
Acids accept an electron pair
(encompasses Brønsted & Arrhenius definitions)
Theories of acid-base chemistry
Solvent system concept: Solvent auto-dissociation produces an acid (cation)
and base (anion). (Extends to aprotic non-aqueous solutions.)
H2O  H+(“H3O+”) + OH–
2 BrF3  BrF2+ + BrF4–
Lewis concept: Acids (bases) accept (donate) electron pairs
(Expands to metal ions and non-aqueous systems and is most general,
including in its scope the other acid/base concepts)
H+ + NH3  NH4+
BF3 + NH3  BF3NH3
Basic, acidic, and amphoteric oxides
Periodic table showing basic (blue), amphoteric (green) and acidic (red) oxides.
The metal-nonmetal boundary is indicated by the gray staircase line.
Periodic Trends
Rank HF, HCl, HBr, and HI in order of increasing H-X bond strength.
Rank HF, HCl, HBr, and HI in order of increasing H+ donating ability.
Rank HF, HCl, HBr, and HI in order of increasing acidity in water.
Non-aqueous solvents
Exercise: Write a balanced chemical equation corresponding to the
reaction of HCl with H2O(l).
Exercise: Write a balanced chemical equation corresponding to the
reaction of HOAc with H2O(l). [HOAc = acetic acid, HC2H3O2]
Exercise: Write a balanced chemical equation corresponding to the
reaction of HCl with HOAc(l). [Note: No water is present]
Non-aqueous solvents
A Brønsted acid that is strong in water (e.g. HCl) may be weak in a
solvent that is a less effective proton acceptor
than water (e.g. HOAc).
Non-aqueous solvents that are good proton acceptors (e.g. NH3)
encourage acids to ionize in them, and all acids can appear to be strong
(i.e., all are completely ionized: their acidity is leveled)
In a basic solvent (e.g. liq. NH3, bp = -33°C), all acids are strong.
In an acidic solvent (e.g. HOAc, H2SO4), ionization of bases is facilitated,
and most acids are therefore weak.
HOAC solvent discriminates HCl, HBr, HI (which are leveled in H2O)
Leveling and discrimination effects
Describe the acid-base behavior of HClO4 dissolved in H2SO4
HClO4, pKa = – 8
H2SO4, pKa = – 3
Which is the stronger acid, HClO4 or H2SO4?
What are the predominant species that are present?
Compare to a solution of HClO4 and H2SO4 in water
Molten salts as solvents
Crystals of metal oxides (e.g., KxBa1-xBiO3 superconductor) are often
grown in solutions of molten NaOH & KOH (mp ~ 250 °C)
autodissociation equilibrium:
2 OH- = H2O + O2• What is the strongest base in this medium? Strongest acid?
• How does added water affect the solubility of KxBa1-xBiO3
crystals?
Bases Stabilize High Oxidation States
Application: Chemical Synthesis of F2 (K. O. Christe, 1986)
First make a stable Mn(IV) fluoride:
4 MnO4-(aq) + 10 H2O(l) + 24 F-(aq) → 4 MnF62-(aq) + 3O2(g) + 20 OH-(aq)
2 K+(aq) + MnF62-(aq) → K2MnF6(s)
Then add Lewis acid:
K2MnF6(s) + 2 SbF5(l) → 2 KSbF6(s)+ "MnF4"(s)
Karl Christe
"MnF4"(s) → MnF3(s) + 1/2 F2(g)
Why is Mn(IV) unstable in acid?
Can we reach higher metal oxidation states
with oxides or fluorides?
Henri Moissan
Lewis acids and bases
Lewis acid: Electron-pair acceptor
Lewis base: Electron-pair donor
Lewis acid + Lewis base  Lewis acid-base adduct
Examples:
AlCl3 + Cl- (in CHCl3)
Al(NO3)3 in H2O
Identifying Lewis acids and bases
Lewis acid:
Positive charge or unfilled octet (H+, BX3)
Highly charged transition metal cations (Fe3+ Fe2+ Co3+)
Group III cations and compounds (Al3+ Ga3+ AlCl3)
Smaller group II cations (Be2+ Mg2+)
Lewis base:
Neutral or anionic, lone pair electrons
OH–, NH3, Bronsted bases
Designing better Lewis acids
Fe3+ is a good Lewis acid. What related species are similar,
but better (or worse) Lewis acids?
Consider these experimental observations…
What does this mean? Can you rationalize it?
Hard-soft acid-base theory
Consider these experimental observations…
SCN– bonds to Hg2+ through its sulfur atom to form [Hg(SCN)4]2–
SCN– bonds to Zn2+ through its nitrogen atom to form [Zn(NCS)4]2–
What does this mean? Can you rationalize it?
Polarizability
Hard-soft acid-base theory
Hard acids and bases
Soft acids and bases
The hard-soft acid-base (HSAB) concept provides a useful guide for
predicting and explaining acid-base chemistry and other chemical
phenomena.
Key concepts:
Electrostatics vs. Covalency
Like likes like
Can you identify trends and/or predictors of hard & soft acid & base behavior?
Hard & soft – periodic trends
Acids
Bases
Applying the HSAB concept
Which are stable: CuF, CuF2, CuI, CuI2?
What do you predict, and how would you verify your prediction?
Which base would you use to stabilize Au(I)?
OH-, NO3-, NH3, CNGold mining: