17–2
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Problems with B-L theory
The theory works very nicely in all protic solvent, but fails to explain acid-base behavior in aprotic solvents and non-solvent situations. A more general concept on acid and base was proposed by G.N. Lewis at about the same time Bronsted-Lowry theory was proposed.
The fundamental concept: (Text PHH: 17-1)
Acid - any substance which delivers hydrogen ion (H+) to the solution. HA H+ + A¯
Base - any substance which delivers hydroxide ion (OH¯ ) to the solution. BOH X+ + OH¯
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Brønsted-Lowry theory of acids and bases
An acid is a substance from which a proton can be removed. Acids are proton donors.
A base is a substance that can remove a proton from an acid. Bases are proton acceptors. Because of strong desire for protons, bases rip protons off acids.
Arrhenius acids and bases
Sevante Arrhenius proposed that substances exists as ions in solution in his dissertation, which was awarded a fourth class (D) in 1884. He was unable to find a job in his native Sweden. He was awarded the Nobel Prize in 1903 for his electrolytic dissociation theory.
Question the established theory.
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Evolution of the acid-base concept
year thinker Acid
1884 Arrhenius ionize H+
Base
ionize OH¯
acid-base reaction H+ + OH¯ = HOH
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Problems with Arrhenius theory
Like Dalton’s atomic theory, Arrhenius theory have problems today: Acidity did not show in other solvent. What is the solvent role? Some salts produce acidic or basic solutions, not neutral. Why? Which is the base, NH3 or NH4OH? Is OH¯ really the only base? How can H+ be stable? Are proton donated?
Acid-base reactions are competitions for protons. HCl + H2O H3O+ + Cl¯ HNO3 + H2O H3O+ + NO3¯ HC2H3O2 + H2O H3O+ + C2H3O2¯ H2O + NH3 NH4+ + OH¯
conjugate acids and bases
Lewis acids are electrophils: H+, Na+, BF3,
Lewis bases are nucleophils: NH3, H2O, PH3
Acid base reactions: BF3 + :NH3 F3B:NH3
Lewis at his desk. He generalized an idea.
1923 Bronsted- Proton Lowry Donor
proton HA + B = HB + A acceptor conjugation
1923 Lewis
electrophil nucleophil E + Nu = E:Nu
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Lewis acids and bases
Gilbert Newton Lewis (1875-1946) influential American chemist. His theories include the Lewis dot structure taught in Chem120 and covalent bond theories.
Some chemists want students to learn Brø nsted-Lowry theory of acids and bases before learning Arrhenius theory, because the former is mon the established theory.
17–9
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HClO4 H2SO4
HI
ClO4 ¯ HSO4¯ I¯