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Chapter 6 Lecture 1 Acid-Base Concepts

Chapter 6 Lecture 1 Acid-Base Concepts. Unifying Concepts The Acid-Base Concept There are many acid-base definitions, each at times useful Acid-Base concepts are not facts or even theories, but are useful generalizations for classification, and organization

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Chapter 6 Lecture 1 Acid-Base Concepts

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  1. Chapter 6 Lecture 1 Acid-Base Concepts • Unifying Concepts • The Acid-Base Concept • There are many acid-base definitions, each at times useful • Acid-Base concepts are not facts or even theories, but are useful generalizations for classification, and organization • Acid-Base concepts are powerful ways to explain data and predict trends • Arrhenius Concept • An acid forms H+ in water; a base forms OH- in water • Applicable to aqueous solutions only • HCl + NaOH H+ + OH- + Na+ + Cl- • Bronsted-Lowery Concept • Acid is a proton donor; Base is a proton acceptor • Conjugate acid/base pairs differ only by a proton • Reactions proceed to produce the weakest acid and base

  2. H3O+ + NO2- H2O + HNO2 • Includes non-aqueous systems NH4+ + NH2- 2 NH3 • Solvent System Concept • Useful for aprotic, non-aqueous systems • Applies to any solvent that can dissociate to cation (acid) and anion (base) • For water: 2 H2O H3O+ + OH- • Any solute increasing [H3O+] is an acid HCl + H2O H3O+ + Cl- • Any solute increasing [OH-] is a base NH3 + H2O NH4+ + OH- • Aprotic, non-aqueous example: 2 BrF3 BrF2+ + BrF4- • Acid: SbF5 + BrF3 BrF2+ + SbF6- • Base: F- + BrF3 BrF4- • The acid-base reaction: acid + base = solvent (reverse of the ionization) • Arrhenius: acid + base = salt + water • Bronsted: acid1 + base2 = base1 + acid2

  3. pKion = -log[acid][base] • pKW = -log[H3O+][OH-] = -log[10-7][10-7] = 14 • pKH2SO4 = -log[H3SO4+][HSO4-] = 3.4 • The smaller the number, the more dissociation has occurred • Lewis Concept • Acid = e- pair acceptor; Base = e- pair donor • Includes metal ions and non-aqueous systems; encompasses other concepts • We will use this concept throughout the rest of the chapter and course • Metal ion Example: Ag+ + 2 NH3 [Ag(NH3)2]+ • Acid-Base product is called an adduct • If the acid is a metal ion, it is also called a coordination compound or a coordination complex or a complex ion

  4. Chapter 6 Lecture 2 Hard-Soft Acid-Base Concepts • Hard and Soft Acids and Bases • Factors other than acid/base strength determine acid/base reactivity • Silver Halide solubility • AgX(s) + H2O Ag+(aq) + X-(aq) • Ksp’s: AgF = 205, AgCl = 1.8 x 10-10, AgBr = 5.2 x 10-13, AgI = 8.3 x 10-17 • Explanation of Solubility • Solvation: F- much better solvated (small, high charge) • Degree of Ag—X Interaction must also play a role • HSAB Theory can help explain this data • Hard acids/bases are small and nonpolarizable • Soft acids/bases are large and polarizable • Hard/Hard and Soft/Soft interactions are the most favorable • Polarizable = easily distorted by other charged ions • AgX data and HSAB • Ag+ is large and polarizable = Soft • Softness of Halides: I- > Br- > Cl- > F- • AgI has the strongest interaction, thus the lowest solubility • Softness is also associated with covalent bonds, not ionic bonds

  5. 5) Coordination of Thiocyanate (SCN-) to metal ions • SCN- binds to large, polarizable metals, through S: Hg2+----SCN • SCN- binds to smaller, less polarizable metals through N: Zn2+----NCS • Explanation: Hard/Hard and Soft/Soft interactions are favored • Exchange Reactions of [CH3Hg(H2O)]+ a) [CH3Hg(H2O)]+ + HCl CH3HgCl + H3O+ K = 1.8 x 1012 • [CH3Hg(H2O)]+ + HF CH3HgF + H3O+ K = 4.5 x 10-2 • Explanation: Hard/Hard and Soft/Soft interactions are favored

  6. 7) LiX solubility: LiBr > LiCl > LiI > LiF • Li+ is a hard ion • LiF would be expected to be very ionic and soluble • Very favorable hard-hard LiF interaction even overcomes solubility • LiBr, LiCl are more soluble because of less favored interactions • LiI is out of order because of poor I- solvation • Pearson’s Hard and Soft Acids and Bases (1963) • Most metal ions are hard acids (class a), some borderline depending on charge • Large polarizable metal ions are soft (class b) • Lewis bases can also be categorized as hard or soft • Reactions favor hardness matches • Hard/hard more energetically favored than soft/soft interactions Soft and borderline soft metal ions

  7. 6) Polarizability = degree to which an atom’s electron cloud is distorted by interactions with other ions • Hard = small, compact charge, nonpolarizable = M3+, O2- • Soft = large, polarizable = M0, S2- • Comparison is easiest within a column of the periodic table

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