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Models of Acids and Bases

Models of Acids and Bases. Arrhenius Concept : Acids produce H + in solution, bases produce OH  ion. Brønsted-Lowry : Acids are H + donors, bases are proton acceptors. HCl + H 2 O  Cl  + H 3 O + acid base. Conjugate Acid/Base Pairs.

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Models of Acids and Bases

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  1. Models of Acids and Bases • Arrhenius Concept: Acids produce H+ in solution, bases produce OH ion. • Brønsted-Lowry: Acids are H+ donors, bases are proton acceptors. • HCl + H2O  Cl + H3O+ • acid base

  2. Conjugate Acid/Base Pairs • HA(aq) + H2O(l)  H3O+(aq) + A(aq) • conj conj conj conj acid 1 base 2 acid 2 base 1 • conjugate base: everything that remains of the acid molecule after a proton is lost. • conjugate acid: formed when the proton is transferred to the base.

  3. Acid Dissociation Constant (Ka) • HA(aq) + H2O(l)  H3O+(aq) + A(aq)

  4. Acid Strength • Its equilibrium position lies far to the right. (HNO3) • Yields a weak conjugate base. (NO3) Strong Acid:

  5. Strong Acid - Weak Acid

  6. Strong Acids

  7. Acid Strength(continued) • Its equilibrium lies far to the left. (CH3COOH) • Yields a much stronger (it is relatively strong) conjugate base than water. (CH3COO) Weak Acid:

  8. Strong Acid Weak Acid

  9. Water as an Acid and a Base • Water is amphoteric (it can behave either as an acid or a base). • H2O + H2O  H3O+ + OH • conj conj acid 1 base 2 acid 2 base 1 • Kw = 1  1014 at 25°C

  10. The pH Scale • pH log[H+] • pH in water ranges from 0 to 14. • Kw = 1.00  1014 = [H+] [OH] • pKw = 14.00 = pH + pOH • As pH rises, pOH falls (sum = 14.00).

  11. Solving Weak Acid Equilibrium Problems • List major species in solution. • Choose species that can produce H+ and write reactions. • Based on K values, decide on dominant equilibrium. • Write equilibrium expression for dominant equilibrium. • List initial concentrations in dominant equilibrium.

  12. Solving Weak Acid Equilibrium Problems (continued) • Define change at equilibrium (as “x”). • Write equilibrium concentrations in terms of x. • Substitute equilibrium concentrations into equilibrium expression. • Solve for x the “easy way.” • Verify assumptions using 5% rule. • Calculate [H+] and pH.

  13. Percent Dissociation (Ionization)

  14. In-Class Problem • Calculate the pH of a 0.20 M solution of Iodic acid (HIO3). Ka = 0.17 • What is the percent dissociation for this acid.

  15. Bases • “Strong” and “weak” are used in the same sense for bases as for acids. • strong = complete dissociation (hydroxide ion supplied to solution) • NaOH(s)  Na+(aq) + OH(aq)

  16. Strong Bases

  17. Bases(continued) • weak = very little dissociation (or reaction with water) • H3CNH2(aq) + H2O(l)  H3CNH3+(aq) + OH(aq)

  18. Polyprotic Acids • . . . can furnish more than one proton (H+) to the solution.

  19. Acid-Base Properties of Salts

  20. Structure and Acid-Base Properties • Two factors for acidity in binary compounds: • Bond Polarity(high is good) • Bond Strength(low is good)

  21. Oxides • Acidic Oxides (Acid Anhydrides): • OX bond is strong and covalent. • SO2, NO2, CrO3 • Basic Oxides (Basic Anhydrides): • OX bond is ionic. • K2O, CaO

  22. Lewis Acids and Bases • Lewis Acid: electron pair acceptor • Lewis Base: electron pair donor

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