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

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.

Acid Dissociation Constant (Ka) HA(aq) + H2O(l) ® H3O+(aq) + A-(aq)

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

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

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

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).

Percent Dissociation (Ionization)

NaOH(s) ® Na+(aq) + OH-(aq) 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)

H3CNH2(aq) + H2O(l) « H3CNH3+(aq) + OH-(aq) Bases (continued) weak = very little dissociation (or reaction with water) H3CNH2(aq) + H2O(l) « H3CNH3+(aq) + OH-(aq)

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

Effect of Cations and Anions An anion that is the conjugate base of a strong acid will not affect the pH. An anion that is the conjugate base of a weak acid will increase the pH. A cation that is the conjugate acid of a weak base will decrease the pH.

Effect of Cations and Anions Cations of the strong Arrhenius bases will not affect the pH. Other metal ions will cause a decrease in pH. When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the Ka and Kb values.

Acid-Base Properties of Salts

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

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

Lewis Acids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor

Solving Weak Acid Equilibrium Problems 1) List major species in solution. 2) Choose species that can produce H+ and write reactions. 3) Based on K values, decide on dominant equilibrium (Strongest Acid Rules!). 4) Write equilibrium expression and reaction for the dominant species. 5) List initial concentrations under reaction equation and react fully to favored side.

Solving Weak Acid Equilibrium Problems (continued) 6) Define change at equilibrium (as “x”). 7) Write equilibrium concentrations in terms of x. 8) Substitute equilibrium concentrations into equilibrium expression. 9) Solve for x the “easy way.” 10) Verify assumptions. 11) Calculate [H+] and pH.