Acid-Base Equilibrium 1 Acid-Base Theory and Equilibrium Constants AP Chemistry

Models of Acids and Bases Arrhenius Concept: Acids produce H+ (or H3O+, hydronium ion) in solution; bases produce OH− ion. HCl + H2O Cl− + H3O+ acid base Brønsted-Lowry: Acids are proton (H+ ) donors bases are proton acceptors. 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) Ka = [H3O+][A-] [HA] = [H+][A -] [HA] 4

Acid Strength Strong Acid: Its equilibrium position lies far to the right. (HNO3) Large Ka value Yields a weak conjugate base. (NO3−) HNO3 + H2O H3O+ + NO3− 5

Acid Strength Weak Acid: Its equilibrium lies far to the left. (CH3COOH) Small Ka value Yields a much stronger (it is relatively strong) conjugate base than water. (CH3COO−) CH3COOH + H2O H3O+ + CH3COO− 6

Base Dissociation Constant, Kb B(aq) + H2O(l) OH-(aq) + HB+(aq) Kb = [OH-][HB+] [B] 7

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) 8

Bases (con’t) weak = very little dissociation (or reaction with water) H3CNH2(aq) + H2O(l) H3CNH3+(aq) + OH−(aq) 9

Water is amphoteric. Water can behave either as an acid or a base. H2O + H2O H3O+ + OH- conj conj acid 1 base 2 acid 2 base 1 Kw = [H+] [OH−] Kw = 1 × 10−14 at 25°C 10

Acid-Base Equilibrium 2 The pH Scale AP Chemistry

The pH Scale pH ≈ −log[H+] and pOH = −log[OH-] 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). 12

Example Problem What is the pH, pOH, and [OH-] of a 0.0020M solution of nitric acid? HNO3 + H2O H3O+ + NO3− pH = −log[H+] pH = −log(0.0020) pH = 2.70 pH + pOH = 14.00 2.70 + pOH = 14.00 pOH = 11.30 [H+] [OH−] = 1.00 × 10−14 (0.0020) [OH−] = 1.00 × 10−14 [OH−] = 5.0 × 10−12 M

Acid-Base Equilibrium 3 Solving Problems AP Chemistry

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. 15

(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.” Calculate [H+] and pH. 16

% Dissociation or % Ionization Amount dissociated (M) x 100% Initial concentration (M) 17

Acid-Base Equilibrium 4 Ka and Kb Relationships & Polyprotic Acids AP Chemistry

Ka and Kb Relationship HA(aq) + H2O(l) H3O+(aq) + A−(aq) Ka A-(aq) + H2O(l) OH-(aq) + HA(aq) Kb H2O(l) + H2O(l) H3O+(aq) + OH-(aq) Kw [H3O+][A-] [HA] [OH-][HA] [A] = [H3O+] [OH-] Ka Kb Kw x =

Polyprotic Acids . . . can furnish more than one proton (H+) to the solution. H2CO3 H+ + HCO3- Ka1 HCO3- H+ + CO32- Ka2 H2CO3 2H+ + CO32- Ka Ka1 x Ka2 = Ka(overall) 20

Ka and Kb Relationship for Polyprotic Acids For diprotic acids Ka1 x Kb2 = Kw Ka2 x Kb1 = Kw For triprotic acids Ka1 x Kb3 = Kw Ka2 x Kb2 = Kw Ka3 x Kb1 = Kw

Acid-Base Equilibrium 5 Miscellaneous Acid-Base Concepts AP Chemistry

Acidic and Basic Salts 23

Molecular Structure and Acidic Properties Two factors affecting acidity in binary compounds Bond Polarity (higher is more acidic) Bond Strength (lower is more acidic) 24

Oxides of Metal and Nonmetals 1. Acidic Oxides (Acid Anhydrides): Nonmetal Oxides (Examples: SO2, CO2, CrO3) O−X bond is strong and covalent. 2. Basic Oxides (Basic Anhydrides): Metal Oxides (Examples: K2O, CaO) O−X bond is ionic. 25

Examples of Anhydrides 1. Acidic Oxides (Acid Anhydrides): H2O + CO2 H2CO3 N2O5 + H2O 2HNO3 2. Basic Oxides (Basic Anhydrides): K2O + H2O 2KOH CaO + H2O Ca(OH)2 26

Coordinate Covalent Bond Lewis Acids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor H H F F B + N F B N H F : H : Lewis Lewis Acid Base F H F H Coordinate Covalent Bond 27