Ch. 19 - Strength of Acids & Bases Strengths of Acids & Bases 19.3 pp. 602 - 611

HA(aq) + H2O(l) → H3O+(aq) + A-(aq) A. Strong & Weak Acids Strong acids are completely ionized in aqueous solution HA(aq) + H2O(l) → H3O+(aq) + A-(aq)

A. Strong & Weak Acids Weak acids ionize only slightly in aqueous solution CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO–(aq) ~99% ~1% Initial Amounts Equilibrium Amounts

B. Acid Dissociation Constant You can write the equilibrium-constant expression from balanced chemical equation CH3COOH(aq) + H2O(l) H3O+(aq) + CH3COO–(aq) The acid dissociation constant, Ka, is ratio of concentrations of dissociated and undissociated form

B. Acid Dissociation Constant Reflects the fraction of the acid in the ionized form If Ka is small, then degree of dissociation is small Weak acids have small Ka values The stronger an acid is, the larger its Ka value Often written in generic form: HA  H+ + A–

NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) C. Strong & Weak Bases Strong bases completely dissociate into metal ions and hydroxide ions in aqueous solution Weak bases react with water to form the hydroxide ion and the conjugate acid of the base NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)

D. Base Dissociation Constant You can write the equilibrium-constant expression from balanced chemical equation NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) The base dissociation constant, Kb, is ratio of concentrations of dissociated and undissociated form

D. Base Dissociation Constant Size of Kb indicates ability of weak base to compete with very strong base OH– for hydrogen ions Weak bases have small Kb values The stronger a base is, the larger its Kb value Often written in generic form: B + H2O  BH+ + OH–

E. Calculating Dissociation Constants A 0.100M solution of acetic acid is only partially ionized. From the pH, [H+] is determined to be 1.34 x 10-3M. What is Ka of acetic acid? Brackets indicate concentrations at equilibrium – you need to find equilibrium values!

E. Calculating Dissociation Constants

F. Percent Dissociation Often useful to specify amount of weak acid or base that has dissociated in achieving equilibrium Ex) 1.00M solution of HF, [H+] = 2.7x10-2 M

G. Weak Acids and Bases WS Find the pH and pOH of a 0.325M acetic acid solution. Ka = 1.8 x 10-5 HA  H+ + A–

G. Weak Acids and Bases WS Find the pH and pOH of a 0.325M acetic acid solution. Ka = 1.8 x 10-5