Determining the Strengths of Acids and Bases

Objectives:• 1. Define the term dissociation constant and

explain what it indicates about and acid or a base

• 2. Explain what happens when acids and bases are mixed together.

Vocabulary• acid dissociation constant, base dissociation

constant, salt hydrolysis reaction

Strength of Acids/Bases

• The strength of acids and bases are based on the amount of ions they form in solution. There properties are based on the amount of dissociation and are tied to the dissociation constant of the acid or base.

AcidsStrong acids• Strong acids almost completely dissociate their H+ ions in solution.

– 1M HCl produces 1M of H+ and Cl- in solution • HCl + H2O g H3O+ + Cl-

– The dissociation constant is greater than 1. (Ka > 1) • Ka = [H+][Cl-]/[HCl] > 1

– its conjugate base (Cl-) is a weak base

Weak Acids• Weak acids release very few H+ ions in solutions

– 1M H2CO3 produces much less than 1M of H+ in solution • H2CO3 + H2O g H3O+ + HCO3

-

– The dissociation constant is less than 1. (Ka < 1) • Ka = [H+][HCO3

-]/[H2CO3] < 1 (4.5 x 10-7)

– its conjugate base (HCO3-) is a strong base

Bases

• strong bases almost completely dissociate to form OH- ions in solution. – 1M NaOH produces 1M of Na+ and OH- in solution

• NaOH + H2O g Na+ + OH-

– The dissociation constant is greater than 1. (Kb > 1) • Kb = [Na+][OH-]/[NaOH] > 1

– its conjugate acid (Na+) is a weak acid • weak bases dissociate to form few OH- ions in solution.

– 1M HF produces much less than 1M of OH- in solution – The dissociation constant is greater than 1. (Kb < 1)

• Kb = [H+][OH-]/[HF] < 1

– its conjugate acid (H+) is a strong acid

Formation of Salts from Neutralization

Acid-Base Properties of Salts• Strong Acid + Strong base g Salt + Water (pH = 7) • Strong Acid + Weak base g Salt + Weak Acid (pH < 7) • Weak Acid + Strong base g Salt + Weak Base(pH > 7)