Unit 7 Acids and Bases

Acids and Bases

Arrhenius Definition of Acids and Bases

• Svante Arrhenius (1859-1927):

  • Proposed that acids produce protons (H+) in solution.

  • Bases produce hydroxide ions (OH-).

  • Example reactions:

    • HCl (aq) → H+ (aq) + Cl- (aq) (Acid)

    • NaOH (aq) → Na+ (aq) + OH- (aq) (Base)

The pH Scale

• pH Calculation:

  • Formula: pH = - log [H+]

• Determining pH:

  • Strong Acids/Bases: HA (aq) → H+ (aq) + A- (aq)

  • Weak Acids/Bases: HA (aq) ←→ H+ (aq) + A- (aq)

• Example: Calculate pH of 0.5 M HCl and corresponding pOH.

Strong Acids and Bases

• List of Strong Acids:

  • HCl, HBr, HI, HNO3, H2SO4, HClO3, HClO4

• List of Strong Bases:

  • NaOH, LiOH, KOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

Bronsted-Lowry Definition of Acids and Bases

• Johannes Brønsted (1879-1947) and Thomas Lowry (1874-1936):

  • Acids are proton donors.

  • Bases are proton acceptors.

  • It allows for equilibrium reactions.

  • Example reaction:

    • HCl (aq) + H2O (aq) ↔ H3O+ (aq) + Cl- (aq)

  • Clarification: H+ is shorthand, actual species is H3O+ (hydronium).

Comparing Relative Strength of Acids and Bases

• Strength Relationship:

  • Stronger acids yield weaker conjugate bases and vice versa.

  • Example: HCl is strong; hence Cl- is weak.

  • Table of examples illustrating strong and weak acids and bases.

Ion Product of Water

• Water can act as both an acid and a base (amphoteric).

• Ion product expression: Kw = [H+][OH-] = 1.0 x 10^{-14} at 25°C.

• Calculate pH and pOH based on ion product.

Polyprotic Acids

• Acids that can donate more than one proton:

  • Example: H2SO3 ↔ H+ + HSO3- (K1), HSO3- ↔ H+ + SO3^{2-} (K2).

• Notable polyprotic acids and their Ka values.

Weak Acids and Bases

• Weak Acid Example:

  • Ka values demonstrate acid strengths.

  • Calculating pH of weak acid solutions (example calculations with values).

Relationship Between Ka and Kb

• Calculation of Ka and Kb for weak acids and bases;

  • For the weak base NH4+, the relationship can be derived: Ka × Kb = Kw = [NH3][H+]/[NH4+].

Lewis Definition of Acids and Bases

• Gilbert N. Lewis (1875-1946):

  • Defines acids as electron pair acceptors and bases as electron pair donors.

  • Reaction representation: A + B → A-B (with curved arrows showing electron movement).

Acid-Base Properties of Salts

• Interaction of salts in water can produce acidic or basic solutions.