CHEM1010 W2 L3

Weak Acids and Bases

• Weak acids and bases only partially ionize in solution.

• Examples:

  • Citric Acid in citrus fruits.

  • Acetic Acid is also a common example of a weak acid.

  • Their importance spans various applications in chemistry.

pH Calculation

• Measuring and predicting the acidity of weak acids and bases involves calculating pH.

• For strong acids, pH is directly related to the concentration of the initial acid, leading to complete ionization

  • Example: Hydrochloric Acid (HCl) fully dissociates into H3O+.

• Weak acids use equilibrium constants (K_a) to determine how much acid has dissociated.

  • Partial ionization results in an equilibrium state represented with the equilibrium constant (K_a).

Acid and Base Interaction

• Equilibrium Constant (K_a) defines the extent of ionization for weak acids:

  • K_a = [H3O+][A-] / [HA]

• In weak acids:

  • The concentration of H3O+ produced is less than that in strong acids.

• Acid Dissociation Constant (K_a)

  • Measured based on equilibrium state when weak acids donate protons.

  • Example of K_a values for different acids:

    • HF: 6.8 x 10^-4 (measurable)

    • Acetic Acid: 1.8 x 10^-5

    • Phenol = one of the weakest acids in comparison.

pKa and its Relationship to Ka

• The pK_a = -log(K_a) provides a more convenient way to compare acidity levels.

  • Example: pK_a of Acetic Acid = 4.75 matches K_a of 1.8 x 10^-5.

• The inversion relationship:

  • Larger K_a signifies a stronger acid, while smaller pK_a values indicate higher acidity.

Calculating pH from Known pKa

• Example with formic acid:

  • Given pH allows for K_a calculation, requiring an equilibrium expression.

  • Use an ICE table to express concentrations of all species involved.

  • From pH, calculate [H+] using:

    • [H+] = 10^-pH

  • Determine concentrations and substitute back to find K_a.

Weak Bases and Their Function

• Weak bases such as ammonia produce hydroxide (OH-) in solution.

• K_b is the base dissociation constant used in similar calculations involving weak bases.

  • E.g., NH3 + H2O ⇌ NH4+ + OH-

• The pK_b also relates similar to pK_a, using:

  • pK_a + pK_b = 14 across conjugate acid-base pairs.

Salt Hydrolysis Effects on pH

• Salts may produce acidic, basic or neutral solutions depending upon their cations and anions:

  • Acidic Cations (e.g., NH4+ from NH3) can lead to an acidic solution.

  • Basic Anions (e.g., acetate from acetic acid) can create a basic solution.

• Example analysis of Sodium Salts:

  • Sodium Fluoride = Basic due to fluoride acting as conjugate base.

  • Sodium Chloride = Neutral, while sodium acetate produces basic conditions.

Titration of Acids and Bases

• Reaction balancing helps find the neutralization point:

  • E.g., HCl + NaOH → NaCl + H2O results in acid-base neutralization.

• Use ICE tables to determine final concentrations following neutralization reactions, allowing for pH analysis.

Buffer Solutions

• Solutions containing weak acids and their conjugate bases help maintain stable pH levels, crucial in biological systems.

• A mixed solution of weak acids and weak bases provides a buffered state, resisting changes in pH upon the addition of strong acids or bases.