Module 1 Lecture 6 2025 2 per page

Module Overview

• Focus: Energetics & Equilibria in Biological Systems

• Key Topics:

  • Weak acids and bases

  • Dissociation constants (Ka, Kb)

  • Relationship between acid/base strengths

  • pH calculations for weak acids/bases

  • Transition metal complex ions as weak acids

Learning Objectives

• Write Ka and Kb expressions for weak acids and weak bases.

• Understand the relationship between Ka and Kb, pKa and pKb.

• Calculate the pH of weak acid and base solutions.

• Assess transition metal complex ions' behavior as weak acids.

Weak Acids

• Majority of acids are weak (incomplete dissociation, equilibrium favoring reactants).

• Example Comparison:

  • Strong: HCl -> [H3O+] = 0.10 mol/L

  • Weak: HNO2 -> [H3O+] = 0.0084 mol/L, [HNO2] = 0.0916 mol/L

Acid Dissociation Constant (Ka)

• Defined by the expression: [ K_a = \frac{[H_3O^+][A^-]}{[HA]} ]

• Indicates strength; larger Ka means stronger acid.

• pKa relation: [ pK_a = -\log(K_a) ]

Base Dissociation Constant (Kb)

• For weak bases: [ K_b = \frac{[BH^+][OH^-]}{[B]} ]

• Stronger base = larger Kb.

• pKb relation: [ pK_b = -\log(K_b) ]

• Example values:

  • CH3COOH: Ka = 1.8×10-5, pKa = 4.74

  • NH3: Kb = 1.8×10-5, pKb = 4.74

Relationship between Ka and Kb

• [ K_a K_b = K_w = 1.0\times10^{-14} ]

• At 25°C, [ pK_a + pK_b = pK_w = 14.00 ]

• Strong acid -> weak conjugate base.

Conjugate Strengths

• General rule: strong acids have very weak conjugate bases, and vice versa.

• Weak acids and their conjugate bases do not have predictable strengths.

pH Calculation Example: Lactic Acid

• Given: [CH3-CH(OH)-COOH] = 1×10-3 mol/L, pKa = 3.86

• Use Ka for calculations:

  • [ K_a = 10^{-pK_a} = 1.38\times10^{-4} ]

• Assumptions for calculation:

  • [CH3-CH(OH)-COO-]e = [H3O+]e

  • [CH3-CH(OH)-COOH]e ≈ [CH3-CH(OH)-COOH]initial

• Final pH calculation yields 3.43.

Weak Base Calculation Example: Caffeine

• Kb = 4.1×10-4 for caffeine.

• To find pH of 0.025 mol/L solution:

  • Assume [OH-]e based on Kb and caffeine concentration.

• pOH calculation gives pH of 11.51.

Transition Metal Compounds as Weak Acids

• Complex ions with water ligands can act as weak acids.

• Example: [Ni(H2O)6]2+ dissociates, forming [Ni(H2O)5(OH)]+ and H3O+.

• Acid strength increases with positive charge on metal ion.

Homework Assignments

• Problems from Brown (15th): 16.81, 16.86, 16.91, 16.92

• Answers are available on Blackboard.