Chapter 16 (3/19/25) Acids, Bases, Equilibrium and pH Concepts

  • Class Announcement

  • No class on Friday; substitute session with Josh to answer questions.

  • Instructor will post a video on Canvas for student learning.

  • Office hours canceled for Thursday and Friday.

  • Acids and Bases Overview

  • Discussion of acids and bases focused on their reactions to create stable responses.

  • Strong species generate weak species (and vice versa).

  • Equilibrium in acid-base reactions allows for reversibility.

  • Conjugate Acid-Base Pairs

  • Strong acids yield weak conjugate bases.

  • Example: HCl (strong acid) dissociates to Cl− (weak conjugate base).

  • Weak acids yield stronger conjugate bases.

  • Example: Water (very weak acid) generates OH− (strong conjugate base).

  • Autoionization of Water

  • Water can act as both an acid and a base, known as autoionization.

  • Lone pair of electrons in water can take up protons from other water molecules.

  • Resulting ions: H3O+ (acidic) and OH− (basic).

  • In pure water, both ions are present at very low concentrations.

  • Equilibrium Constant of Water (Kw)

  • Kw = [H3O+][OH−], where Kw = 1 x 10^-14 at 25°C.

  • Very few ions are present in pure water, indicating that equilibrium favors reactants.

  • Impact of Adding Acids and Bases

  • Adding acid increases [H3O+] and shifts equilibrium left, consuming OH−.

  • Adding base increases [OH−], causing a shift left to consume H3O+.

  • Determining Acidity/Basicity Based on Ion Concentrations

  • Acidic solution: [H3O+] > [OH−]

  • Neutral solution: [H3O+] = [OH−]

  • Basic solution: [H3O+] < [OH−]

  • pH Scale

  • pH measures [H3O+] and is calculated as pH = -log[H3O+].

  • Inverse relationship between pH and H3O+ concentration.

  • A 10-fold increase in [H3O+] results in a decrease of 1 pH unit.

  • Typical pH values:

    • Acidic (pH < 7)
    • Neutral (pH = 7)
    • Basic (pH > 7)

  • Example Calculations

  • Given [H3O+] = 3.0 x 10^-4 M, calculate [OH−]:

    • Use Kw = [H3O+][OH−] to find [OH−] = 3.3 x 10^-11 M; solution is acidic.

  • Given [OH−] = 0.042 M, calculate [H3O+]:

    • Calculate [H3O+] = 2.3 x 10^-12 M; solution is basic.

  • Noteworthy Acidic and Basic Solutions

  • HCl (1 M): pH 0

  • Blood: slightly basic (pH ~7.4)

  • Household ammonia: basic

  • Strong bases like NaOH can have pH 14.

  • Final Thoughts on pH

  • Example of 1985 Rothschild wine, [H3O+] = 8.1 x 10^-4 M gives pH = 3.09.

  • Rainwater pH = 5.22, [H3O+] = 6.0 x 10^-6 M.