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Chemistry Core/AP

Chemistry 16: 1-4

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WebLecture: Acids and Bases Revisited

Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 16: Sections 1-4.

There are three common definitions of acids and bases, the Arrhenius theory, the Brønsted-Lowry theory, and the Lewis theory. In all three, acids donate or promote the concentrations of hydrogen ions (H+ protons). Bases may accept these ions, or promote the concentration of hydroxide ions (OH-) .

  1. 16.1 The Arrhenius definition describes an acid as any substance which, when dissolved in water, increases the H+ ion concentration. The Brønsted-Lowry concept identifies acids as proton donors. In Brønsted-Lowry terms, hydrochloric acid HCl acts as a proton donor. Some molecules, such as H2SO4 have multiple protons which they can donate, and so are polyprotic acids.
    When an acid and base interact, the resulting products are also an acid-base pair. If HCO3- reacts to accept a proton, it is a base, but the H2CO3 molecule it forms is an acid capable of donating an H+ in the reverse reaction.
  2. 16.2 The reaction for dissociation of water molecules is 2 H2O → H3O+ + OH-.
    The equilibrium constant K2 thus equals [H3O+] [OH-] / [H2O], but our rules define [H2O] = 1. So Kw simplifies to [H3O+][OH-], which experimentally is 1 * 10-14.
    In a neutral solution, [H3O+] = [OH-], so the concentrations at neutralization must be 10-7 to be equal and give Kw = 1 * 10-14. If we express the concentrations [H3O+] as 10-a and [OH-] as 10-b, a + b = 14. Increasing b decreases a. Chemists use these exponents as shorthand for the concentration of acids and bases: pH = a = -log[H3O+] and pOH = b = -log[OH-].
  3. 16.4 In a acid reaction where HA → + H2O = A- H3O+, the equilibrium constant will be Ka = [H3O+][A-] / [HA].
    In a base reaction where B + H2O → = BH+ OH-, the equilibrium constant will be Kb = [BH+][AOH-] / [B].
    How much an acid dissociates determines whether it is a weak acid (there is only a small change in HA concentration) or a strong acid (the HA concentration drops to near zero). The strength of the conjugate base will be inversely related to the strength of the acid: the stronger the acid, the weaker the conjugate base.
  4. 16.4 In an acid-base reaction, the cation of the base and the anion of the acid form a salt, and the donated H+ and OH- form neutral water: NaOH + HCl → NaCl + H2O.

Videos for Chapter 16: Principles of Chemical Reactivity: The Chemistry of Acids and Bases

Review the Videos at Thinkwell Video Lessons.

Homework problems: See your Moodle assignment!

AP #12 GUIDED INQUIRY — Determining the rate law — Phase III

Complete your data analysis and submit a formal report of your results.