Chapter 14: Acids and Bases: The pH Scale, Strength, and Properties

Students begin by examining the foundational definitions of acids and bases, progressing from the Arrhenius model where acids produce hydrogen ions and bases produce hydroxide ions, through the more versatile Brønsted-Lowry framework that identifies acids as proton donors and bases as proton acceptors. The Lewis acid-base model further expands this conceptual foundation by defining acids as electron pair acceptors and bases as electron pair donors, enabling analysis of reactions that traditional models cannot explain. The distinction between strong and weak acids forms a central focus, with strong acids such as hydrochloric acid, hydrobromic acid, and nitric acid undergoing complete dissociation in aqueous solution, while weak acids including acetic acid establish equilibrium systems between molecular and ionic forms. Periodic trends in acid strength and the influence of molecular structure, particularly bond polarity and bond dissociation energy, help explain why certain acids are stronger than others. The pH scale is introduced as a logarithmic measure of hydrogen ion concentration, and students develop proficiency in calculating pH, pOH, and hydrogen ion concentration for various acidic and basic solutions. The chapter emphasizes equilibrium constants Ka and Kb as quantitative measures of acid and base strength, demonstrating how these constants enable predictions about solution behavior. Polyprotic acids such as sulfuric acid and phosphoric acid, which donate multiple protons sequentially, receive detailed treatment including calculations of concentrations at each ionization stage. Salt hydrolysis is examined to explain how certain salts produce acidic, basic, or neutral solutions depending on whether their constituent ions undergo hydrolysis reactions. The structural factors affecting acidity of oxoacids and organic acids are explored through periodic table comparisons and bond strength considerations. The chapter concludes by consolidating problem-solving strategies for equilibrium calculations, reinforcing the central role acids and bases play in laboratory contexts and applied fields ranging from environmental science to biological systems.