Chapter 4: Acids and Bases

Beginning with the Arrhenius model, the chapter transitions to the Brønsted–Lowry conception, which emphasizes proton transfer equilibria and introduces the fundamental relationships between acid dissociation constants, base dissociation constants, and the water ionization constant. Students learn to distinguish strong acids and bases that completely dissociate from weak acids and bases that establish equilibrium, with particular attention to polyprotic species like hydrogen sulfide and phosphoric acid that release multiple protons sequentially. Distribution diagrams become essential tools for predicting which molecular forms dominate across different pH ranges. The chapter then explores the molecular factors governing acid and base strength, including proton affinity, bond dissociation enthalpies, solvation effects quantified through the Born equation, and periodic trends that reflect changes in electronegativity and atomic size. Beyond aqueous solutions, the solvent system definition reveals how acid–base behavior shifts dramatically in nonaqueous media such as liquid ammonia, hydrogen fluoride, and anhydrous sulfuric acid, demonstrating that acidity and basicity are relative concepts dependent on solvent properties. Systematic categorization of Brønsted acids into aqua acids, hydroxoacids, and oxoacids, combined with Pauling's rules, allows rational prediction of strength trends and explains anomalies in the periodic table. The relationship between anhydrous oxides and amphoterism connects acid–base chemistry to periodic metallic and nonmetallic character. The Lewis electron-pair donor–acceptor framework dramatically expands the scope of acid–base chemistry beyond proton transfer, encompassing diverse species like boron trifluoride, aluminum chloride, silicon tetrafluoride, and antimony pentafluoride. Group trends in Lewis acidity and basicity across the periodic table illuminate mechanisms of catalysis, hypervalence, and coordination chemistry. Pearson's hard and soft acid–base principle provides predictive power for understanding reactivity patterns and selectivity in complex formation, while the Drago–Wayland equation offers a thermodynamic quantification of Lewis acidity. The chapter concludes by examining practical applications including superacids and superbases that extend the conventional strength scale, heterogeneous acid–base catalysis on solid surfaces critical to industrial processes, and solvent participation effects that rationalize diverse reaction mechanisms.