4.1: Arrhenius Acids and Bases (Review) Arrhenius acids form hydrogen ions in aqueous solution with Arrhenius bases forming hydroxide ions. 4.2: Brønsted-Lowry Acids and Bases (Review) A Brønsted-Lowry acid is a proton donor, while a Brønsted-Lowry base is a proton acceptor. 4.3: Distinguishing between pH and pKa The pKa of a compound is empirically determined similar to the melting point or boiling point. The pH communicates the environment of the sample as a measure of the hydrogen ion concentration and can be changed. 4.4: Predicting Relative Acidity To determine the relative acidity of compounds, we compare the relative stabilities of their conjugate bases. This skill can be applied to organic reactions when comparing leaving group stability to evaluate reaction pathways. 4.5: Lewis Acids and Bases Lewis acids are electron acceptors and Lewis bases are electron donors. The electron flow in Lewis acid-base chemistry prepares us for the electron flow of many organic reactions.