Acids and Bases
Among the many ions, two of the most important are the hydrogen cation (H+) and the hydroxide anion (OH-). Since a hydrogen atom contains one proton and one electron, a hydrogen cation is simply a proton. A hydroxide ion, by contrast, is a polyatomic anion in which an oxygen atom is covalently bonded to a hydrogen atom. Although much of Chapter 15 is devoted to the chemistry of H+ and OH- ions, it’s worthwhile taking a preliminary look at these two species now.
The importance of the H+ cation and OH- anion is that they are fundamental to the concept of acids and bases. In fact, one useful definition of an acid is a substance that provides H+ ions when dissolved in water, and one definition of a base is a substance that provides OH- ions when dissolved in water.
Hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), and phosphoric acid (H3PO4) are among the most common acids. When any of these substances is dissolved in water, H+ ions are formed along with the corresponding anion. For example, HCl gives H+ ions and Cl- ions when it dissolves in water. We sometimes attach the designation (aq) to show that the ions are present in aqueous solution. In the same way, we often attach the designation (g) for gas, (l) for liquid, or (s) for solid to indicate the state of other reactants or products. For example, pure HCl is a gas, HCl(g), and pure HNO3 is a liquid, HNO3 (l).
Depending on their structure, different acids can provide different numbers of H+ ions. Hydrochloric acid and nitric acid provide one H+ ion each per molecule. Sulfuric acid can provide two ions per molecule, and phosphoric acid can provide three ions per molecule.
Hydrochloric acid: HCl -> H+ (aq) + Cl- (aq)
Nitric acid: HNO3 -> H+ (aq) + NO3- (aq)
Sulfuric acid: H2SO4 -> H+ (aq) + HSO4- (aq)
HSO4- (aq) -> H+ (aq) + SO42- (aq)
Sodium hydroxide (NaOH; also known as lye or caustic soda), potassium hydroxide (KOH; also known as caustic potash), and barium hydroxide [Ba(OH)2] are examples of bases. When any of these compounds dissolves in water, OH- anions go into solution along with the corresponding metal cation. Sodium hydroxide and potassium hydroxide provide one ion each, and barium hydroxide provides two OH- ions, as indicated by its formula.
Sodium hydroxide: NaOH (s) -> Na+ (aq) + OH- (aq)
Potassium hydroxide: KOH (s) -> K+ (aq) + OH- (aq)
Barium hydroxide: Ba(OH)2 (s) -> Ba2+ (aq) + 2OH- (aq)