Acid–Base Reactions for AP Chemistry
Practice problems for these concepts can be found at:
- Reactions and Periodicity Review Questions for AP Chemistry
- Reactions and Periodicity Free-Response Questions for AP Chemistry
Acids and bases are extremely common, as are the reactions between acids and bases. The driving force is often the hydronium ion reacting with the hydroxide ion to form water. The chapter on Equilibrium describes the equilibrium reactions of acids and bases, as well as some information concerning acid–base titration. After you finish this section, you may want to review the acid–base part of the Equilibrium chapter.
Properties of Acids, Bases, and Salts
At the macroscopic level, acids taste sour, may be damaging to the skin, and react with bases to yield salts. Bases taste bitter, feel slippery, and react with acids to form salts.
At the microscopic level, acids are defined as proton (H+) donors (Brønsted–Lowry theory) or electron-pair acceptors (Lewis theory). Bases are defined as proton (H+) acceptors (Brønsted–Lowry theory) or electron-pair donors (Lewis theory). Consider the gas-phase reaction between hydrogen chloride and ammonia:
- HCl + :NH3(g) → HNH3+Cl– ( or NH4+Cl–)
HCl is the acid, because it is donating an H+ and the H+ will accept an electron pair from ammonia. Ammonia is the base, accepting the H+ and furnishing an electron pair that the H+ will bond with via coordinate covalent bonding. Coordinate covalent bonds are covalent bonds in which one of the atoms furnishes both of the electrons for the bond. After the bond is formed, it is identical to a covalent bond formed by donation of one electron by both of the bonding atoms.
Acids and bases may be strong, dissociating completely, or weak, partially dissociating and forming an equilibrium system.(See Chapter 15 for the details on weak acids and bases.) Strong acids include:
- Hydrochloric, HCl
- Hydrobromic, HBr
- Hydroiodic, HI
- Nitric, HNO3
- Chloric, HClO3
- Perchloric, HClO4
- Sulfuric, H2SO4
The strong acids above are all compounds that ionize completely in aqueous solution, yielding hydrogen ions and the anions from the acid.
Strong bases include:
- Alkali metal (Group IA) hydroxides (LiOH, NaOH, KOH, RbOH, CsOH)
- Ca(OH)2, Sr(OH)2, and Ba(OH)2
The strong bases listed above are all compounds that dissociate completely, yielding the hydroxide ion (which is really the base, not the compound).
Unless told otherwise, assume that acids and bases not on the lists above are weak and will establish an equilibrium system when placed into water.
Some salts have acid–base properties. For example, ammonium chloride, NH4Cl, when dissolved in water will dissociate and the ammonium ion will act as a weak acid, donating a proton. We will examine these acid–base properties in more detail in the next section.
Certain oxides can have acidic or basic properties. These properties often become evident when the oxides are dissolved in water. In most case, reactions of this type are not redox reactions.
Many oxides of metals that have a +1 or +2 charge are called basic oxides (basic anhydrides), because they will react with acids.
Many times they react with water to form a basic solution:
Many nonmetal oxides are called acidic oxides (acidic anhydrides), because they react with water to form an acidic solution:
- CO2(g) + H2O(I) → H2CO3(aq)
H2CO3(aq) is named carbonic acid and is the reason that most carbonated beverages are slightly acidic. It is also the reason that soft drinks have fizz, because carbonic acid will decompose to form carbon dioxide and water.
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