Advanced Placement Chemistry
Lesson Plan #8
1.5 class periods
Acids are substances that are able to ionize in aqueous solutions to form a hydrogen ion and thereby increase the concentration of H+(aq) ions. Because a hydrogen atom consists of a proton and an electron, H+ is simply a proton. Thus, acids are often called proton donors.
Molecules of different acids can ionize to form different numbers of H+ ions. Both HCl and HNO3 are examples of monoprotic acids, which yield one H+ per molecule of acid. Sulfuric acid, H2SO4, is an example of a diprotic acid, one that yields two H+ per molecule of acid. The ionization of H2SO4 and other diprotic acids occurs in two steps:
H2SO4(aq) ß H+(aq) + HSO4-(aq)
HSO4-(aq) £ H+(aq) + SO42-(aq)
Although H2SO4 is a strong electrolyte, only the first ionization is complete. Thus, aqueous solutions of sulfuric acid contain a mixture of H+(aq), HSO4-(aq), and SO42-(aq).
Bases are substances that accept H+ ions. Hydroxide ions, OH-, are basic because they readily react with H+ ions to form water.
H+(aq) + OH-(aq) ß H2O(l)
Any substance that increases the concentrations of OH-(aq) when added to water is a base. Compounds that do not contain OH- ions can also be bases. Ammonia, NH4, is a common base.
Strong and Weak Acids and Bases
Acids and bases that are strong electrolytes (completely ionized in solution) are called strong acids and strong bases. Those that are weak electrolytes (partly ionized) are called weak acids and weak bases. Strong acids are more reactive than weak acids when the reactivity depends only on the concentration of H+(aq). The reactivity of an acid, however; can depend on the anion as well as on H+(aq). For example, hydrofluoric acid, HF, is a weak acid, being only partly ionized in aqueous solution. However, HF is very reactive and vigorously attacks many substances, including glass. This reactivity is due to the combined action of H+(aq) and F-(aq).
As you examine this table, notice first that some of the most common acids, such as HCl, HNO3, and H2SO4, are strong. Second, three of the strong acids result from combining a hydrogen atom and a halogen atom. (HF, however, is a weak acid.) Third, the list of strong acids is very short. Most acids are weak. Fourth, the only common strong bases are the hydroxides of Li+, Na+, K+, Rb+, and Cs+ (the alkali metals, group 1A) and the hydroxides of Ca2+, Sr2+, and Ba2+ (the heavy alkaline earths, group 2A). The most common weak base is NH3.
Identifying Strong and Weak Electrolytes
Neutralization Reactions and Salts
Solutions of acids and bases have very different properties. Acids have a sour taste, whereas bases have a bitter taste. Acids can change the colors of certain dyes in a specific way that differs from the effect of a base. For example, the dye known as litmus is changed from blue to red by an acid, and from red to blue by a base.
When a solution of an acid and that of a base are mixed, a neutralization reaction occurs. The products of the reaction have none of the characteristic properties of either the acidic or the basic solutions. In general, a neutralization reaction between an acid and a metal hydroxide produces water and a salt.
Acid-Base Reactions with Gas Formation
There are many bases besides OH- that react with H+ to form molecular compounds. Two of these that you might encounter in the laboratory are the sulfide ion and the carbonate ion. Both of these anions react with acids to form gases that have low solubilities in water.
Both NaHCO3 and Na2CO3 are used as acid neutralizers in acid spills. The bicarbonate or carbonate salt is added until the fizzing due to the formation of CO2(g) stops. Sometimes sodium bicarbonate is used as an antacid to soothe an upset stomach.