ACIDS, BASES, AND SALTS

BASIC CONCEPT

Substances known as acids and bases are commonly encountered and have important chemical properties. Therefore, this group of substances deserves separate treatment.

According to the Arrhenius model, an acid is any substance that, when dissolved in water, produces or causes to be produced hydrogen ions, H+. A base is any substance that, when dissolved in water, produces or causes to be produced hydroxide ions, OH-. The Bronsted-Lowry model stresses the fact that acids are proton donors and bases are proton acceptors, as well as the notion of conjugate (meaning “related”) acids and bases. The Lewis model states that acids are substances that are electron-pair acceptors and bases are substances that are electron-pair donors. This concept broadens the meaning of acids and bases considerably to include many salts that would otherwise be excluded. Realistically, since a hydrogen ion is a bare proton, it is too reactive to exist as a stable species by itself. Hence, the real acid species is the hydronium ion, H3O+, a protonated water molecule, that is, [H+(H2O)]. Acids are powerful dehydrating agents.

When an acid and a base are added together, a neutralization (a double-replacement reaction) ensues, with water and a salt as the neutralization products. Quantitatively, one mole of acid, H3O+, exactly neutralizes one mole of base, OH-. Thus, this is true for monoprotic acids and monobasic bases. Monoprotic acids contain one H+ ion, such as HNO3, while diprotic acids contain two H+ ions, such as H2SO4, and thus require two OH- ions to neutralize them. In general, polyprotic acids contain more than one ionizable H+ ion; examples include phosphoric acid, H3PO4, with three H+ ions, oxalic acid, C2O4H2, with two H+ ions, as well as the aforementioned sulfuric acid.

Acids and bases may be strong or weak. This property is determined by molecular structure and is different from concentration. A strong acid is one that dissociates and ionizes completely or 100% into its hydrogen or hydronium ions and corresponding anions, known as its conjugate base. Similarly, a strong base is one that dissociates and ionizes completely into hydroxide ions and its corresponding cations, known as its conjugate acid. Weak acids and bases, in contrast, dissociate and ionize less than 100%, conventionally less than 10%, and typically less than 5%, into their respective ions. Recall that these substances are weak electrolytes, while strong acids and bases are strong electrolytes.

There are four common strong acids. All others can be assumed, by exclusion, to be weak. [1]

  • Sulfuric acid, H2SO4 [This is a diprotic acid since it has two hydrogen ions.]
  • • Perchloric acid, HCIO4

Examples of common weak acids include acetic acid (in vinegar), boric acid (in the medicine cabinet), hydrofluoric acid (used as an agent to etch glass), ascorbic acid (vitamin C), carbonic acid, and phosphoric acid (the latter two in many carbonated beverages). Calculations for weak acids and bases are treated in Section 2.3 of Chapter 2.

Examples of strong bases are sodium hydroxide (NaOH) and potassium hydroxide (KOH). Examples of weak bases are ammonia, NH3, which when dissolved in water becomes ammonium hydroxide, NH4OH, and organic amines, such as methyl amine, CH3NH2.

When dissolved in water, oxides of metals (e.g., CaO, K2O, MgO) undergo hydrolysis and are generally basic, while oxides of nonmetals (e.g., CO2, SO2, SO3) also undergo hydrolysis and are generally acidic. Hydrolysis simply means reaction with water.

  • [1] Hydrochloric acid, HCl [HBr and HI are also strong but uncommon; HF is weak.] • Nitric acid, HNO3 [This is a monoprotic acid since it has only onehydrogen ion.]
 
Source
< Prev   CONTENTS   Source   Next >