208

Unit 2
Elements of Chemical Change

2-8. SALTS

Previously, it has been stated that one of the properties associated with acids and bases is the neutralization reaction. This reaction involves the production of a salt and water from the reaction of an acid and a base. We will now examine various types of salts produced in neutralization reactions. Salts are the third major classification of inorganic compounds (acids and bases being the first two). They are important in the physiology of the body and are often used as therapeutic agents.

Definition. We have already given one definition of a salt in our discussion, that is, the product of a reaction between an acid and a base. A more specific definition, however, would be an ionic compound formed by the replacement of part or all of the acid hydrogen of an acid by a metal or a radical acting like a metal. It is an ionic compound that contains a positive ion other than hydrogen and a negative ion other than hydroxyl (OH-) or "O-2," as in MgO.

Types of Salts. There are four types of reactions possible between acids and bases as we classified them (strong or weak) earlier. These are as follows:

Table 2-1. Relative strength of common acids and bases.

Relative strength of common acids and bases

STRONG ACIDS
HCl Hydrochloric acid
H2SO4 Sulfuric acid
H3PO4 Phosphoric acid
WEAK ACIDS
HC2H3O2 (HAC) Acetic acid
H2CO3 Carbonic acid
H3BO3 Boric acid
STRONG BASES
KOH Potassium hydroxide
NaOH Sodium hydroxide
WEAK BASES
Fe(OH)2 Ferrous hydroxide
Al(OH)3 Aluminum hydroxide
NH3 Ammonia
* Ca(OH)2 Calcium hydroxide
Mg(OH)2 Magnesium hydroxide
MgO Magnesium oxide

Notice that Ca(OH)2, Mg(OH)2, and MgO which forms Mg(OH)2 in water are chemically classified as strong bases because of their high degree of dissociation. Because they are only slightly soluble in water, they produce low concentrations of the hydroxide (OH-) ion in solution. Since calcium hydroxide and magnesium hydroxide do not produce tissue damage, they can be safely used as therapeutic agents (e.g., antacids).

Reaction 1. Strong acid and strong base: HCl+ NaOH NaCl + H2O
Reaction 2. Weak acid and weak base: 2H2CO3 + Fe(OH)2 Fe(HCO3)2 + 2H2O
Reaction 3. Strong acid and weak base: 2HCl + Fe(OH)2 FeCl2 + 2H2O
Reaction 4. Weak acid and strong base. H2CO3 + NaOH  NaHCO3 + H2O
NOTE: These four reactions result in three types of salts. Reactions 1 and 2 result in neutral salts (that is, in terms of pH), which means a solution of the salt in water will be a neutral solution. Reactions such as 3 result in acidic salts, which produce acidic solutions. Reaction 4 results in basic salts, which produce basic solutions.

Determination of Salt Type. To determine the type of salt from a chemical formula, we employ the following steps:

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Example. Al2(SO4)3.

Example. FeBO3.

Importance of Type of Salt. The type of salt is very important when a salt is used medicinally, since the body maintains a specific acidity in the tissues and fluids. The type of salt is also important in the prediction and understanding of incompatibilities. It is important for you to identify the type of salt from its formula. The importance and use of the type will become clear to you as you progress through the course.

Table 2-2. Salt types resulting from various acid-base combinations.

Reactants
Weak Acid Strong Acid
Weak Base Neutral Salt Acidic Salt
Strong Base Basic Salt Neutral Salt