Acids, Bases, and Salts

Acids
In everyday life we deal with many compounds that chemists classify as acids. For example, orange juice and grapefruit juice contain citric acid. These juices, and others, also contain ascorbic acid, a substance more commonly known as Vitamin C. Salads are often flavored with vinegar, which contains dilute acetic acid. Boric acid is a substance that is sometimes used to wash the eyes.
In any chemistry laboratory, we find acids such as hydrochloric acid, sulfuric acid, and nitric acid. These acids are called mineral acids because they can be prepared from naturally occurring compounds called minerals. Mineral acids are generally stronger than household acids, and should be handled with great care because they can burn skin and clothing.

Properties of Acids:

Acids taste sour. Citric acid is responsible for the sour taste of lemons, limes, grapefruits, and oranges. Acetic acid is responsible for the sour taste of vinegar.

Acids turn litmus (or indicator papers) red. Litmus is a vegetable dye that may be either red or blue, depending on the acidity. When a sample of an acid is placed on red litmus paper, the color of the litmus does not change. Red litmus has been previously treated with acid. Adding more acid does not change the red color. However, when the same acid is placed on blue litmus paper, the color turns from blue to red. (Blue litmus has been treated with a base).

Acids contain combined hydrogen. When a sample of zinc, a fairly reactive metal, is dropped into a test tube containing an acid such as hydrochloric acid, a reaction occurs. The bubbling in the tube indicates that a gas is released. When we test this gas by inserting a burning splint into the test tube, the gas burst into flame and produces a small popping sound. This is the characteristic test for hydrogen gas. In general, when certain acids react with metals, hydrogen gas is released. See following reactions:
Zn (s) + 2HCl (aq)  H2 (g) + ZnCl2 (aq)
Zn (s) + H2SO4 (aq)  H2 (g) + ZnSO4 (aq)

Acids release hydrogen in water solutions. When an acid dissolves in water, the acid ionizes, releasing both hydrogen ions and ions of a nonmetal or nonmetallic polyatomic ion. Thus, when hydrochloric acid is dissolved in water, the acid ionizes, forming hydrogen ions and chloride ions, as shown in the following equation:
HCl (aq)  H+ (aq) + Cl1- (aq)
Other examples:
H2SO4 (aq)  2 H+ (aq) + SO42- (aq)
H3PO4 (aq)  3 H+ (aq) + PO43- (aq)
Special example:
HC2H3O2 (aq) <--> H+ (aq) + C2H3O21- (aq)
Note the use of the double arrow in the ionization of acetic acid. We know that acetic acid is a weak acid. The smaller arrow pointing to the right indicates that the change to the right (ionization) is relatively small. This means that, in a solution of acetic acid, we have a large number of acetic acid molecules and few hydrogens and few acetate ions.
Thus acids are defined as substances that release hydrogen ions in solution. It is these H+ (aq) that are responsible for the properties of acids.

How Acids are Prepared:
Acids may be prepared when certain gases, related to acids, are dissolved in water and when certain salt compounds, again related to acids, are allowed to react with sulfuric acid.
CO2 (g) + H2O (l)  H2CO3 (aq) carbonic acid
2 NaCl (s) + H2SO4 (aq) + heat  2 HCl (g) + Na2SO4 (aq)
Uses of Acids:
Sulfuric acid is the chemical most widely used in industry. Sulfuric acid is also used to make other acids such as hydrochloric and nitric acid because the boiling point of sulfuric acid is higher that that of other acids. This allows the acid being produced to be distilled and collected separately from the starting material. Sulfuric acid is also used to remove the surface oxide layers on metals (pickling) before the metals are coated with materials that prevent rusting. For example, before iron is coated with chromium (in chromium plating), the iron is dipped into dilute sulfuric acid to remove the iron oxide normally present on the surface of the iron. Another important use of sulfuric acid is the storage cell. In a lead storage cell, dilute sulfuric acid serves as the electrolyte through which ions move between the lead plates, acting as the cathode, and the spongy lead dioxide, acting as the anode. Several such cells connected together make up the type of storage battery used in automobiles.
Nitric acid, another important industrial acid, is used in the manufacture of fertilizers, plastics, photographic film, and dyes. Nitric acid is also used in the preparation of such explosives as dynamite and TNT. Hydrochloric acid, like sulfuric acid, is used to clean metals. Hydrochloric acid is also used to clean brick and tile; it is used in the manufacture of sugar and glue. Hydrochloric acid is produced in small quantities in the stomach where the acid aids digestion.

Bases:
Ammonium hydroxide, or ammonia water, is very irritating to the nose and the eyes. This substance, called a hydroxide, or a base, is often used in the home for cleaning because bases generally dissolve grease. Milk of magnesia (magnesium hydroxide), which is used as an antacid, is a base; lye (sodium hydroxide), which is used in the manufacture of soap, is another familiar example of base.
Bases are ionic compounds containing metal ions and hydroxide ions. For example, sodium hydroxide contains sodium ions and hydroxide ions. When sodium metal is placed in water, sodium hydroxide is formed and hydrogen gas is released. Since the formula for water can be written as HOH instead of H2O, the reaction involves single replacement:
2 Na (s) + 2 HOH (l)  2 NaOH (aq) + H2 (g)

Properties of Bases: (in water solutions)
1. Bases taste bitter. A bitter taste is characteristic of all bases. It is the presence of a base that give unflavored milk of magnesia its bitter taste.
2. Bases feel slippery. If you rub a drop or two of household ammonia between your fingers, you experience the slippery feeling of a base. Wet soap is also slippery because of the presence of a base.
3. Bases turn red litmus blue. A common indicator, used to detect the presence of a base, is phenolphthalein which, when mixed with a base, turns pink.
4. Bases release hydroxide ions in water solutions. When dissolved in water, bases ionize releasing metal ions (or metallic polyatomic ions) and hydroxide ions. For example: when sodium hydroxide is dissolved in water, it ionizes as:
NaOH (s) + H2O (l)  Na1+ (aq) + OH1- (aq)
NH4OH (aq) <--> NH41+ (aq) + OH1- (aq) double arrow indicates ionization of weak base ammonium hydroxide
Thus bases are defined as substances that release hydroxide ions in solution. It is these OH1- (aq) ions that are responsible for the properties of bases.

How Bases are Prepared:
Bases may be prepared by the reaction of water and very reactive metals, related to the base, and by the reaction of water and certain oxides, again related to the base.
2 K (s) + 2 HOH (l)  2 KOH (aq) + H2 (g)
CaO (s) + HOH (l)  Ca(OH)2 (aq) plus considerable heat released

Uses of Bases:
Ammonium hydroxide, frequently called ammonia, is used in the preparation of important related compounds such as nitric acid and ammonium chloride. Ammonia is also used as a cleaning agent.
Sodium hydroxide is used in the manufacture of soap, rayon, and paper. Strong solutions of this base are very caustic; that is, they are extremely harmful to the skin.
Calcium hydroxide, commonly known as slaked lime, is used in the preparation of plaster and mortar. Water solutions of calcium hydroxide, called limewater, can be used in the lab as a test for the presence of carbon dioxide.


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Sebastian

G.M. Technical

Nunes Instruments

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