Acids and bases

Acids and bases are chemical compounds that have distinctive properties in water solution. The sour taste of a lemon, lime, or grapefruit, for example, is caused by citric acid. The slippery feel of ammonia, a common base, is characteristic of all bases. One of the most interesting properties of acids and bases is the way they react with indicators. An indicator is a material that changes color in the presence of an acid or a base. For instance, the hydrangea flower can be either pink or blue, depending on the amount of acid or base present in the soil in which it is planted.

Acids and bases have been known since prehistoric times. Vinegar, for example, is a water solution of acetic acid that has been used for centuries. The first modern definitions for acids and bases were suggested by Swedish chemist Svante Arrhenius (1859–1927). Arrhenius proposed that acids be defined as chemicals that produce positively charged hydrogen ions, H⁺, in water. By comparison, he suggested that bases are compounds that produce negatively charged hydroxide ions, OH⁻, in water. Acids and bases react with each other in a reaction called neutralization. In a neutralization reaction, the hydrogen ion from an acid and the hydroxide ion from a base react to form a molecule of water:

H⁺ + OH⁻ → H₂O

Other definitions of acids and bases

Since the time of Arrhenius, chemists have adopted other ways of defining acids and bases. In 1923, English chemist Thomas Lowry (1874–1936) and Danish chemists J. N. Brønsted (1879–1947) and N. Bjerrum (1879–1958) suggested defining acids as chemicals that donate a proton (specifically H⁺) in a chemical reaction and bases as chemicals that accept a proton. This definition is slightly more comprehensive than Arrhenius's definition and, in many cases, more useful to chemists.

Another definition of acids and bases was suggested in 1923 by American chemist Gilbert Newton Lewis (1875–1946). According to Lewis, an acid could be thought of as any compound that accepts a pair of electrons from another substance; bases, on the other hand, could be thought of as compounds that donate a pair of electrons.

Most acids and bases fit all three of these definitions, but some are covered by only one or both of the more modern definitions.

Strong and weak acids and bases

One of the most important characteristics of acids and bases is their strength. The strength of an acid or base depends on the number of hydrogen ions or hydroxide ions produced in water solution. For example, suppose that 100 molecules of an acid are added to water. Of those 100 molecules, imagine that 99 release hydrogen ions. That acid is said to be a strong acid. In comparison, suppose that 100 molecules of a second acid release only 10 hydrogen ions in water. That acid is said to be a weak acid.

Examples of strong acids are hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). Among the best-known strong bases are sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (CaOH₂). The weak acids include acetic acid (HC₂H₃O₂), lactic acid (CH₃CHOHCOOH), and oxalic acid (H₂C₂O₄). The most familiar weak base is ammonia (NH₃).

Neutralization

Acids and bases are sometimes described as the chemical opposites of each other. If equivalent quantities of an acid and a base are combined, the two compounds react to form a salt and water. For example:

HCl + NaOH → NaCl + H₂O

hydrochloric acid + sodium hydroxide → sodium chloride + water

This process is known as neutralization.

Neutralization has many practical applications. For example, agricultural land is often too acidic or too basic to grow certain crops. Farmers can add either a weak acid or a weak base to produce the level of acidity or basicity in which various plants grow best. Reclamation (restoration) of land once used for mining also involves neutralization reactions. Such land is often too acidic for plants to grow. Treating the land with calcium oxide neutralizes acids remaining in the soil and restores its fertility.

Neutralization is also used to deal with environmental problems. Gases produced in factories, power generating plants, and other industrial facilities are usually acidic. When they escape into the air, they react with water to form acid rain. Scrubbers that contain bases can be attached to the inside of smokestacks in such plants to neutralize acids in escaping gases. Treatment with acids or bases is also used to neutralize hazardous chemicals produced by a variety of manufacturing operations.

Uses of acids and bases

Acids and bases can be found everywhere in the world around us. Lactic acid occurs in sour milk, citric acid in citrus fruits, oxalic acid in rhubarb, malic acid in apples, and tartaric acid in wine. Baking soda, antacids, and lye all contain bases.

Acids and bases are also used widely in industry. Three of the top ten chemicals produced in the United States each year are acids or bases. In 1994, 40 billion kilograms (or about 90 billion pounds) of sulfuric acid were manufactured in the United States, making it the number one chemical in the chemical industry. In addition, 12 billion kilograms (about 26 billion pounds) of sodium hydroxide and 11 billion kilograms (about 25 billion pounds) of phosphoric acid were produced.

The most important single use of acids and bases is in the manufacture of other chemicals. Fertilizers, synthetic fabrics, pigments, petroleum, iron and steel, explosives, dyes, plastics, pesticides, soaps and detergents, paper, film, and many other chemicals are produced from acids and bases. They are also used for various other purposes, including cleaning surfaces, refining oil and sugar, electroplating metals, and treating food products.

acids and bases two related classes of chemicals; the members of each class have a number of common properties when dissolved in a solvent, usually water.

Properties

Acids in water solutions exhibit the following common properties: they taste sour; turn litmus paper red; and react with certain metals, such as zinc, to yield hydrogen gas. Bases in water solutions exhibit these common properties: they taste bitter; turn litmus paper blue; and feel slippery. When a water solution of acid is mixed with a water solution of base, water and a salt are formed; this process, called neutralization , is complete only if the resulting solution has neither acidic nor basic properties.

Classification

Acids and bases can be classified as organic or inorganic. Some of the more common organic acids are: citric acid , carbonic acid , hydrogen cyanide , salicylic acid, lactic acid , and tartaric acid . Some examples of organic bases are: pyridine and ethylamine. Some of the common inorganic acids are: hydrogen sulfide , phosphoric acid , hydrogen chloride , and sulfuric acid . Some common inorganic bases are: sodium hydroxide , sodium carbonate , sodium bicarbonate , calcium hydroxide , and calcium carbonate .

Acids, such as hydrochloric acid, and bases, such as potassium hydroxide, that have a great tendency to dissociate in water are completely ionized in solution; they are called strong acids or strong bases. Acids, such as acetic acid, and bases, such as ammonia, that are reluctant to dissociate in water are only partially ionized in solution; they are called weak acids or weak bases. Strong acids in solution produce a high concentration of hydrogen ions, and strong bases in solution produce a high concentration of hydroxide ions and a correspondingly low concentration of hydrogen ions. The hydrogen ion concentration is often expressed in terms of its negative logarithm, or p H . Strong acids and strong bases make very good electrolytes (see electrolysis ), i.e., their solutions readily conduct electricity. Weak acids and weak bases make poor electrolytes.

See buffer ; catalyst ; indicators, acid-base ; titration .

Acid-Base Theories

There are three theories that identify a singular characteristic which defines an acid and a base: the Arrhenius theory, for which the Swedish chemist Svante Arrhenius was awarded the 1903 Nobel Prize in chemistry; the Brönsted-Lowry, or proton donor, theory, advanced in 1923; and the Lewis, or electron-pair, theory, which was also presented in 1923. Each of the three theories has its own advantages and disadvantages; each is useful under certain conditions.

The Arrhenius Theory

When an acid or base dissolves in water, a certain percentage of the acid or base particles will break up, or dissociate (see dissociation ), into oppositely charged ions. The Arrhenius theory defines an acid as a compound that can dissociate in water to yield hydrogen ions, H ⁺ , and a base as a compound that can dissociate in water to yield hydroxide ions, OH ^-  . For example, hydrochloric acid, HCl, dissociates in water to yield the required hydrogen ions, H ⁺ , and also chloride ions, Cl ^-  . The base sodium hydroxide, NaOH, dissociates in water to yield the required hydroxide ions, OH ^- , and also sodium ions, Na ⁺ .

The Brönsted-Lowry Theory

Some substances act as acids or bases when they are dissolved in solvents other than water, such as liquid ammonia. The Brönsted-Lowry theory, named for the Danish chemist Johannes Brönsted and the British chemist Thomas Lowry, provides a more general definition of acids and bases that can be used to deal both with solutions that contain no water and solutions that contain water. It defines an acid as a proton donor and a base as a proton acceptor. In the Brönsted-Lowry theory, water, H ₂ O, can be considered an acid or a base since it can lose a proton to form a hydroxide ion, OH ^- , or accept a proton to form a hydronium ion, H ₃ O ⁺ (see amphoterism ). When an acid loses a proton, the remaining species can be a proton acceptor and is called the conjugate base of the acid. Similarly when a base accepts a proton, the resulting species can be a proton donor and is called the conjugate acid of that base. For example, when a water molecule loses a proton to form a hydroxide ion, the hydroxide ion can be considered the conjugate base of the acid, water. When a water molecule accepts a proton to form a hydronium ion, the hydronium ion can be considered the conjugate acid of the base, water.

The Lewis Theory

Another theory that provides a very broad definition of acids and bases has been put forth by the American chemist Gilbert Lewis. The Lewis theory defines an acid as a compound that can accept a pair of electrons and a base as a compound that can donate a pair of electrons. Boron trifluoride, BF ₃ , can be considered a Lewis acid and ethyl alcohol can be considered a Lewis base.

acid–base balance The regulation of the concentrations of acids and bases in blood and other body fluids so that the pH remains within a physiologically acceptable range (see pH scale). This is achieved by the presence of natural buffer systems, such as the haemoglobin, hydrogencarbonate ions, and carbonic acid in mammalian blood. By acting in conjunction, these effectively mop up excess acids and bases and therefore prevent any large shifts in blood pH. The acid–base balance is also influenced by the selective removal of certain ions by the kidneys and the rate of removal of carbon dioxide from the lungs.

acid-base balance n. the balance between the amount of carbonic acid and bicarbonate in the blood, which must be maintained at a constant ratio of 1:20 in order to keep the hydrogen ion concentration of the plasma at a constant value (pH 7.4).