Chemistry: Exploring Matter, Properties & Reactions

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Introduction to Chemistry

Chemistry, the science of the composition of matter, its properties and characteristics, and the changes it undergoes. Chemistry deals with the properties that distinguish one substance from another.

Many people think of chemistry and chemical changes in terms of mysterious experiments conducted in laboratories with strange chemicals and complex apparatus. This is only one aspect of chemistry. Chemical changes are also natural processes and take place all about us.

The burning of coal, gas, and wood; the cooking of meat and other foods; the rusting of a kitchen knife and the tarnishing of silver-all these things which we take for granted involve chemical changes. Life processes such as growth, digestion, and breathing are also examples of chemical change. Many of these changes, which take place in our bodies, are so complex that they cannot yet be duplicated in chemical laboratories.

Important dates in chemistryc. 3500 B.C. People learned to make bronze.c. 400 B.C. Democritus proposed an atomic theory.A.D. 600's Alchemy began to spread from Egypt to the Arabian Peninsula and reached western Europe in the 1100's.1600's Robert Boyle taught that theories must be supported by careful experiments.Early 1700's Georg Ernst Stahl developed the phlogiston theory.1750's Joseph Black identified carbon dioxide.1766 Henry Cavendish identified hydrogen as an element.1770's Carl Scheele and Joseph Priestley discovered oxygen.Late 1700's Antoine Lavoisier stated the law of the conservation of mass and proposed the oxygen theory of combustion.1803 John Dalton proposed his atomic theory.1811 Amedeo Avogadro suggested that equal volumes of all gases at the same temperature and pressure contain equal numbers of particles.Early 1800's Jons J. Berzelius calculated the masses of a number of elements.1828 Friedrich Wohler made the first synthetic organic substance from inorganic compounds.1856 Sir William H. Perkin made the first synthetic dye.1869 Dmitri Mendeleev develops the first modern periodic table. Julius Lothar Meyer independently creates a similar table the following year.1910 Fritz Haber patented a process to produce synthetic ammonia.1913 Niels Bohr proposed his model of the atom.1916 Gilbert N. Lewis described electron bonding between atoms.1950's Biochemists began to discover how such chemicals as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) affect heredity.Early 1980's Chemists began working to develop a solar-powered device that produces hydrogen fuel by means of the chemical breakdown of water.1985 Richard E. Smalley, Robert F. Curl, Jr., and Harold W. Kroto discover buckminsterfullerene, a ball-shaped molecule consisting only of carbon.

Chemistry In Everyday Living

There is hardly a phase of daily life in which chemistry does not play a part. The foods we eat, the clothing we wear, the homes in which we live, and the countless material things we enjoy have nearly all been influenced and improved by chemical research. In many fields of endeavor, including agriculture, manufacturing, and medicine, there are countless examples of the contributions of chemistry.

Food

Chemistry plays an important role in the preservation and preparation of food. Many foods are kept from spoiling through the use of chemical preservatives. Most refrigerators use fluids that were discovered through chemical research. Chemical research has also contributed to the analysis of foods and a knowledge of their nutritive values. The discovery of vitamins resulted from chemical science.

Clothing

An important contribution to clothing has been the development of synthetic fibers, such as nylon, Oron, and Dacron. Some natural fibers are chemically processed to increase their strength and durability. Chemistry has also made it possible to bleach, dye, or dry-clean materials used in clothing.

Health

The development of the science of chemistry has made possible better diagnosis, prevention, and cure of diseases. Antiseptics, anesthetics, and antibiotics have saved countless lives.

Branches of Chemistry

The organized knowledge of the science of chemistry is so vast that it has been subdivided into several specialized fields. Inorganic chemistry is mainly concerned with the properties and preparation of compounds of the chemical elements contained in minerals. The element carbon forms such a vast variety of compounds that study of these constitutes a separate branch of chemistry, organic chemistry.

Biochemistry is a division of organic chemistry that is specifically concerned with the carbon compounds found in living organisms. It combines biological knowledge with that of chemistry. Physiological chemistry studies the chemical changes occurring in the life functions of living organisms.

Analytical chemistry deals with the identification of the constituents of a substance. It is divided into two main branches, qualitative and quantitative analysis. The methods used in qualitative analysis determine the constituents of substances; those of quantitative analysis determine the amount of each constituent present in the substance.

Physical chemistry is concerned with the physical properties of chemical substances and with interpreting chemical phenomena in terms of underlying physical processes.

Some other specialized branches of chemistry are radiochemistry, which deals with the use of radioactive substances in the study of chemical changes; geochemistry, which deals with the chemical composition of rocks and minerals; electrochemistry, which deals with the relation between electricity and chemical changes; and photochemistry, which deals with chemical changes produced by the action of light.Chemical thermodynamics studies conversion of energy during chemical reactions and effects of pressure and temperature on reactions. Applied chemistry involves the use of theoretical chemistry concepts in practical applications. This includes industrial chemistry, the science of chemical reactions on an industrial scale.

Stereochemistry involves the study of the way atoms and molecules are aligned in a substance and how the alignment affects the properties of the substances. Related to this is polymer chemistry, where chemists study molecules made of a chain of small molecules. Surface chemistry involves the study of the surface properties of substances.

Major branches of chemistryAnalytical chemistry determines the properties of chemical substances and the structure and composition of compounds and mixtures.Qualitative analysis identifies the types of elements and compounds that make up substances.Quantitative analysis measures the amounts of the different chemicals that make up substances.Radiochemistry involves the identification and production of radioactive elements and their use in the study of chemical processes.Applied chemistry refers to the practical use of the knowledge of chemical substances and processes.Agricultural chemistry develops fertilizers and pesticides and studies the chemical processes that occur in the soil and that are involved in crop growth.Environmental chemistry studies, monitors, and controls chemical processes and other factors in the environment and their relationships to living things.Industrial chemistry involves the chemical production of raw materials and the development, study, and control of industrial chemical processes and products.Biochemistry deals with the chemical processes of living things.Inorganic chemistry concerns chemical substances that do not contain carbon-to-carbon bonds.Organic chemistry is the study of chemical substances that contain carbon-to-carbon bonds.Physical chemistry interprets chemical processes in terms of physical properties of matter, such as mass, motion, heat, electricity, and radiation.Chemical kinetics studies the sequence of steps in chemical reactions and the factors that affect the rates at which chemical reactions proceed.Chemical thermodynamics deals with the energy changes that occur during chemical reactions and how temperature and pressure differences affect reactions.Nuclear chemistry is the use of chemical techniques in the study of nuclear reactions.Quantum chemistry analyzes the distribution of electrons inmolecules and interprets the chemical behavior of molecules in terms of their electron structure.Radiation chemistry concerns the chemical effects of high-energy radiation on substances.Solid-state chemistry deals with the composition of solids and the changes that occur within and between solids.Stereochemistry studies the arrangement of atoms in molecules and the properties that follow from such arrangements.Surface chemistry examines the surface characteristics of chemical substances.Polymer chemistry deals with chainlike molecules formed by linking smaller molecules; and with plastics, which consist of chainlike molecules, often combined with other materials.Synthetic chemistry involves combining chemical elements and compounds to duplicate naturally occurring substances or to produce compounds that do not occur naturally.

The Chemist

The chemist's work depends on his or her specialty-analytical chemistry, biochemistry, etc. Today's chemists work, often in teams, in large, well-equipped laboratories. Many are employed by private industry or by government. Others teach. Some chemists concentrate on research, on testing, or on selling chemical products.

A successful chemist must have an orderly mind, curiosity, initiative, patience, and keen powers of observation. Training includes four years or more of college with a major in chemistry or chemical engineering and a strong emphasis on mathematics. Those who wish to pursue research as a career need to have a doctoral degree. Chemists and chemical engineers have been in great demand since World War II and are generally well-paid. The professional association of chemists and chemical engineers is the American Chemical Society.

Chemists use many kinds of techniques and instruments. A mass spectrometer is used for calculating the mass of molecules. For isolating minute traces of components in a mixture, chemists use chromatography. Spectroscopy is another technique related to the study of arrangement of atoms in a molecule.

The Fundamentals of Chemistry

Nature and Structure of Matter

Anything that occupies space and has mass is called matter. Matter can exist in any one of three physical states: solid, liquid, or gaseous.

Matter undergoes changes that may be either chemical changes or physical changes. When water is transformed into steam it still retains all the chemical properties of water; that is, only its physical state is changed. Upon cooling, it returns to its original liquid state. However, when a piece of wood is burned, the original substance disappears, and new substances are formed. This is a chemical change, the kind of change with which chemistry is primarily concerned.

Elements

All matter is made up of fundamental substances called elements. An element cannot be broken down or decomposed by ordinary chemical means. The smallest part of an element that can take part in chemical reactions is the atom, and the chemical properties of an element are really the properties of its atoms. Chemists study the structure of atoms of different elements to gain a better understanding of their chemical behavior.

An atom has a nucleus made up of protons, which are positively charged, and neutrons, which are uncharged. Outside the nucleus are one or more electrons, which are negatively charged and may be thought of as revolving about the nucleus. Chemical reactions between atoms involve only the arrangement of the electrons in the atom; the arrangement of the protons and neutrons in the nucleus is not affected. All the atoms of one element have the same chemical properties, but two or more atoms of an element may differ slightly in weight. These atoms, called isotopes of the element, have different numbers of neutrons.

The elements are classified into two general categories: metals and nonmetals. However, since some elements have properties of both classes, there is no sharp dividing line between the two groups. The metals are characterized by such physical properties as luster; high conductivity of heat and electricity; and high tensile strength, malleability, and ductility (that is, they may be hammered or rolled into sheets or drawn into wire). All the nonmetals except one are either brittle solids or gases at ordinary temperatures. The exception is bromine, a liquid. The solid nonmetals have lower tensile strength and are poorer conductors of heat and electricity than metals.

There are two distinctive properties that determine the chemical characteristics of the atoms of an element: (1) atomic number, and (2) atomic weight.

Atomic Number

The positive charge on the nucleus is its atomic number. This is the number of protons in the nucleus and, since the whole atom is neutral, it also is the number of electrons an atom holds outside the nucleus. Isotopes of an element have the same atomic number. The range of atomic numbers is from 1 (for hydrogen) to more than 100.

Atomic Weight

Atomic weight values are determined by comparing the average weight of an element's atoms with the weight of the most abundant isotope of carbon, C-12, which is arbitrarily given the value 12. Since naturally occurring carbon is about 99 per cent C-12 and slightly more than 1 per cent C-13, the atomic weight of carbon is 12.011. A gram-atomic weight of an element is the amount whose weight in grams is numerically equal to the atomic weight. A gram-atomic weight of any element always contains the same number of atoms-6.02 x 1023 atoms. This number is called Avogadro's number.

Compound

A compound is a substance composed of two or more elements that are chemically combined. The elements may be present either as electrically charged atoms or groups of atoms called ions, or as atoms that are joined to form molecules.

Each molecule of a pure compound contains the same relative number of atoms. Thus, the water molecule is made up of 2 atoms of hydrogen combined with 1 atom of oxygen and is written symbolically H2O. Hydrogen and oxygen are gases; water is a liquid.

Molecular Weight

Just as atoms can be assigned a relative atomic weight, so molecules have a molecular weight. The molecular weight is obtained by adding together the atomic weights of the atoms present in the molecule. In the example above, the molecular weight of a molecule of water is 18 (16 for oxygen plus 1 for each of the two hydrogen atoms). A gram-molecular weight of a substance is the amount whose weight in grams is numerically equal to the molecular weight. A gram-molecular weight of any substance contains 6.02 x 1023 molecules.

Chemical Compositionof the earth's crust ElementPer cent by weightOxygen46.60Silicon27.70Aluminum8.10Iron5.00Calcium3.60Sodium2.80Potassium2.60Magnesium2.10Titanium0.50Hydrogen0.10Phosphorus0.10Manganese0.10Fluorine0.10Sulfur0.05Carbon0.03Chlorine0.03All others0.49Total100.00of seawater ElementPer cent by weightOxygen85.50Hydrogen10.80Chlorine1.90Sodium1.05Magnesium0.13Sulfur0.09Calcium0.04Potassium0.04Bromine0.007Carbon0.003Nitrogen0.002Strontium0.0008Boron0.0005Silicon0.0003Fluorine0.0001Argon0.000005All others0.436295Total100.00of the human body ElementPer cent by weightOxygen65.00Carbon18.00Hydrogen10.00Nitrogen3.00Calcium2.00Phosphorus1.00Potassium0.35Sulfur0.25Chlorine0.15Sodium0.15Magnesium0.05Iron0.004All others*0.046Total100.00* Includes fluorine, iodine, copper, manganese, vanadium, zinc, molybdenum, nickel, cobalt, cadmium, aluminum, lithium, and barium.Mixture

A mixture is a material composed of two or more substances that are not chemically joined. These substances retain their individual properties and characteristics even though they have been mixed together. The proportions of a mixture, unlike those of a compound, can be varied. Furthermore, its constituents can be separated from each other by nonchemical means. For example, a mixture of sand and salt will have some of the characteristic taste of the salt and the gritty feel of the sand. It can be separated by dissolving out the salt in water to leave the sand. The water then can be evaporated away from the salt solution to reclaim the salt.

Chemical Symbols, Formulas, and Equations

For the sake of convenience, chemists have developed a kind of shorthand system by which they can record chemical reactions. This system consists of: (1) symbols, which represent the atoms of the elements; (2) formulas, which show how these atoms are combined into molecules; and (3) equations, which express chemical changes between molecules and atoms.

Symbols

Every element-except for new elements that have not yet been given official names-is represented by a definite symbol universally used in chemical literature and the same in all languages. The symbol stands for one atom of an element.

Formulas

Every pure substance, either an element or a compound, can be chemically identified by a formula. The formula indicates what elements are present and how many atoms of each are combined in the molecule of the substance. Thus in the example above, H2O is the formula for the water molecule. Some formulas are very complicated. The formula for vitamin B12, for example, is C63H90CoN14O14P. There is only one atom each of phosphorus and cobalt but larger numbers of the carbon, hydrogen, oxygen, and nitrogen atoms in the molecule.

Sometimes elements exist as molecules that are combinations of like atoms. Hydrogen gas and oxygen gas are diatomic, H2 and O2. Some can be polyatomic, for instance, S8 or P4.

Equations

A chemical equation is a way of describing chemical changes. An equation indicates: (1) what substances enter into the reaction; (2) what substances result from the reaction; and (3) the number of molecules of each substance that enter into and result from the reaction. For example, when methane is mixed with oxygen, they combine, forming water and carbon dioxide. This reaction is expressed by the following equation:

CH4 + 2O2 2H2O + CO2

The equation indicates that one molecule of methane (CH4) combines with two molecules of oxygen (as indicated by the coefficient 2 in front of the formula for free oxygen, (O2) to form two molecules of water (H2O) and one molecule of carbon dioxide (CO2). Since the equation is balanced (has the same number of atoms of each element on both sides of the arrow), the reaction is complete-all the methane and all the oxygen combine into water and carbon dioxide.

Because gram-molecular weights of all substances have the same number of molecules, the equation also indicates that one gram-molecular weight of methane (16 grams, approximately) will combine with two gram-molecular weights of oxygen (64 grams) to form two gram-molecular weights of water (36 grams) and one gram-molecular weight of carbon dioxide (44 grams).

Valence

The capacity of an atom to combine with other atoms is referred to as its valence. This property of an atom results in its ability to combine with different numbers of atoms of different elements. Hydrogen is assigned a valence of +1; oxygen, of -2. The + and - designations are convenient to help visualize the properly constructed molecular formula as being neutral. Thus, H2O implies 2 x (+1) is balanced by 1 x (-2). In the case of lime, calcium oxide, the formula CaO implies that the valence of the calcium atom is +2, balancing oxygen's valence of -2. If the correct formula for hydrogen chloride is HCl, then chlorine must have a valence of -1. The prediction then can be made that the compound of calcium and chlorine must be CaCl2. Two atoms of chlorine, each -1, are needed to satisfy the +2 valence of calcium.

Modern chemical theory interprets valence in terms of the number and arrangement of atoms' electrons. A useful way of picturing the arrangement of the electrons is in terms of the shell model. According to this concept, an atom's electrons are arranged in groups (called shells) whose members are equidistant from the atom's center. The way in which an element combines with another generally depends on the number of electrons that occupy the outermost shell of an atom of the element. The outermost shell is thus called the valence shell and the electrons in it are called valence electrons.

In forming compounds, atoms tend to lose their valence electrons or to gain extra electrons to form a complete valence shell. In general, valence shells may contain up to eight electrons. An important exception is the valence shell of hydrogen; it can contain no more than two electrons. Most metals have only a small number of valence electrons and tend to lose them. In losing electrons, an atom becomes positively charged, forming a positive ion. Other elements, notably active nonmetals, have a tendency to capture extra electrons to complete the valence shell. In gaining electrons, an atom becomes a negative ion. Positive and negative ions may then be held together by electrical attraction to form a chemical compound. Common salt, NaCl is an example of this ionic, or electrovalent, type of bonding. Under other circumstances, atoms are thought of as being arranged into molecules in such a way that electrons are shared between them. The bonds that occur in this manner are called covalent bonds.

Laws of Chemical Combination

Modern chemistry has many laws. These laws are basic principles that appear to govern the chemical behavior of atoms and molecules. Although chemists generally have no means of watching or measuring individual atoms, their theories account very satisfactorily for the behavior of large numbers of atoms. Consequently they often speak in terms of atoms when describing chemical phenomena. The most important chemical laws are as follows:

• Law of Conservation of Mass. The atom is indestructible in chemical reactions. According to this principle, a chemical reaction leaves the total mass of the substances involved unchanged. Strictly speaking, the mass is the same only if the energy gained or lost in the reaction is taken into account. However, in chemical reactions, the energies are relatively small and the changes in mass are too small to be detected.
• The Law of Constant Composition. In making any given compound, the atoms of the elements involved always combine in exactly the same way. In other words, a compound can have only one formula. Thus the proportion of the atomic weights of the elements in a compound will always be the same. For example, the compound hydrogen peroxide, H2O2, must be different from H2O, water.
• Law of Multiple Proportions. Atoms combine with each other only in whole numbers; no molecule will hold only a fraction of an atom. A formula such as HO1/2 is not possible.

Chemical Changes

In a chemical change the substances taking part in the reaction lose their original identity. The substances formed as a result of the reaction have new physical and chemical properties. There are vast numbers of different chemical reactions. It is possible to classify some of the simplest under the following headings:

Combination, or Synthesis

In this type of reaction, an element or compound combines with another element or compound. For example, copper can react with chlorine to form copper chloride:

Cu + Cl2 CuCl2

Combination often results in the release of heat.

Decomposition

In this type of reaction, a compound breaks down into simpler substances. Decomposition can be considered the reverse of combination. For example, water can be broken down by electricity to form the gases hydrogen and oxygen:

2H2O + electricity 2H2 + O2

Some forms of decomposition result in the release of energy. Explosions are violent decomposition reactions that release large amounts of energy.

Single Replacement

In this type of reaction, one element or radical replaces another element or radical in a compound. A radical is a group of atoms that behaves chemically as a single atom. For example, zinc can replace the hydrogen in hydrochloric acid to form zinc chloride and hydrogen gas:

Zn + 2HCl ZnCl2 + H2

Metathesis, or Double Replacement

In this type of reaction, an element or radical of one compound changes places with an element or radical of another compound. For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water:

HCl + NaOH NaCl + H2O

Chemical Energy

Every substance has chemical energy because of its chemical composition. This energy is latent, or inactive, when there is no chemical activity. However, when a chemical change takes place, the energy becomes kinetic, or active. For example, a fuel such as coal has latent energy because of its high combustibility. When the coal is ignited the energy becomes kinetic and is released in the form of heat.

Chemical energy is measured in the form of heat, the unit of which is the calorie. One calorie, also called the small calorie, is the quantity of heat required to raise the temperature of one gram of pure water one degree Celsius. In the laboratory, a more convenient unit is the kilocalorie (kcal.), which is equal to 1000 calories. Chemical engineers often use the British thermal unit (Btu), which is defined as the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. One Btu equals 252 calories.

Classification of Elements

In the 18th century scientists realized that certain groups of elements had similar physical and chemical properties. The classification of elements began in the early 19th century when the German chemist Johann Dbereiner arranged elements with similar properties in groups of three, called triads. He also was the first to investigate the numerical relations between the atomic weights of elements.

By the mid-1850s larger groups of related elements had been discovered. It also was found that certain numerical relations exist between the atomic weights of related elements. The study of atomic weights was further developed in the late 1850's largely through the work of William Odling in England and Jean Baptiste Dumas in France. Their contributions laid the basis for the modern system called the periodic classification of elements.

In 1865 John Newlands, an English chemist, found that if the elements were arranged in order of increasing atomic weights, similar properties tended to be repeated at intervals of eight. This relationship, called the law of octaves, did not hold for all elements, and therefore found no acceptance at that time. However, Newlands' idea of a periodic recurrence of properties was basically correct, and its importance was realized a few years later after the discovery of the periodic law.

Periodic Law

The Russian chemist Dmitri Mendeleev and the German chemist Julius Lothar Meyer independently discovered the periodic law. It states that the properties of elements are periodic functions of their atomic weights. Mendeleev published his report in 1869-a year earlier than Meyer. Both scientists, who were unaware of Newlands' work, arranged the elements in order of increasing atomic weights and in groups, or families, of related elements. In this way they obtained a natural classification based on the properties of elements.

Mendeleev went even further and predicted with great accuracy the properties of undiscovered elements. Within 20 years three new elements-gallium, scandium, and germanium-were discovered; they had properties almost exactly as Mendeleev predicted.

Modern Periodic Classification

Probably the most significant implication in the periodic law was that atoms must have similarities in their structure, which are responsible for similarities in properties. Progress in physics and chemistry was so rapid that by 1914 it became apparent that the correct basis for putting the elements in order was the atomic number, rather than the atomic weight. This basis for classification permits the chemist to deduce something about an atom's structure from its position in the periodic table. Vertical groupings of elements that have similar properties are called families. The properties of elements in a horizontal grouping, or period, show gradual change from metallic on the left to nonmetallic on the right.

Inorganic Compounds

Inorganic chemists put many of the compounds they study into one of four classifications: (1) oxides; (2) acids; (3) bases; and (4) salts.

Oxides

An oxide is a chemical compound formed by oxygen and another element, either metallic or nonmetallic. The compound so formed is said to be an oxide of that element. For example, when zinc (Zn) combines with oxygen (O) it forms an oxide called zinc oxide. The balanced equation is:

2Zn + O2 2ZnO

When carbon (C), a nonmetallic element, combines with oxygen, carbon dioxide (C + O2 CO2) or carbon monoxide (2C + O2 2CO) is formed.

The oxides of metallic elements, such as zinc oxide, are called basic anhydrides. In solution with water, these compounds form bases. The oxides of nonmetallic elements, such as carbon dioxide, are called acid anhydrides. When these oxides are dissolved in water, they form acids.

Acids

Acids are hydrogen-containing compounds whose water solutions have certain characteristic properties. These solutions usually taste sour, react with metals to release hydrogen gas, and conduct electric current. The simplest test for an acid is to use an indicator, such as litmus. A piece of blue litmus paper turns red when moistened with an acid.

The ability of a water solution of an acid to conduct electricity is due to the presence of ions in the solution. The way in which the ions are formed is represented in the following equation using hydrochloric acid (HCl):

HCl + H2O H3O+ + Cl-

The ion H3O+ consists of a hydrogen ion (H+) fastened to a water molecule. It is called a hydronium ion.

An example of the reactiob between a metal and an acid is:

Zn + H2SO4 ZnSO4 + H2

One way of judging the strength of an acid is to note the speed with which bubbles of hydrogen gas are produced by this reaction.

Bases

Bases are compounds that are also called alkalies. Chemists usually apply the name alkali to the strongest bases, which are caustic, bitter, soapy substances. Bases that dissolve in water make solutions that will also carry electric current. This ability is due to the presence of OH ions. This charged pair of atoms is an example of a radical ion and usually acts as a single unit. As with acids, indicators show a characteristic color with bases. Red litmus paper turns blue.

Reaction Between Acids and Bases

An acid contains hydrogen that can be exchanged for a metal. A base, or alkali, contains a metal, or a combination of elements that acts like a metal, to exchange for hydrogen. When acid and base meet, the exchange occurs, and sometimes it is quite violent.

The result of such a reaction is the formation of water and a salt. For example:

HCl + NaOH H2O + NaCl

acid base water salt

This reaction can be written in terms of the ions present, as follows:

H2O+, Cl- + Na+, OH- 2H2O + Na+, Cl-

water solution of the acid + water solution of the base water + water solution of the salt

This reaction is called neutralization because the ions typical of the acid, H3O+, are removed from the solution by the ions typical of the base, OH-, to form water, H2O.

Salts

The pair of ions formed in a neutralization reaction is called a salt. Salts in general are composed of the positive ions of a base and the negative ions of an acid. Examples in addition to NaCl are Na2SO4, sodium sulfate, formed from NaOH and H2SO4; and NH4Cl, ammonium chloride, formed from NH4OH and HCl.

Oxidation and Reduction

Oxidation and redaction are two fundamental chemical reactions. Oxidation originally meant the union of a compound with oxygen. Reduction meant the opposite, the loss of oxygen by a compound. The meanings of both terms have been extended, however, and now apply even when oxygen has no part in a reaction. Oxidation refers to any increase in the positive valence of an atom (caused by a loss of one or more electrons to an atom of a different element). Likewise, reduction occurs when there is a decrease in the positive valence of an atom because of electrons gained from another element.

An important industrial use of reduction is in the separation of metals from their ores. Carbon monoxide is commonly used as the reducing agent in separating iron (Fe) from iron ore having the formula Fe2O3. The carbon monoxide removes the oxygen from the ferric oxide, leaving iron and carbon dioxide: Fe2O3 + 3CO = 2Fe + 3CO2

Oxidation and reduction always take place together. It will be noticed that while the iron decreased its valence from +3 (in Fe2O3) to 0 (in the free element), the carbon increased from +2 (in CO) to +4 (in CO2).

Sometimes the combination of oxygen with a substance is so rapid and vigorous that heat and light are given off. This form of oxidation is called combustion. Other forms of oxidation are so slow that the release of heat is not easily noticed. Oxidation of this kind occurs when a metal rusts. In the living body, energy is released by the slow oxidation of foodstuffs. These contain C and H, which the body converts to CO2 and H2O.

Organic Compounds

All organic compounds contain carbon. There are more than 1,000,000 different organic molecules whose formulas are known and thousands more are identified each year. The reason for this tremendous variety is the unique ability of carbon atoms to hook together into rings or long chains. The bonds they form are covalent in type. Hydrogen, nitrogen, oxygen, and other non-metals, such as chlorine, are most frequently combined with carbon; the metals seldom. Relatively few organic molecules are soluble in water. Inorganic materials, by contrast, frequently are water-soluble. Among the most important classes of organic compounds are hydrocarbons and plastics. Other important classes of organic compounds include alcohols, aldehydes, amino acids, ethers, and ketones.

Structural Formulas

The spatial arrangement of atoms in a molecule is called the structural formula of a compound. For many organic compounds, this structure is well known and much information about the properties and ability of a molecule to react can be inferred. Organic chemists build many molecular models to help them visualize structure. It is often possible to show the arrangement of atoms graphically by representing an atom with the element's symbol and a bond by a dash (-).

Hydrocarbons

There is a group of organic compounds consisting only of carbon and hydrogen that are called hydrocarbons. These are the chief constituents of petroleum. Many series of similar compounds exist. The simplest such series is the alkane series. The smallest molecule of this series is the gas methane, which has a molecular formula of CH4. The structural formula for this compound is as follows:

The second compound in the alkane series is ethane, the molecular formula of which is C2H6. The structural formula shown below represents two atoms of carbon, each with 4 valence bonds, and six atoms of hydrogen, each with a single bond.

Propane, the third in the alkane series, has the molecular formula of C3H8. The structural formula is as follows:

Plastics

One of the largest industries based on organic chemistry involves the building of giant molecules commonly known as plastics. The ability of carbon to link into long chains is taken advantage of by the organic chemist who makes carbon-containing molecules which will hook together into tremendously long chains, sheets, or blocks.

A relatively simple plastic, vinyl chloride, can be represented as:

The symbol n means that the structure in parentheses is repeatedly bonded together with other identical structures for what may be millions of times.

The History of Chemistry

Alchemy

A forerunner of the science of chemistry flourished during the Middle Ages. This was alchemy, a mixture of black magic and scientific knowledge flavored with much superstition. Alchemists sought a mythical philosopher's stone with which they could transform the base metals, such as iron and lead, into gold. They also tried to compound an elixir of life that would make them live forever. They studied the classical Greek philosophers, especially Aristotle, who argued that all substances originated in some way from the four basic elements. The alchemists tried to find a fifth element (the quintessence), which, they believed, could control the changing of one substance into another.

Although their efforts were largely misdirected, the alchemists contributed much useful information as a result of their experiments. Special kinds of equipment, such as the test tube, the closed crucible, and the retort, still used in chemical laboratories today were devised by the alchemists. During the 16th century, certain alchemists and physicians developed the theory that disease must be treated by experimental use of chemicals accompanied by observation. The chief proponent of this theory was Philippus Aureolus Paracelsus, a Swiss physician and alchemist.

Rise of Modern Chemistry

At the end of the 16th century only 10 substances were definitely known to Europeans in pure form-carbon, sulfur, copper, iron, gold, silver, tin, lead, mercury, and antimony. All these had been known to the ancients. Arsenic may have been discovered by Albertus Magnus in the middle of the 13th century, but the arsenic and bismuth known to the alchemists were impure and were often confused with other substances. Zinc alloys had been used for thousands of years, but metallic zinc was known only in the Far East. Platinum was known to the Indians of Mexico and Central and South America, but probably not in pure form.

In the 17th century, people began to question older ideas and started doing experiments. This method led to discoveries from which the fundamental concepts of scientific chemistry were evolved. In 1661 Robert Boyle, an Englishman, published a book called The Sceptical Chemist. In it he defined elements as substances that could not be broken down into simpler substances nor formed by combining simpler substances.

At about the same time the theory prevailed that when a piece of wood or some other combustible material burned, an invisible substance escaped into the air. The theory was put into its final form about 1700 by Georg Stahl, a German physician and chemist, who called the invisible substance phlogiston. The English scientists Joseph Priestley and Henry Cavendish were among its chief proponents. Around the same time, other scientists found ways to examine gases. Carbon dioxide was isolated by Joseph Black.

Antoine-Laurent Lavoisier, a French chemist, discovered by careful weighing that metallic substances are heavier after they are burned than before. This led him to conclude that a burning substance combines with a gas (which he named oxygen) in the air, rather than losing something called phlogiston. Lavoisier's experiments, performed in the late 18th century, conclusively disproved the phlogiston theory. Lavoisier's theory of combustion and his method of careful experimentation are so important that he is often called the father of modern chemistry. Lavoisier also did experiments that pointed out that chemical reactions that take place during respiration in animals resemble combustion.

The 19th century saw further revolutions in chemical science. John Dalton, in 1808, formulated the atomic theory of matter. In 1828, Friedrich Whler, a German chemist, prepared an organic compound, urea, in the laboratory. It was the first time that an organic compound had been synthesized. This synthesis proved that there was no essential difference between organic and inorganic substances, as had previously been believed. Dmitri Mendeleev, a Russian chemist, compiled the periodic table of elements in 1869, and predicted the discovery of several new elements. The discovery of radium in 1898 by Pierre and Marie Curie was another important development in chemistry. Justus von Liebig made contribution to the field of organic chemistry in the 1830s.

Wilhelm Oswald and Avante Arrhenius proposed that electrical charge in a solution is transferred by charged particles, or ions. Niels Bohr in 1913 developed the atomic model where electrons orbited a positively charged nucleus.

Parallel to the growth of theoretical chemistry was the growth of industry. In the 19th century, factories were making chemicals such as bleaching powder and sulfuric acid, as well as dyes.

20th-century and Current Developments

Many important advances in chemistry occurred in the 20th century. Organic chemistry expanded in scope to include biochemical reactions. Melvin Calvin discovered the chemical reactions involved in the process by which plants made food. In 1919, the British physicist Lord Rutherford bombarded nitrogen atoms with alpha particles (helium nuclei) to form oxygen and hydrogen atoms. This was the first man-made transmutation of an element, and marked the birth of a new field of study-nuclear chemistry. In 1938, Otto Hahn and Fritz Strassmann, German scientists, discovered the process of nuclear fission. Since World War II, radioactive isotopes produced in nuclear reactors have aided research in chemistry, biology, and medicine.

Chemical research has led to the development of thousands of synthetic materials that have made possible the establishment of new industries. Bakelite, the earliest synthetic resin, was produced in 1909. Since then, plastics such as polyethylene, Teflon, Lucite, and the silicones have found wide-spread application. Drugs such as the sulfa drugs, penicillin, antihistamines, and antibiotics have also been synthesized. The two world wars saw an increase in output by these industries to meet shortfalls of ammunition, metals, and rubber. The food industry also expanded.

New instruments and techniques have made it possible to determine the molecular structure of such complex substances as proteins, nucleic acids, and sterols.

Space exploration, which began in the 1950s, placed new demands on chemical research. Heat-resistant materials, for space vehicles, and liquid and solid propellants, for rocket engines, are among the contributions of chemistry to the space age.

In the 1970s, some commonly used chemicals were found to be harmful to humans and damaging to the environment. Chemicals in certain pesticides were found to accumulate in the tissues of birds, fish, and other wildlife; and chemicals in aerosol sprays, detergents, antifreeze, and other products were found to pollute the environment. Certain chemicals in food products and drugs were found to cause cancer and birth defects. In the United States, cities, states, and the federal government passed laws banning some chemicals, limiting the use of others, and placing restrictions on the handling and disposal of still others.

Biochemistry is an active area of research today. New instruments have enabled biochemists to study the action of chemicals within an organism without harming the organism. Biochemists are studying substances suspected of causing cancer or genetic damage to determine the molecular features responsible for the harmful effects. Other chemists are investigating how chemical pollutants affect the environment and how they decompose into other substances.

Synthetic chemistry is another area of active research. Chemists synthesize many thousands of new compounds each year. They have discovered chemical agents that can be used in reactions to add special groups of atoms to specific parts of other molecules. Researchers design new molecules and use such agents in a series of reactions to build the new compounds. Their techniques have led to the creation of many drugs.

The study of the surface properties of chemical compounds-called surface chemistry-is another field of present-day research. Chemists have learned that surface characteristics cause certain substances-called catalysts-to accelerate the rate of chemical reactions. Chemists today are also working to develop a chemical cell that would use the energy of sunlight to break up water molecules into oxygen and hydrogen. The hydrogen thus produced could be used as fuel.

The metallurgy of ancient peoples involved some of the earliest chemical discoveries. They discovered around 3500 BC that certain ores if heated would produce metals. They created weapons made of copper and jewelry made of gold. Ancient metalsmiths by 3000 BC discovered that by combining ores they could create new metals, or alloys. The first alloy was bronze, made of copper and tin. Tools and weapons made of bronze were sharper and more durable than those made of copper.

Around 1400 BC, the Hittites perfected the smelting of iron ore, a process that required much higher heat and the presence of carbon to draw off the oxygen that is combined with the iron in the ore. Iron was a stronger material than bronze.

The ancients also learned that clay vessels could be made more durable if baked or fired. They also developed a crude glass.

The records left by early civilizations show no knowledge of chemistry as the science we know today. The ancients, however, were acquainted with many useful substances and the methods for their preparation. Certain plants and shellfish were found to yield dyes with which fabrics could be colored. Many herbs and roots were used as medicines. The bark of some kinds of trees provided a substance that tanned leather.

Some early people learned how to make paper, bricks, and glazed pottery. These and other crafts were developed to a high degree, yet none of the ancient artisans understood the nature of the chemical processes involved. Each craft and process was attributed to one god or another and regarded as a miraculous gift bestowed upon mankind.

Some of the philosophers of ancient Greece, however, attempted to explain the nature of matter. They believed that all substances were made up of four basic elements: air, earth, fire, and water. These people advanced several theories as to why substances differ from one another. Some of these theories were remarkably logical; many were almost correct, but none was ever tested by doing an experiment. Democritus of Greece was the first to consider the existence of atoms in the 4th century BC. A hundred years later, Aristotle said that transmutation occurred when a substance changed its state; for example, a solid became liquid or gas. Chemistry did not become a science until people began to investigate systematically the characteristics of the materials and natural forces making up their surroundings.