Chemical Reaction

I

Introduction

Chemical Reaction, process by which atoms or groups of atoms are redistributed, resulting in a change in the molecular composition of substances. An example of a chemical reaction is formation of rust (iron oxide), which is produced when oxygen in the air reacts with iron.

The products obtained from a given set of reactants, or starting materials, depend on the conditions under which a chemical reaction occurs. Careful study, however, shows that although products may vary with changing conditions, some quantities remain constant during any chemical reaction. These constant quantities, called the conserved quantities, include the number of each kind of atom present, the electrical charge, and the total mass.

II

Chemical Symbols

In order to discuss the nature of chemical reactions, certain basic facts about chemical symbols, nomenclature, and the writing of formulas must first be understood. All substances are made up of some combination of atoms of the chemical elements. Rather than full names, scientists identify elements with one- or two-letter symbols. Some common elements and their symbols are carbon, C; oxygen, O; nitrogen, N; hydrogen, H; chlorine, Cl; sulfur, S; magnesium, Mg; aluminum, Al; copper, Cu; silver, Ag; gold, Au; and iron, Fe.

Most chemical symbols are derived from the letters in the name of the element, most often in English, but sometimes in German, French, Latin, or Russian. The first letter of the symbol is capitalized, and the second (if any) is lowercase. Symbols for some elements known from ancient times come from earlier, usually Latin, names: for example, Cu from cuprum (copper), Ag from argentum (silver), Au from aurum (gold), and Fe from ferrum (iron). The same set of symbols in referring to chemicals is used universally. The symbols are written in Roman letters regardless of language.

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Symbols for the elements may be used merely as abbreviations for the name of the element, but they are used more commonly in formulas and equations to represent a fixed relative quantity of the element. Often the symbol stands for one atom of the element. Atoms, however, have fixed relative weights, called atomic weights, so the symbols often stand for one atomic weight of the element.

The atomic weights (atomic wt.) of the elements (see Elements, Chemical) are average atomic weights of the elements as they occur in nature. Every chemical element consists of atoms the weights of which vary because of varying numbers of neutrons in their nuclei. Atoms of the same element that differ in weight are called isotopes of the element. An isotope's weight may be indicated by a superscript to the left of the abbreviation that indicates the total number of nucleons (protons plus neutrons) in the nucleus. The symbols ²³⁵U and ²³⁸U, for example, represent two uranium isotopes of weight 235 and 238. The symbols ¹H, ²H, and ³H represent three hydrogen isotopes of weights 1, 2, and 3. If no isotopic weight is indicated, the mean (weighted average) atomic weight is indicated. All of these weights are in atomic mass units (amu). One amu is defined as u of the mass of a ¹²C atom, the most common isotope of carbon. See Atom.

An electrically neutral atom has equal numbers of protons and electrons. Electrically charged atoms and groups of atoms are called ions. When an atom is electrically charged—that is, when it has lost or gained one or more electrons, and thereby become an ion—that state may be indicated by a superscript to the right of the symbol, as in H⁺, Mg⁺⁺, or Cl^-. The symbol H⁺ indicates a singly positive hydrogen ion, Mg⁺⁺ a doubly positive magnesium ion, and Cl^- a singly negative chlorine ion. See Ionization.

The atomic number of an element is equal to the number of protons in the nucleus of an atom of the element. All isotopes of a particular element have the same number of protons in their nuclei. The atomic number is sometimes indicated by a lower-left subscript. The symbol °U³⁺ represents a uranium ion of triply positive charge (that is, an atom that has lost 3 electrons), with 92 protons and 146 neutrons (238 nucleons - 92 protons = 146 neutrons) in its nucleus, which is surrounded by 89 electrons (92 - 3 = 89).

III

Chemical Formulas

An individual atom can be represented by the symbol of the element, with the charge and mass of the atom indicated when appropriate. Most substances, however, are compound, in that they are composed of combinations of atoms. The formula for water, H₂O, indicates that two atoms of hydrogen are present for every atom of oxygen. The formula shows that water is electrically neutral, and it also indicates (because the atomic weights are H = 1.01, O = 16.00) that 2.02 unit weights of hydrogen will combine with 16.00 unit weights of oxygen to produce 18.02 unit weights of water. Because the relative weights remain constant, the weight units can be expressed in pounds, tons, kilograms, or any other unit so long as each weight is expressed in the same unit as the other two.

Similarly, the formula for carbon dioxide is CO₂; for gasoline, C₈H₁₈; for oxygen, O₂; and for candle wax, CH₂. The subscripts in each case (with a 1 understood if no subscript is given) show the relative number of atoms of each element in the substance. CO₂ has 1 C for every 2 Os, and CH₂ has 1 C for every 2 Hs. But why write O₂ and C₈H₁₈ rather than simply O and C₄H₉, which show the same atomic and weight ratios? Experiments show that atmospheric oxygen consists not of single atoms (O) but of molecules made up of pairs of atoms (O₂); molecules of gasoline consist of carbon and hydrogen ratios of C₈ and H₁₈ rather than any other combinations of carbon atoms and hydrogen atoms. The formulas of atmospheric oxygen and gasoline are examples of molecular formulas. Water consists of H₂O molecules, and carbon dioxide consists of CO₂ molecules. Thus, H₂O and CO₂ are molecular formulas. Candle wax (CH₂), on the other hand, is not made up of molecules each containing 1 carbon atom and 2 hydrogen atoms. It actually consists of very long chains of carbon atoms, with most of the carbon atoms bonded to 2 hydrogen atoms in addition to being bonded to 2 neighboring carbon atoms in the chain. Such formulas, which give the correct relative atomic composition but do not give the molecular formula, are called empirical formulas.

All formulas that are multiples of simpler ratios can be assumed to represent molecules: The formulas N₂, H₂, H₂O₂, and C₂H₆ represent nitrogen gas, hydrogen gas, hydrogen peroxide, and ethane. However, formulas that show the simplest possible atomic ratios must be assumed to be empirical unless evidence exists to the contrary. The formulas NaCl and Fe₂O₃, for example, are empirical; the former represents sodium chloride (table salt) and the latter iron oxide (rust), but no single molecules of NaCl or Fe₂O₃ are present.

IV

Naming Inorganic Compounds

All organic and inorganic compounds can be given systematic names based on the elementary composition and often the structure of the substance. See Chemistry, Organic.

Binary inorganic compounds contain two different elements and are written with the more metallic (more electrically positive) element first. Such compounds are named by taking the name of the first element followed by the main part of the name of the second, more negative, element combined with the suffix -ide: NaCl, sodium chloride; CaS, calcium sulfide; MgO, magnesium oxide; SiN, silicon nitride. When the atomic ratio differs from 1:1, a prefix to the name often makes this clear: CS₂ carbon disulfide; GeCl₄, germanium tetrachloride; SF₆, sulfur hexafluoride; NO₂, nitrogen dioxide; N₂O₄, dinitrogen tetraoxide.

Many groups of elements occur so often as ions that they are given names: nitrate, NO₃^-; sulfate, SO₄^2-; and phosphate, PO₄^3-. The suffix -ate usually indicates the presence of oxygen. The positive ion, NH₄⁺, is called ammonium, as in NH₄Cl, ammonium chloride, or (NH₄)₃PO₄, ammonium phosphate.

Rules for naming more complicated compounds exist, but many compounds have been given trivial names—for example, Na₂B₄O₇·10 H₂O, borax—or proprietary names—F(CF₂)_nF, Teflon. These nonsystematic names may be convenient in some usages but they are often difficult to interpret.

The accompanying table lists names and formulas of the most common polyatomic inorganic ions. They form compounds by combining in such a way that the net charge for the entire molecule is zero. The sum of the charges on the positive ions equals the sum of the charges on the negative ions. When formed from water solutions, the compounds (termed hydrates) often contain water molecules, as does borax, the systematic name of which is disodium tetraborate decahydrate—a good example of the advantages and disadvantages of trivial names.

In the table, the suffix -ite indicates fewer oxygen atoms than in the corresponding -ate ion, with the prefix hypo- used with the suffix -ite indicating still fewer. The prefix per- indicates more oxygen, or less negative charge, than the corresponding -ate ion.

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