Chemistry

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Ionic Reactions

Simple ionic reactions—ionic reactions that do not involve the transfer of electrons—take place when ions are removed from solution to form insoluble (ionic) solids, gases, or covalently bonded molecules. When two soluble compounds each dissociate (in solution) into ions, and these ions subsequently combine to form an insoluble product, the reaction is driven forward to completion. This drive to completion occurs because the insoluble product cannot participate in the reverse reaction (due to its insolubility). For example, silver ions (Ag⁺) and chloride ions (Cl^-) combine in solution to form silver chloride: Ag⁺_(aq) + Cl^-_(aq) → AgCl_(s). Because AgCl_(s) is only slightly soluble in water, this reaction does not proceed in the opposite direction.

Double decomposition reactions occur when two reactants are each decomposed, or broken up, into a cation (positive ion) and an anion (negative ion). These ions recombine to form two or more products. The formation of insoluble AgCl_(s) drives the following double decomposition reactions to completion: AgNO_3(aq) + NaCl_(aq) → NaNO_3(aq) + AgCl_(s) and Ag₂SO_4(aq) + 2NH₄Cl_(aq) → (NH₄)₂SO_4(aq) + 2AgCl_(s).

Complete reactions between soluble ionic compounds can also be driven by the formation of gases. Once a gas bubbles and escapes from an ion-containing solution, the gas cannot participate in a reverse reaction. In the following reaction, carbonic acid (H₂CO₃) dissociates into water and carbon dioxide gas. Because carbon dioxide (CO₂) is not very soluble in water, the CO₂ gas bubbles away, driving the ionic reaction forward: H₂CO_3(aq) → H₂O + CO_2(g).

Formation of a covalently bonded product may also drive an ionic reaction to completion. Such a reaction may be generally represented as follows: X⁺ + :Y^- → X:Y (where : represents an electron pair).

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When an acid ionizes in solution and produces hydrogen ions (H⁺), and these hydrogen ions subsequently react with hydroxide ions (OH^-), which are provided by a base, water is produced (see Acids and Bases). Because water is a covalently bonded liquid and will not participate in the reverse reaction, the reaction proceeds essentially to completion: H⁺ + OH^- → H₂O_(l).

Strong acids are those that dissociate completely in water, producing many more H⁺ ions per mole than a weak acid produces. Similarly, strong bases are those that dissociate almost completely in water, producing many more OH^- ions per mole than a weak base. A strong acid, such as hydrochloric acid (HCl), will dissociate almost completely. When HCl is involved in a reaction with a strong base (such as NaOH), the large amounts of H⁺ ions and OH^- ions will combine to produce water. Because water molecules are joined by covalent bonds, water will not participate in the reverse reaction, and the reaction proceeds nearly to completion: HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l).

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Oxidation-Reduction Reactions

Oxidation-reduction reactions combine a chemical wanting to gain electrons with a chemical willing to give up electrons. Such a reaction may be generally represented as follows: X·+ Y ⇄ XY· (where · represents an electron). The material that loses electrons is said to be oxidized and is called a reducing agent; the material that gains electrons is reduced and is called an oxidizing agent (see Chemical Reaction). The most common examples of oxidation are those reactions involving the combination of materials with the element oxygen, such as the rusting of iron or the burning of any combustible material in air. The equation for the burning of magnesium is: 2Mg_(s) + O_2(g) → 2MgO_(s).

When magnesium reacts with oxygen, each magnesium atom gives two electrons to oxygen. The positive magnesium ions (Mg²⁺) then combine with negative oxygen ions (O^2-) to form solid magnesium oxide (MgO). In this reaction, magnesium (the reducing agent) is oxidized, and oxygen (the oxidizing agent) is reduced.

The reaction between metallic sodium and chlorine gas is an oxidation-reduction reaction that does not involve oxygen:

This way of writing the oxidation-reduction reaction illustrates that both elements attain a noble-gas configuration (completely filled outer shells). Sodium loses an electron, achieving the noble gas configuration of neon, and chlorine gains an electron, achieving the noble gas configuration of argon.

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Electron-Sharing Reactions

Electron-sharing reactions involve breaking the covalent bonds between atoms in the reactants to form new covalent bonds with different atoms. The reaction between iodine and chlorine is an example of such a reaction: I₂ + Cl₂ → 2ICl.

Another type of electron-sharing reaction is an addition reaction, which increases the number of groups bonded to a molecule by breaking a double or triple bond (see Chemical Bond). An example of this type of reaction is the following: CH₂ = CH₂ + Br₂ (combined in CCl₄) → CH₂Br-CH₂Br.

Substitution reactions, which redistribute the way electrons are shared, occur when one chemical group replaces another group on a compound: CH₃-Cl + NaOH (combined in H₂O_(l)) → CH₃-OH + NaCl.

Hydrolysis reactions are a type of electron-sharing reaction that involves the cleavage of a molecule by water: HCl_(g) + H₂O_(l) → H₃O⁺_(aq) + Cl^-_(aq).

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Branches of Chemistry

Chemists have divided chemistry into a number of different branches. These branches are somewhat arbitrary and do not have sharply defined boundaries. They often overlap with each other or with other sciences, such as physics, geology, or biology.

Inorganic chemistry is the study of the chemical nature of the elements and their compounds (except hydrocarbons—compounds composed of carbon and hydrogen).

Organic chemistry is the study of compounds consisting largely of hydrocarbons, which provide the parent material of all other organic compounds. Since carbon atoms can form rings and long branched chains, hundreds of thousands of carbon-based molecules exist. Organic compounds are of special importance, because they make up the majority of compounds in living organisms. Organic compounds form coal and petroleum. Organic chemists have learned how to convert raw materials from coal, petroleum, and grain into synthetic textiles, pesticides, dyes, drugs, plastics, and many other products.

Radiochemistry is the study of the chemical effects of high-energy radiation and the behavior of radioactive isotopes, atoms of the same element that vary in the number of neutrons they contain. For example, the heaviest known element, Element 112 (ununbium, or Uub) was first created by scientists at the Heavy-Ion Research Laboratory in Darmstadt, Germany in 1996. These scientists created an atom of ununbium containing 165 neutrons, labeled ununbium-277 (112 protons + 165 neutrons = ununbium-277). Because the ununbium nucleus contains so many particles, the atom becomes unstable and splits into smaller, so-called daughter components. As the atom breaks apart, energy is released in the form of electromagnetic waves and electrically charged bits of matter. This energy is known as radiation (Radioactivity; Nuclear Energy).

Physical chemistry is fundamental to all chemistry and deals with the application of physical laws to chemical systems and chemical change. Much of physical chemistry is concerned with the role of energy in chemical reactions; this branch of physical chemistry is known as thermodynamics. Other major areas of study in physical chemistry are the rates and mechanisms of reactions, called chemical kinetics. A third area of physical chemistry studies molecular structure. Physical chemists study molecular structure by examining the spectrum of electromagnetic energy emitted by molecules and explain structure using principles of quantum mechanics (see Quantum Theory).

Important subfields of physical chemistry include electrochemistry, which deals with the behavior of chemical substances subjected to electric current and the production of electrical energy by chemical systems. Other subfields of physical chemistry are colloid chemistry, which is concerned with the behavior of finely divided particles of matter; surface chemistry, which deals with the nature of surfaces and adsorption on them (see Photochemistry); and statistical mechanics, which applies the laws of probability to large numbers of particles.

Analytical chemistry is the science of separating complex materials into simpler ones and detecting and measuring the constituents. In a sense, analytical chemistry is the oldest branch of chemistry. A major feature of chemical analysis today is the wide use of physical instruments and computer control to automate the analysis of complex materials.

Biochemistry is the chemistry of living organisms and life processes. Even the simplest living thing is a complex chemical factory. Biochemists must have a detailed knowledge of organic chemistry. In some aspects of biochemistry, advanced physical chemistry is used, and biophysics and molecular biology are companion sciences.

Geochemistry is the application of chemistry (and, inevitably, physics) to processes taking place in the earth, such as mineral formation, the metamorphosis of rocks, and the formation and migration of petroleum.

Fields such as biochemistry, geochemistry, and materials science reveal the unity of the sciences. The divisions between chemistry, physics, biology, and geology are arbitrarily created for the convenience of humans—nature takes little account of these divisions.

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