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Photochemical reactions

In many cases, the energy to initiate a reaction comes from heat. The rate of most reactions increases regu­larly with a rise in tempera­ture. The rate roughly dou­bles for each 18" F. (10° C) rise in temperature. How­ever, other forms of energy can also initiate reactions. Photochemistry is the branch of chemistry con­cerned with the interaction between light (photons) and molecules. Such reactions are essential to life on earth. The trapping of solar en­ergy in chemical form by plants is the first step in the

Atoms, elements, and molecules: Key chemical reactions 21

down into more than one product is known as a decomposition reaction. For example, heat­ing the compound calcium carbonate pro­duces carbon dioxide gas and quicklime. Quicklime is used in making mortar and glass. If two reactants break down into other prod­ucts, the reaction is known as a double de­composition reaction. An example of such a reaction is one involving an acid and a base. These compounds react to form a salt and water.

The opposite of decomposition is an addi­tion reaction. The reactants join together in­stead of breaking down. There is no removal of any part of the reacting molecule. Such a re­action is common in organic chemistry, dis­cussed later in its own section.

If a reaction requires a continuous source of energy (usually heat) to keep it going, it is termed endothermic. In other cases, once a re­action starts, it produces enough energy to sustain itself. There is no need for continued external supplies of energy. Such reactions are termed exothermic.

In some cases, a molecule may break down only partly. It then reforms chemical bonds in a different manner. Such reactions are called rearrangements. They are often significant in synthetic chemistry (organic chemistry) and in the chemistry of living systems (biochemistry). Both organic chemistry and biochemistry are discussed later in this book.

+

2-methyi-2-bromopropane (CH₃)₃CBr

+

o

O-o

_aiX₀ + OO

Potassium

bromide

KBr

Elimination reactions are

important in synthetic chemistry. They involve the formation of a small, simple molecule such as water or ammonia /left). The elimina­tion of water (3) accompa­nies the conversion of an organic bromide (1) into a double-bonded hydrocar­bon (2). in the manufacture of nylon (below), the elimi­nation of water accompa­nies the reaction that creates the synthetic fiber.

food chain for many living species.

Photochemical reactions are also the basis of photog­raphy. In black-and-white photography, the key reac­tion is the conversion of sil­ver salts to finely-divided particles of silver. These particles appear black when light strikes them. Color photography depends upon a range of complex light-sensitive carbon com­pounds.

In some cases, the effect of a very small amount of light can be dramatic. A mix­ture of hydrogen and chlo­rine gases kept in the dark

at room temperature does not react. However, expo­sure to light starts a series of reactions known as a chain reaction. These reac­tions are very explosive.

Chemical elements In the

following section, all the chemical elements are dis­cussed. There are fact en­tries at the end of each arti­cle. The following abbreviations are used: at. no.—atomic number; at. mass—atomic mass (weight); m.p—melting point (freez­ing point); b.p.—boiling point.