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Chemical reactions

A chemical reaction is a process of change during which either two or more substances both change or one substance changes into at least two other substances.

A chemical reaction is any type of chemical process in which substances are changed into different substances, as differentiated from other kinds of changes – those of position or form of matter. Chemical reactions are manifested by the disappearance of proper­ties characteristic of the starting materials and the appearance of new properties that distinguish the products, the mass of the products formed is equal to the mass of the substances consumed.

Chemical reactions are often classified according to some prominent feature of the process under study. These categories are not mutually ex­clusive, however, for each may combine aspects of two or more catego­ries. A polymerization reaction, for example, may also be a chain reac­tion.

In simple synthesis reaction two reactants, which may be elements or compounds, form a compound as the product. In a decomposition reac­tion a compound breaks up into at least two simpler parts. A polymeriza­tion reaction is a type of synthesis in which the product is formed from hundreds or thousands of simpler reagent species. A chain reaction is a series of reactions in which the product of each step is a reagent for the next. A reaction in which a compound loses one or more atoms and sub­sequently gains one or more other atoms is a substitution reaction. One in which a compound loses one or more atoms is an elimination reaction.

One in which a compound gains one or more atoms is an addition reac­tion. Oxidation-reduction reactions, a large and important class of chemi­cal processes, are characterized by the transfer of oxygen or hydrogen atoms, or electrons, from one substance to another. A substance that loses electrons is said to be oxidized; one that gains electrons is said to be re­duced. Of all the other types of reactions, perhaps the most important is the acid-base reaction.

There are several other useful principles of classification that can be noted. When two atoms (or groups of atoms) combine and each supplies an electron to their covalent bond, the process is called colligation, and the reverse reaction is called homolysis. If, on the other hand, two atoms (or groups of atoms) combine to form a covalent bond and both elec­trons come from only one of the reactants, the process is called coordi­nation, and the reverse reaction is called heterolysis.

Before most reactions will occur, some form of energy must be added to the system. This energy is used to raise the reactants to a minimum energy level needed to convert the reactants to an activated complex. The formation of products then proceeds without requiring any more energy; indeed, energy is often released when the products form.

The de­tailed study of the mechanisms of chemical reactions enable chemists to prepare reaction conditions that favour a particular (usually more economical) pathway when a reaction can proceed by more than one reaction path. Furthermore, knowledge of reaction mecha­nisms often makes it possible to successfully predict the course of reactions. A detailed description of reaction mecha­nisms must cover energy requirements and any changes in the bonding patterns of molecules (including shifts in the bonding electrons).

A complete study of a reaction must also take into account those factors that complicate the process. The reversibility of reactions is one such complication; the existence of compet­ing reactions and the presence of intermediate products are others.

Many factors can influence the rate and course of a reaction, including the solvent, properties of the solvent, temperature, pressure and the presence of catalysts. Thus, the properties of the solvent, or reaction medium, can greatly influence the course of a reaction. Solvent molecules may interact with reactant or product molecules or with the activated complex. Raising the temperature of a re­acting system generally increases the rate of reaction by supplying energy to the reactants. Increasing the pressure on a reacting system usually changes the reaction rate. In the case of gaseous reactants, an increase in pressure always increases the concentration of reactants. Catalysts, though they do not affect the position of equilibrium of a chemical reaction, accelerate both forward and backward reactions.