Knowledge database: Chemical reactions: Basics

A chemical reaction is a process in which a number of reactants are being converted to a number of products whereby a change of physical and chemical properties occurs. A chemical reaction which includes the decomposition of substances into several products is called chemical analysis, while the reaction that joins two or more substances into a specific product, is known as a chemical synthesis.

Accordingly, the reaction 2H₂ + O₂ -> 2H₂O is a synthesis in which two molecules of hydrogen and one oxygen molecule give two molecules of water. On the other side, the reaction 2H₂O -> 2H₂ + O₂ is an analysis in which two water molecules decompose into two molecules of hydrogen and one molecule of oxygen.

As most already know, a chemical reaction is represented by a chemical reaction equation, whereby on the left side we have the reactants, and on the other, right side, we have the products. The numbers in front of the symbols (which indicate an element) and formulas (indicating compounds) let us know how much of each particle participates in the reaction, or is being produced by it. These numbers can also be understood as the number of moles, because a mole of any substance has the same number of particles.

Chemical reactions can be further divided into several types. There are chemical reactions in which there are no changes of oxidation states, reactions in which changes of oxidation states are present (redox reactions), complex reactions, reactions of association and dissociation and disproportionation reactions.

Chemical reactions which in which there is no change of oxidation states are among the simpler ones. A typical example is the neutralization of hydrochloric acid with sodium hydroxide: HCl + NaOH -> H₂O + NaCl.

All the other mentioned types of reactions are further described in the following chapters.