The Definition of a Chemical Reaction

Chemical Reactions

• A chemical reaction is a process during which one or more chemical substances undergo a change into a different chemical substance or substances. Some chemical reactions can occur spontaneously, without the input of energy, while other chemical reactions require that light, heat or energy in some other form be applied for the reaction to proceed. Reactions also can take varying amounts of time to go to completion, from fractions of a second to years. When a chemical reaction occurs, matter and energy are never lost; they are merely transformed from one form into another.

Synthesis and Decomposition

• You can think of synthesis and decomposition reactions as opposites of each other. In a synthesis, two or more chemical species combine to form a new product compound. For example, hydrogen and oxygen gas can combine to form water. In a decomposition, on the other hand, a compound breaks down into different chemical species. For example, in electrolysis, a current is passed through water, which causes water molecules to decompose back into oxygen and hydrogen gas molecules.

Single and Double Replacement

• Single and double replacement reactions are a sort of interchange. In a single replacement, a compound exchanges one of its elements for another element. This can be represented as element A reacting with compound BC to give compound AC and element B. In a double replacement, two compounds react and exchange elements. This can generally be represented as compounds AB and CD reacting to form compounds AD and BC. Basically, the cationic species, A and C, switch places with the anionic species, B and D.

Combustion

• In a combustion reaction, a compound that acts as the fuel reacts with an oxidizing element, and the result is the formation of product species, as well as heat. The most familiar combustion reactions involve oxygen as the oxidizing molecule and some sort of organic, or carbon-containing, compound as the fuel. For example, the burning of wood as fuel results in the formation of carbon dioxide, water and other compounds as the products. Of course, the reaction produces heat in the form of flames, too.