Chemicals are elements of the Periodic Table and their compounds. Reactions are responses to actions. So, chemical reactions are responses of chemicals to specific actions under specific conditions. A general symbolic representation of a chemical reaction is as follows:
a1x1 + a2x2...amxm ----> b1y1 + b2y2...bnyn -------(1) where n and m are finite real numbers, ≥ 1.
In equation (1), x1 thru xm and y1 thru yn are chemicals (substances). The coefficients a1 thru am and b1 thru bn are the number of molecules of each chemical involved in the chemical reaction. The arrow indicates that given amounts of x1 thru xm (called reactants) have undergone a chemical reaction to produce some amounts of y1 thru yn (called products of the chemical reaction). The values of the coefficients are such that the number of atoms of elements on both sides of the arrow must be equal. This is called balancing the equation, a concept that is a consequence of the law of the conservation of matter. A primary characteristic of a chemical reaction is that the reactants and products are different chemicals or substances. In other words, the reactants were changed into chemically different products during the chemical reaction.
The specific conditions underwhich chemical reactions occur are usually conditions of pressure (force per unit area), temperature (degree of hotness), volume or mass which implicitly indicates the amount of molecules of the substances involed in the chemical reaction.
There are many kinds of chemical reactions. Nonetheless, a good number of these reactions can be grouped into the following five types of chemical reactions:
(1) Single Displacement
In this type of reaction, one element displaces another element in a compound. For example, in the reaction:
Cu + H2O ----> CuO + H2
Copper (an element) displaces another element, oxygen from water (a compound) to form cupric oxide (another compound and hydrogen (another element).
(2) Double Displacement
In this type of reaction, the positive and negative components of two compounds are interchanged. For example, in the reaction:
Al(NO3)3 + 3NaOH ----> Al(OH)3 + 3NaNO3
Aluminum and sodium exchange positions while the hydroxide and the nitrate exchange positions.
In this type of reaction, energy (e.g. heat, electricity) is used to break up the compound. For example, in the reaction:
2KNO3 -------> 2KNO2 + O2
Potassium nitrate is decomposed into potassium nitrite and oxygen.
In this reaction, two or more substances combine to form one new substance. For example, in the reaction:
2H2(g) + O2(g) ----> 2H2O(g)
Hydrogen gas combine with oxygen gas to form water vapor.
This type of reaction s also called an oxidation reaction. It is commonly identified as the burning of a compound with the oxygen in air. Most organic and some inorganic compounds will burn in air. For example, in the reaction:
CH4 + 2O2 ----> CO2 + 2H2O
Methane reacts with oxygen to produce carbon dioxide and water vapor.
Not all reactions can be grouped into the preceeding five types of chemical reactions. For example, nuclear reactions have some unique characteristics. Nonetheless, a large number of important chemical reactions such as reactions used to make or synthesize new products, belong to at least one of the groups.
The atoms involved in a chemical reaction, the orbital space wherein the grouping/interaction of atoms occur, the force that effects the grouping/interaction and the resultant bonds of the grouping/interaction are all Pj Problems. Conseqently, chemical reactions are expressions of Pj Problems.