Chemical reactions are the heart of chemistry.   A chemical reaction is when one or more substances are changing into other substances.  Chemical reactions are evidenced by the disappearance of characteristics of the starting substances and the appearance of new properties that identify the products;

Typical chemical reactions include burning, decay, fermentation, corrosion of steel and digestion of food.

The law of conservation of mass states there is no loss of mass in the reaction process.

Reactions are identified in chemistry using simple equations.  An example reaction equation, of iron reacting with oxygen in the corrosion process, is provided below;

The reactants are shown on the left of the equation and the products are shown on the right.    The number of atoms of each element will therefore be the same on both sides of the equation.

It is important first to define the components of a reaction.   It is also necessary to understand that several competing reactions may take place in a given system, and their relative velocities (rates) will generally influence the composition of the products.   

A reaction can be exothermic (exoergic) or endothermic (endoergic) or for very rare cases athermic (aergic).

Exothermic reactions give out energy, usually as heat.   The combustion reactions such as gas or wood burning, are examples of exothermic reactions.   Endothermic reactions take in energy from the surroundings.  Reactions that need a continuous supply of heat or electrical energy, such as thermal decomposition reactions and electrolysis, are endothermic reactions.  Athermic reactions involve no exchange of energy

During the course of a reaction, chemical bonds are broken and new bonds formed.   The process of making bonds releases energy and the process of breaking bonds requires energy input.  

A reaction process where more energy is evolved in bond making than is absorbed in the bond breaking is exothermic.  Conversely a reaction where more energy is absorbed in the bond breaking than is evolved in the bond breaking is endothermic.  The majority of spontaneous reactions are exothermic

In the process of the reaction the reactants become activated as they gain energy and it is only when they reach a minimum activation energy (E_a) that the products are formed..

This activation energy value is a barrier to be overcome before the successful compeletion of the reaction to the product stage.  The figures below show the energy levels throughout a typical reaction process. E_a1 represents the activation energy of the forward direction and E_a2 is the activation energy of the reverse process..

There are four basic types of reactions.

The main types of reactions are:..

This is typically a reaction where one chemical is oxidised and another is reduced i.e one chemical gains oxygen and the other loses oxygen.   This definition has developed into a more general view of the process in which oxidation is the loss of electrons and reduction is the gain of electrons. Ref. Oxidation - Reduction

A reaction between an acid and a base to form a salt and water as the only products. A typical Acid- Base reaction is that between reactants Sulphuric acid and potassium hydroxide resulting in products potassium sulphate and water i.e.

H₂SO₄ + 2KOH --> K₂SO₄ + 2H₂O

More notes are found on webpage Acid and Bases

A reaction between an acid and a metal oxide forms a salt and water as the only products.

A reaction between an acid and a metal, forming a metal salt and hydrogen as the only products.

A reaction between an acid and a carbonate forming a salt, carbon dioxide and water as the only products.

A reaction forming an ester. Usually this is a reaction between an organic acid and and an alcohol forming an ester and water as the only products. Ref Organic Esters

A reaction where reactants water and a larger molecule are split into two smaller product molecules, one of which has the hydrogen from the water and the other has the OH group from the water.

Just having water present as the solvent does NOT make a reaction hydrolysis.   Hydrolysis is actually a special type of substitution reaction.

A reaction where hydrogen is added across a double bond or even a triple bond. Example

Ethene and hydrogen --> ethane

CH₂=CH₂ + H₂ ---> CH₃CH₃