A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between
atoms , with no change to the nuclei (no change to the elements present), and can often be described by a chemical equation.

The substance (s) initially involved in a chemical reaction are called
reactants or reagents. Chemical reactions are usually characterized by a chemical change , and they yield one or more products , which usually have properties different from the reactants.

NOTE: Antoine Lavoisier developed the theory of combustion as a chemical reaction with oxygen.

The general equation for a reaction is:
For example; When sulfur dioxide is added to oxygen, sulfur trioxide is produced. In the chemical equation shown below, sulfur dioxide and oxygen (SO 2 and O2 ) are reactants, and sulfur trioxide (SO 3 ) is the product.
It can be represented mathematically as;
2SO2 + O2 > 2SO3

Some types of Chemical reactions are;
1. •Direct Combination or Synthesis Reaction
In a synthesis reaction two or more chemical species combine to form a more complex product.
A + B → AB
The combination of iron and sulfur to form iron (II) sulfide is an example of a synthesis reaction:
8 Fe + S8 → 8 FeS

2. Oxidation-Reduction or Redox Reaction
In a redox reaction the oxidation numbers of atoms are changed.
Redox reactions may involve the transfer of electrons between chemical species.
The reaction that occurs when In which I2 is reduced to I- and S2O32- (thiosulfate anion ) is oxidized to S4O62- provides an example of a redox reaction :
2 S2O32−(aq) + I2(aq) → S4O62−(aq) + 2 I−(aq)

NOTE: Rust Is a Common Chemical Reaction

3. •Acid-Base Reaction
An acid-base reaction is type of double displacement reaction that occurs between an acid and a base. The H+ ion in the acid reacts with the OH- ion in the base to form water and an ionic salt:
HA + BOH → H2O + BA
The reaction between hydrobromic acid (HBr) and sodium hydroxide is an example of an acid-base reaction:
HBr + NaOH → NaBr + H2O

The following are factors that affect the rate of a reaction;
Concentration of Reactants, Temperature, Presence of Catalysts and Competitors, State of Matter, Pressure and Mixing reactants together increases their ability to interact, thus increases the rate of a chemical reaction.

Reference: Wikipedia,chemistry.about.com, ck12.org

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