Types of Chemical Reactions  
The vast number of chemical reactions can be classified in any number of ways. Under one scheme they can be categorized either as oxidation-reduction (electron transfer) reactions or non-oxidation-reduction reactions. Another completely different but common classification scheme recognizes four major reaction types:
(1)  combination or synthesis reactions
(2)  decomposition reactions
(3)  substitution or single replacement reactions
(4)  metathesis or double displacement reactions

The Four Major Types of  Reactions
      Name                                           General Reaction Pattern
Combination or synthesis                                 A  +  B ----> AB

Decomposition                                                      AB ---->  A  +  B

Substitution or Single Replacement            A  +  BC ---->  B  +  AC

Metathesis or Double Displacement        AB  +  CD ---->  AD  +  CB

Combination or Synthesis Reactions   Two or more reactants unite to form a single product.

                                     S   +   O2 --------->  SO2
                               sulphur    oxygen            sulphur dioxide

                                  2 S      +    3 O2 --------->  2 SO3
                                sulphur            oxygen                 sulphur trioxide

                                2 Fe   +   O --------->  2 FeO
                                iron        oxygen                    iron (II)  oxide

Decomposition Reactions   A single reactant is decomposed or broken down into two or more

                                       CaCO3   ---------->    CaO     +      CO2
                                calcium carbonate                calcium oxide     carbon dioxide

                                                     2 H2O ----------->  2 H2    +     O2
                                                         water                           hydrogen        oxygen

                                                 2 KClO3 ----------->  2 KCl   +     3 O2
                                           potassium chlorate          potassium chloride   oxygen

Substitution or Single Replacement Reactions  A single free element replaces or is substituted for one of the elements in a compound.  The free element is more reactive than the one its replaces.

                          Zn    +     2 HCl   ---------->  H2    +  ZnCl2
                                  zinc      hydrochloric acid             hydrogen    zinc chloride

                       Cu   +   2 AgNO3  ----------->  2 Ag    +    Cu(NO3)2
                   copper        silver nitrate                             silver          copper (II) nitrate

                               H2    +   2 AgNO3 ----------->  2 Ag   +   2 HNO3
                           hydrogen     silver nitrate                            silver           nitric acid

                                  2 Na   +  2 H2O  ----------->  2 NaOH    +     H2
                                 sodium          water                          sodium hydroxide     hydrogen

Metathesis or Double Displacement Reactions  This reaction type can be viewed as an "exchange of partners."  For ionic compounds, the positive ion in the first compound combines with the negative ion in the second compound, and the positive ion in the second compound combines with the negative ion in the first compound.

                       HCl    +     NaOH  ----------->  NaCl   +  HOH
                  hydrochloric        sodium                              sodium         water
                              acid               hydroxide                           chloride

                          BaCl2   +   2 AgNO3 ---------->  2 AgCl    +   Ba(NO3)2
                            barium                  silver                                    silver                barium
                            chloride                nitrate                                 chloride              nitrate

                      CaCO3   +   2 HCl   ----------->  CaCl2   +  H2CO3
                              calcium         hydrochloric                        calcium          carbonic
                            carbonate             acid                                 chloride            acid