Endothermic and Exothermic

• Endothermic reactions absorb energy while exothermic reactions release it. An exothermic reaction may require a little "nudge" of energy to get started, but its net energy release is positive. For example, 2H2O'2H2+O2 requires electrical energy (hydrolysis), while 2H2+O2'2H2O is explosive enough to power rockets.
  
  

Combination Reactions

• In combination reactions, two or more molecules react to produce one substance. Represented as A+B'AB, an exothermic example is the burning of carbon: C+O2'CO2.
  
  

Decomposition Reactions

• In decomposition reactions, molecules separate--for example, the electrolysis reaction mentioned above. They are also called "analysis reactions" because the simpler products help identify the larger reactant.
  
  

Single and Double-Displacement

• Reactions that replace one atom with another are called displacement reactions. Single-replacement reactions can be represented as A+BC'AC+B. Double-replacement reactions, also called metathesis reactions, are represented as AB+BC'AD+BC. An aqueous metathesis reaction leads to precipitation if any product is insoluble.
  
  

Neutralization Reactions

• Neutralization reactions are (usually) displacement reaction in which an acid gives up an H+ to an acid, producing a salt as a product. For example, HCl(aq)+NaOH(aq) ' NaCl(aq)+H2O.