Types of Chemical Reactions

A chemical reaction is a process that is usually characterized by a chemical change in which the starting materials (reactants) are different from the products. Chemical reactions tend to involve the motion of electrons, leading to the formation and breaking of chemical bonds. There are several different types of chemical reactions and more than one way of classifying them. Here are some common reaction types:

• 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

• Chemical Decomposition or Analysis Reaction
  In a decomposition reaction a compound is broken into smaller chemical species.
  
  AB --> A + B
  
  The electrolysis of water into oxygen and hydrogen gas is an example of a decomposition reaction:
  
  2 H₂O --> 2 H₂ + O₂

• Single Displacement or Substitution Reaction
  A substitution or single displacement reaction is characterized by one element being displaced from a compound by another element.
  
  A + BC --> AC + B
  
  An example of a substitutions reaction occurs when zinc combines with hydrochloric acid. The zinc replaces the hydrogen:
  
  Zn + 2 HCl --> ZnCl₂ + H₂

• Metathesis or Double Displacement Reaction
  In a double displacement or metathesis reaction two compounds exchange bonds or ions in order to form different compounds.
  
  AB + CD --> AD + CB
  
  An example of a double displacement reaction occurs between sodium chloride and silver nitrate to form sodium nitrate and silver chloride.
  
  NaCl(aq) + AgNO₃(aq) --> NaNO₃(aq) + AgCl(s)

• 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

• 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 S₂O₃^2- (thiosulfate anion) is oxidized to S₄O₆^2- provides an example of a redox reaction:
  
  2 S₂O₃²⁻(aq) + I₂(aq) --> S₄O₆²⁻(aq) + 2 I⁻(aq)

• Combustion
  A combustion reaction is a type of redox reaction in which a combustible material combines with an oxidizer to form oxidized products and generate heat (exothermic reaction). Usually in a combustion reaction oxygencombines with another compound to form carbon dioxide and water. An example of a combustion reaction is the burning of naphthalene:
  
  C₁₀H₈ + 12 O₂ ---> 10 CO₂ + 4 H₂O

• Isomerization
  In an isomerization reaction, the stuctural arrangement of a compound is changed but its net atomic composition remains the same.

• Hydrolysis Reaction
  A hydrolysis reaction involves water. The general form for a hydrolysis reaction is:
  
  X^-(aq) + H₂O(l) <--> HX(aq) + OH^-(aq)