Classifying Chemical Reactions Chemical reactions are often classified to highlight patterns in chemical behavior. In some cases, categorizing a reaction can allow its products to be predicted. Many chemical reactions fall into one of five classes:

Combination Reactions build a compound from simpler compounds or elements. General form: A + B  C General form: A + B  C Recognition: usually a single product and several reactants Recognition: usually a single product and several reactants Example: Example: Zn+ S  ZnS Zn+ S  ZnS

Decomposition Reactions break a compound into simpler fragments. General form: C  A + B General form: C  A + B Recognition: usually a single reactant and several products Recognition: usually a single reactant and several products Example: Example: 2H 2 O 2  2H 2 O + O 2 2H 2 O 2  2H 2 O + O 2

Single-replacement Reactions involve replacement or swapping of parts of a compound. General form: A + BC  AC + B General form: A + BC  AC + B Features: number of reactants is usually equal to the number of products Features: number of reactants is usually equal to the number of products atoms or ions migrate from one compound to another atoms or ions migrate from one compound to another Example: Example: K + H 2 O  H 2 + KOH K + H 2 O  H 2 + KOH

Double-replacement Reactions involve replacement or swapping of parts of a compound. General form: AB + CD  AD + CB General form: AB + CD  AD + CB Features: number of reactants is usually equal to the number of products Features: number of reactants is usually equal to the number of products atoms or ions migrate from one compound to another atoms or ions migrate from one compound to another Example: Example: 3AgNO 3 + FeCl 3  3AgCl + Fe(NO 3 ) 3 3AgNO 3 + FeCl 3  3AgCl + Fe(NO 3 ) 3

Combustion Reactions are burning compound made of carbon, hydrogen (and sometimes oxygen) in oxygen gas. General form: CxHy + O 2  H 2 O + CO 2 General form: CxHy + O 2  H 2 O + CO 2 Recognition: probable products are CO 2 and H 2 O Recognition: probable products are CO 2 and H 2 O Example: Example: 2CH 3 CH 2 CH 2 OH + 9O 2  8H 2 O + 6CO 2 2CH 3 CH 2 CH 2 OH + 9O 2  8H 2 O + 6CO 2