We know the signs that a chemical reaction has occurred, but how do we know what is going on the molecular/atomic level (i.e., how do we know what compounds formed)? We have a system in order to help us classify chemical reactions into different types.

Types of Reactions Combination/Synthesis:Two reactants combine to form a single product. Reactants may be elements or compounds. A + B => AB Example: Zn(s) + I 2 (s) => ZnI 2 (s)

Types of Reactions Decomposition: One reactant breaks down to give two or more products. AB => A + B Example: 2H 2 O 2 (aq) => 2H 2 O(l) + O 2 (g)

Types of Reactions Single Replacement: An element reacts with a compound and replaces one of the elements in the compound. Metals replace hydrogen or other metals; nonmetals replace nonmetals. A + BC => AC + B Example: Zn(s) + 2HCl(aq) => H 2 (g) + ZnCl 2 (aq)

Types of Reactions Double Replacement: Two ionic compounds exchange ions to form new compounds. AB + CD => AC + BD Examples: 1. precipitation (solid formed): NaCl(aq) + AgNO 3 (aq) => AgCl(s) + NaNO 3 (aq) 2. acid-base (water formed): H 2 SO 4 (aq) + 2NaOH(aq) => Na 2 SO 4 (aq) + H 2 O(l) 3. gas-forming: Na 2 SO 3 (aq) + 2HCl(aq) => 2NaCl(aq) + H 2 O(l) + SO 2 (g)

Types of Reactions Combustion: A compound burns in the presence of oxygen, producing heat and light. Combustion of organic compounds produces carbon dioxide and water. Example: C 4 H 8 (l) + 6O 2 (g) => 4CO 2 (g) + 4H 2 O(l)