Classifying Chemical Reactions Chapter 10.2

Categories There are 5 major categories of chemical reactions. Each has a specific way that the atoms rearrange themselves during the reaction. You must be able to recognize each general pattern.

Synthesis Reaction Two or more substances combine to form one new substance A + B  AB 2 H2 + O2  2 H2O

Synthesis Reactions (Marriage) two elements can combine to form a compound two compounds can combine to form a larger compound monomers can combine to form polymers glucose + glucose = cellulose polystyrene (foam cups) polyvinyl acetate (chewing gum)

Decomposition A substance breaks down into two or more simpler substances AB  A + B 2 H2O  2H2 + O2

Decomposition (Divorce) Decomposition is the reverse of Addition If the only reactant is a compound, it is most likely a decomposition reaction Most decomposition reactions require heat, light or electricity to provide energy to break the bonds Airbags work through a decomposition reaction. 2 NaN3  2 Na + 3 N2

Single Replacement a more active element replaces a less active element A + BC  AC + B Fe(s) + CuSO4(aq)  FeSO4(aq) + Cu(s)

Single Replacement (Homewrecker) Usually a more active metal replaces a less active metal in a salt. An activity series will be used to determine whether an element is more active. A more active halogen can replace a less active halogen. Reactivity of halogens can be read directly from the periodic table.

Double Replacement Two elements from two different compounds swap places AB + CD  CB + AD AgNO3 + NaCl  AgCl + NaNO3

Double Replacement (Wife Swap) Double replacement reactions usually occur in aqueous solutions Double replacement reactions usually produce a precipitate, a gas or water A cation can only replace another cation

Combustion A hydrocarbon (fuel) in the presence of oxygen breaks down into carbon dioxide and water. Combustion reactions release large amounts of heat and light C3H8(g) + 5 O2(g)  3 CO2(g) + 4 H2O(g) Other materials, such as metals, will form different products