Classifying Reactions 9.2

5 Chemists classify reactions in order to organize the many reactions that occur daily. Knowing the categories helps you recognize patterns and predict products of many chemical reactions.

When two or more substances react to form one a single product, this is called a synthesis reaction. A + B AB 2Mg(s) + O2(g) 2MgO(s)

When a single compound breaks down into two or more elements or new compounds, this is called a decomposition reaction. This often requires energy. AB A + B 2C7H5N3O6 3N2 + 7CO + 5H2O + 7C

In contrast, many chemical reactions involve the replacement of an element in a compound. These are also called displacement reactions. 1) Single replacement is when the atoms of one element replace the atoms of another element. Metals may replace hydrogen or other metals. Halogens may replace halogens. You will use an activity series to predict whether or not these reactions occur. A metal/halogen can replace any metal/halogen below it or less active. A + BC AC + B 3Ni + 2AuBr3 3NiBr2 + 2Au

These produce either water, a precipitate, or a gas. 2) Double replacement is when ions are exchanged between two compounds. The anions will switch places and bond to the other cations in the reaction. These produce either water, a precipitate, or a gas. You will use a solubility chart to determine if the new compound is soluble or insoluble. AB + CD AD + CB

When oxygen combines with a substance releasing energy, this is called combustion. The most common occurs between a hydrocarbon compound which produces carbon dioxide and water. CH compound + O2 CO2 + H2O CH4 + 2O2 CO2 + 2H2O