Chemical reactions

Types of Reactions Types of Reactions There are four types of reactions. Synthesis Decomposition Single replacement Double replacement Occurs when two substances unite Synthesis A + B AB Synthesis 2 Mg + O2 2 MgO magnesium(element) oxygen(element) magnesium oxide(compound) 4 Occurs when a substance splits into parts Decomposition AB A + B Decomposition hydrogen(element) oxygen(element) water(compound) 2 H2O 2 H2 + O2 6 Occurs when a more active substance replaces one part of a compound that is less active Single Replacement Y X + X Y Z Z + Single Replacement hydrogen(element) potassiumhydroxide(compound) water(compound) potassium(element) 2 K + 2 H2O H2 + 2 KOH 8 These reactions are also called displacement or substitution reactions. Single Replacement

Chemical Composition &Reactions Oxidation Numbers Keep track of electrons during bonding Tell how many electrons are involved in the bond Tell whether electrons are gained, lost, or unequally shared Oxidation Numbers Help in predicting formulas The more electronegative element gains electrons. The oxidation number of atoms and elements is zero. (free-element) Rule 1 Oxidation Numbers Examples: He ? no bonds O2 ? equal sharing (free-element) Rule 1 Oxidation Numbers The oxidation number of a monatomic ion is equal to the charge of the ion. (ions) Rule 2 Oxidation Numbers Example: Mg loses two electrons, so its charge is +2; therefore, its oxidation number is also +2. (ions) Rule 2 Oxidation Numbers Example: Cl gains one electron, so its charge and oxidation number are ?1. (ions)