Chemical equations represent chemical reactions

1. Chemical Equations and Reactions Chemical equations represent chemical reactions Chemical formula are used and the arrow takes the place of an equals sign. The arrow says “yields” C10H8 + 12 O2 ---> 10 CO2 + 4 H2O

2. The number in front of an element, molecule, or compound represents the number of the element, molecule, or compound. Ex: 2C (two carbon atoms), 8Fe (eight Iron atoms), 2H2O (two water molecules) The subscript number represents the number of atoms or Ions immediately preceding the subscript. Ex: S8 (eight sulfur atoms), (NH4)2SO4 Four oxygen atoms One sulfur atom Two Ammonium Ions (NH4) total 2 nitrogen 8 hydrogen Four Hydrogen atoms One Nitrogen atom

3. Chemical Reactions Four signs of a chemical reaction: Formation of a gas, formation of a solid (precipitate), release of energy, or change of color. Reactants go into the reaction and Products come out of the reaction. AB + CD ---> AD + CB reactants ---> products Energy is conserved in chemical reactions. It is stored in the bonds as chemical energy. Exothermic Reactions more energy is released from the reaction than is put into the reaction. Ex: Combustion reactions Endothermic Reactions more energy is put into the reaction than is released from the reaction. Ex: Chemical Ice Packs