Signs of Chemical Reactions There are five main signs that indicate a chemical reaction has taken place: change in colorchange in odorproduction of new gases or vapor input or release of energy difficult to reverse release input

Subscripts vs. Coefficients The subscripts tell you how many atoms of a particular element are in a compound.The subscripts tell you how many atoms of a particular element are in a compound. The coefficient tells you about the quantity, or number of molecules of the compound.The coefficient tells you about the quantity, or number of molecules of the compound.

CHEMICAL REACTIONS Reactants: Zn + I 2 Product: Zn I 2

Word Equations A WORD EQUATION describes chemical change using the names of the reactants and products. Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water. methane + oxygen Reactant Product CH 4 O2O2 CO 2 H2OH2O + + carbon dioxide + water

Balancing Equations

Visualizing a Chemical Reaction Na + Cl 2 NaCl 2 2

Visualizing a Chemical Reaction Na + Cl 2 NaCl 2 2

Chemical Equations 4 Al(s) + 3 O 2 (g) ---> 2 Al 2 O 3 (s) This equation means 4 Aluminum atoms + 3 oxygen molecules produces 2 molecules of aluminum oxide 2 molecules of aluminum oxide

Balancing Equations

Cl H H H HH H H H 2 + Cl 2  HCl H 2 + Cl 2  2 HCl reactants products H Cl reactants products H Cl (unbalanced) (balanced) Unbalanced and Balanced Equations

Chemical Equations Because of the principle of the conservation of matter, an equation must be balanced. It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788

Chemical Equations reactantsproducts Depict the kind of reactants and products and their relative amounts in a reaction. 4 Al(s) + 3 O 2 (g) 2 Al 2 O 3 (s) The letters (s), (g), and (l) are the physical states of compounds. reactants product

Symbols Used in Equations Solid ___ Liquid (l) Gas ___ Aqueous solution (aq) Catalyst H 2 SO 4 Change of temperature (  )