1. Describing Reactions What is a chemical equation? What is a balanced equation? How can equations be checked for balance?

2. Chemical Formula Tells us: 1.elements in the compound 2.# of atoms of each element ► Subscript – small number written below & after the symbol ► no subscript = 1 atom H2OH2OH2OH2O Element H = hydrogen Element O = oxygen Subscript 2 – two hydrogen atoms No subscript 1 oxygen atom

3. Parts of an equation Reactants –substance that changes before the chemical reaction. Products –substance formed after the chemical reaction. Coefficient – # of molecules (number in front) 2H 2 + O 2  2H 2 O reactantsproducts “yield” arrow

4. How to represent: Element ► symbol Compound ► formula Reaction ► equation Na = Sodium NaCl = Sodium chloride (salt) Na + Cl  NaCl

5. Law of Conservation of Matter ► Matter or energy can never be created or destroyed. ► Matter is only REARRANGED

6. How to check for balance: ► List the number of atoms of each element on both sides of the equation. Make sure they are equal. H 2 O  H 2 +O 2 H - 2 O - 1 H - 2 O - 2 NOT BALANCED

7. Add a COEFFICIENT in front of molecules to increase the number of atoms. This can be done on both sides until balance is achieved. YOU MAY NOT CHANGE SUBSCRIPTS! H 2 O  H 2 +O 2 H - 4 O - 2 H - 4 O - 2 22 How to balance an unbalanced equation: BALANCED

8. Check for balance: AuCl 3  Au + Cl 2 Au - 1 Cl - 3 Au - 1 Cl - 2 NOT BALANCED

9. balance: AuCl 3  Au + Cl 2 Au - 2 Cl - 6 Au - 2 Cl - 6 2 23