2. Oxidation Process in which atoms or ions attain a more + oxidation state Ex. Na → Na + + e Cl - → Cl + e Ca + → Ca 2+ + e

3. Reduction Atoms or ions of elements attain a more negative oxidation state Ex. Cl + e → Cl - Mnemonic: “OIL RIG” Oxidation involves loss, reduction involves gain

4. Oxidation Number Uncombined elements have oxidation # of 0 Monoatomic ions have oxidation # equal to their ion charge F is always –1 in a compound O is almost always –2 in a compound H is +1 in all compounds except those with metals (then its –1) More electronegative element is assigned a number equal to its ion charge Algebraic sum of oxidation numbers in a compound is always zero

5. Practice Determine the oxidation numbers for each element in the compounds below NaCl H 2 SO 4 NO 3 - CO 2 SF 6 AgNO 3

6. Redox Reactions Redox reactions are among the most common of all chemical processes Combustion, synthesis, decomposition, and single replacements are all redox reactions

7. Definition Reaction in which a change in oxidation number occurs Oxidation – becomes more positive Reduced – becomes more negative Oxidation and reduction always come in pairs Electrons are transferred from the substance being oxidized to the substance being reduced

8. Example In the reactions below, identify what is oxidized and what is reduced 2PH 3 + 4O 2  P 2 O 5 + 3H 2 O 2H 2 O + Al + MnO 4 -  Al(OH) 4 - + MnO 2