1. Oxidation Numbers p.232-234

2. Assigning Oxidation Numbers Oxidation Number: positive or negative number assigned to an atom according to a set of rules

3. Rules for Assigning Oxidation Numbers 1.The oxidation number of any free element is 0 - Na or O 2 2.The oxidation number of a monatomic ion is equal to the charge on the ion. Some atoms have several possible oxidation numbers. - Iron can be Fe 2+ or Fe 3+

4. Rules for Assigning Oxidation Numbers 3.The oxidation number of each hydrogen atom in most of its compounds is +1, EXCEPT in metal hydrides (LiH) where it is -1. 4.The oxidation number of oxygen IN A COMPOUND is -2, EXCEPT in peroxides (H 2 O 2 ) where it is -1.

5. Rules for Assigning Oxidation Numbers 5.The sum of the oxidation numbers of ALL THE ATOMS in a particle must equal the apparent charge of that particle. SO 4 2-, S 2 O 3 2-, Na 2 SO 4 6.In compounds, the elements of Group 1, 2, and aluminum have positive oxidation #’s of +1, +2, and +3 respectively. AlBr 3, CaO

6. Now you try some! Find the oxidation numbers of iodine in HIO 4, HIO 3, HIO, HI Section Review p. 235 #1 a-i

7. Oxidation-number Changes in Chemical Reactions If the oxidation number increases for an atom –Oxidation has occurred If the oxidation number decreases for an atom –Reduction has occurred 2AgNO 3 (aq) + Cu(s)  Cu(NO 3 ) 2 (aq) + 2Ag(s) What is being oxidized? What is being reduced? +1 +5 –2 0+2 +5 –2 0

8. Now you try some! Use the changes in oxidation numbers to identify which atoms are oxidized, and which atoms are reduced: 2H 2 (g) + O 2 (g)  2H 2 O (l) 2KNO 3 (s)  2KNO 2 (s) + O 2 (g) NH 4 NO 3 (s)  N 2 (g) + 2H 2 O (g) (HINT: consider each N separately) PbO 2 (aq) + 4HI (aq)  I 2 (aq) + PbI 2 (s) + 2H 2 O (l)