1. Multiple Oxidation States
Truman Chemistry Dept.

2. Oxidation States can change during a reaction
Example: Oxidation state of oxygen
CH4 + 2O2  CO2 + 2H2O
In O2 the oxidation state of O is zero
In CO2 and H2O the oxidation state of O is -2
So the main question here is how can we predict how the oxidation state will change?
There are rules to predict this….

3. Rules
All elements in a diatomic molecule have an oxidation state of zero (they are neutral) ie. Cl2
If an element is monatomic (by itself) the charge is only as specified (ie. Cl-). Free elements have a charge of zero (ie. Fe)
All neutral compounds must have a net (total) oxidation charge of zero and the element with the highest electronegativity (furthest right on table) is the negative element.

4. Rules continued…
Hydrogen always has a +1 charge except in hydrides where it is negative one (ie. LiH or CaH2)
Oxygen always has a -2 charge except in peroxides (H2O2) where it is -1
Group 1 elements are normally +1
Group 2 elements are normally +2
Group 17 elements are often -1

5. Polyatomic ion rules…
Polyatomic ions have a net charge that is equal to the sum of all the charges.

6. Lets try some:
1. The oxidation number of nitrogen in N2 is (1) +1    (2) 0    (3) +3      (4) -3
What is the oxidation number of hydrogen in CaH2? 
(1) +1     (2) +2      (3) -1     (4) -2
What is the oxidation number of carbon in NaHCO3? 
(1) -2     (2) +2      (3) -4   (4) +4

7. A few more?
What is the oxidation number of chlorine in HClO4? (1) +1      (2) +5    (3) +3     (4) +7
What is the oxidation number of sulfur in H2SO4? (1) 0     (2) -2     (3) +6   (4) +4
What is the oxidation number of chromium in K2Cr2O7? (1) +12     (2) +2   (3) +3    (4) +6

8. Lets look at some reactions:
Al + O2  Al2O3
What happened to the oxidation states?
Half reactions?
S + NO3-  SO2 + NO