1. Oxidation-Reduction Reactions
LEO SAYS GER

2. Remember Oxidation Numbers

3. Oxidation Numbers What is an oxidation number:
An oxidation number is a positive or negative number assigned to an atom to
keep track of electron transfers & electron sharing

4. Rules for Assigning Oxidation Numbers
The oxidation number of any uncombined element is zero.
2.The oxidation number of a monatomic ion equals its charge. +1 -1
2Na + Cl2 2NaCl

5. Rules for Assigning Oxidation Numbers
3. The oxidation number of O in compounds is -2,
Exceptions:
in hydrogen peroxide: H2O2 where it is -1,
in compounds with the more electronegative fluorine F2O +1 -2 +1 -1 -1 +2
F2O
H2O
H2O2
2(+1) + 1(-2) = 0
2(-1) + 1(+2) = 0
2(+2) + 2(-2) = 0

6. Rules for Assigning Oxidation Numbers
4. The oxidation number of H in compounds is +1, Exception: in metal hydrides it is -1. +1 -2 +1 -1
H2O
NaH
Warning!
Don't get too bogged down in these exceptions. In most of the cases you will come across, they don't apply!

7. Rules for Assigning Oxidation Numbers
5. For any neutral compound, the sum of the oxidation numbers of the atoms in the compound must equal 0. +1 -2 +2 -2 +1
Ca(OH)2
H2O
(+2) + 2(-2) + 2(+1) = 0
Ca O H
2(+1) + (-2) = 0
H O

8. Rules for Assigning Oxidation Numbers
6. For a polyatomic ion, the sum of the oxidation numbers in the formula is equal to the ionic charge of the ion ? -2 ? -2
SO42-
NO3-
X + 3(-2) = -1
N O
X + 4(-2) = -2
S O
THUS X = +6
THUS X = +5

9. Oxidation and Reduction
We already learned why salt is spread on roads during winter months
Lowers the freezing point of water preventing the buildup of slippery ice
But…what about the affect of the salt that clings to the metallic parts of cars?
RUST
The corrosion of metal is one example of an oxidation-reduction reaction

10. Oxidation and Reduction Defined
old definition - substance oxidized reacts with oxygen
new definition – loss of electrons
Reduction
old definition – substance reduced loses oxygen
new definition – gain of electrons

11. Oxidation and Reduction
Until now our view of a reaction:
Mg + S  MgS
Reaction viewed as Redox:
Mg S → Mg S2-
The magnesium atom changes to a more stable magnesium ion by losing 2 electrons, and is thus oxidized
The sulfur atom is changed to a more stable sulfide ion by gaining 2 electrons, and is thus reduced.

12. Oxidation and Reduction
Mg + S MgS
The over all process is represented as the two component processes below:
Mg Mg e-
S + 2e- S2-
Oxidation – loss of electrons
Reduction – gain of electrons

13. Example 2: Oxidation and Reduction
Each sodium atom loses one electron – oxidation
Each chlorine atom gains one electron – reduction

14. LEO says GER Gain Electrons = Reduction Lose Electrons = Oxidation
Sodium is oxidized
Gain Electrons = Reduction
Chlorine is reduced

15. But there are two Cl atoms
So, if
Cl e-  2Cl-1
Then,
2Na0  2Na e-
___________________

16. Terminology for Redox Oxidation Reduction Oxidizing Agent
Loss of electrons
Increase in oxidation number
Increase in oxygen
Oxidizing Agent
Electron acceptor
Species that is reduced
Reduction
- Gain of electrons
- Decrease in oxidation
number
- Decrease in oxygen
Reducing Agent
- Electron donor
- Species that is oxidized

17. You can’t have one… without the other!
Reduction (gaining electrons) can’t happen without an oxidation to provide the electrons.
You can’t have 2 oxidations or 2 reductions in the same equation. Reduction has to occur at the cost of oxidation
GER!

18. Practice Problem
Identify these processes as either oxidation or reduction.
S2- S + 2e-
Zn2+ + 2e- Zn
Oxidation – is loss
Reduction – is gain

19. Practice Problem
Determine what is oxidized and what is reduced in each reaction.
2Na + S Na2S
4Al + 3O2 2Al2O3
Na is oxidized
(Na1+ S2-)
S is reduced
Al is oxidized
(Al3+ O2-)
O is reduced

20. Redox in Covalent Compounds
It is easy to see the loss and gain of electrons in ionic compounds, but what about covalent compounds? (complete electron transfer does not occur)
Consider: 2H2 + O H2O
Oxygen is highly electronegative
In H there is a shift of bonding electrons away from H
Hydrogen is oxidized
In O there is a shift of electrons toward O
Oxygen is reduced
loss
gain