1. West Valley High School
Chapter 19 Oxidation-Reduction Reactions (Redox Reactions)
West Valley High School
Chemistry
Mr. Mata

2. Standard 3G
Students will know how to identify reactions that involve oxidation and reduction and how to balance redox reactions.

3. How do electrons behave in a redox reaction?
Essential Question
How do electrons behave in a redox reaction?

4. Oxidation-Reduction Reactions (Redox):
Chemical change.
Involves the transfer of electrons.
Principal source of energy on earth.
Examples:
Combustion of gasoline
Burning of wood
Burning of food in your body

5. Oxidation reactions are always accompanied by a reduction reaction
-    originally meant combining with oxygen
-    iron rusting (iron + oxygen)
Reduction
-    originally meant the loss of oxygen from a compound
removing iron from iron ore ( iron II oxide)

6. Today
OXIDATION means:
complete or partial LOSS of ELECTRONS
REDUCTION means:
complete or partial GAIN of ELECTRONS
Memory Device :
LEO the lion says GER

7. Reduction is… Oxidation is… the gain of electrons
a decrease in oxidation state
the loss of oxygen
the addition of hydrogen
MgO + H2 ® Mg + H2O
notice the Mg2+ in MgO is gaining electrons
Oxidation is…
the loss of electrons
an increase in oxidation state
the addition of oxygen
the loss of hydrogen
2 Mg + O2 ® 2 MgO
notice the magnesium is losing electrons

8. Oxidation States
Oxidation states are numbers assigned to atoms that reflect the net charge an atom.
They are assigned according to the following set of rules: Oxidation Number = ON

9. #1
The ON of a simple ion is equal to its
ionic charge.
Na + Cu 2+ N3-

10. #2
The ON of hydrogen is always +1. +1
HCl

11. #3
The ON of oxygen is always –2. -2
H2O  

12. #4
The ON of an uncombined element or diatomic molecule is always zero.
Na Cu N2

13. #5
For any neutral(zero charge) compound, the sum of the ON’s is always zero.
+4-2
CO2
+4 + (-4) = 0

14. #6
For a complex ion, the sum of the ON’s equals the charge of the complex ion.
+7 -2
MnO41-
+7 + (-8) = - 1

15. +2 to +4 0 to -1 Oxidation Number Changes
an increase in oxidation number of an atom signifies oxidation
+2 to +4
a decrease in oxidation number of an atom signifies reduction
0 to -1

16. Identifying Redox Reactions
Oxidation and reduction always occur together.
These reactions are called “redox”.
Different ways to ID a redox reaction.
The best way is to look for a change in oxidation state:

17. SnCl2 + PbCl4 SnCl4 + PbCl2 CuS + H+ + NO3- Cu+2 + S + NO + H2O
+2 = LEO OA +2 -1 +4 -1 +4 -1 +2 -1
SnCl PbCl SnCl PbCl2 RA
-2 = GER
-3 = GER RA +2 -2 +1 +5 -2 +2 +2 -2 +1 -2
CuS + H NO Cu S NO H2O OA
+2 = LEO

18. Examples - labeling redox reactions
In each reaction, look for changes in oxidation state.
If changes occur, identify the substance being reduced, and the substance being oxidized.
Identify the oxidizing agent and the reducing agent.
= +1 (H is oxidized)
(reducing agent) +2 -2 +1 -2
H2 + CuO ® Cu + H2O
= -2 (Cu is reduced)
(oxidizing agent)

19. 5 Fe2+ + MnO4- + 8 H+ ® 5 Fe3+ + Mn2+ + 4 H2O
Try These!!
+1 = Fe 2+ is oxidized (reducing agent)
5 Fe2+ + MnO H+ ® 5 Fe3+ + Mn H2O
Zn HCl ® ZnCl2 + H2
- 5 = Mn 7+ is reduced (oxidizing agent)
+2 = Zn 0 is oxidized (reducing agent)
- 1 = H 1+ is reduced (oxidizing agent)

20. Balancing Redox Equations
There are two methods used to balance redox reactions
1)the oxidation number change method *(the one we will use)
2)the half reaction method

21. Oxidation Number Change Method
Balance the following: Fe2O3 + CO Fe + CO2
1)Assign ON to all atoms +3 -2 +2 -2 +4 -2
Fe2O3 + CO Fe + CO2
2)Identify which atoms are oxidized and which are reduced
-3 (Fe reduced) +3 -2 +2 -2 +4 -2
Fe2O3 + CO Fe + CO2
+2 (C oxidized)

22. 3) Make the total increase in oxidation number equal the total
decrease in oxidation number by using appropriate coefficients
on the reactant side only. -3
(x 2 atoms) = 6 electrons gained +3 -2 +2 -2 +4 -2
Fe2O3 + CO Fe + 3CO2 3 +2
(X 3 atoms) = 6 electrons lost
4) Finally check to be sure that the equation is balanced both for
atoms and charge.
Fe2O3 + CO Fe + CO2 3 2 3

23. Chapter 19 SUTW Prompt
Describe the differences between oxidation and reduction in a redox chemical reaction.
Complete a sentence paragraph using the SUTW paragraph format. Hilight using green, yellow, and pink.
Due Date: Tomorrow (start of class).