1. Redox Reactions
Chapter 18
+ O2 

2. Oxidation-Reduction (Redox) Reactions
“redox” reactions: rxns in which electrons are transferred from one species to another
oxidation & reduction always occur simultaneously
we use OXIDATION NUMBERS to keep track of electron transfers

3. Rules for Assigning Oxidation Numbers:
1) the ox. state of any free (uncombined) element is zero.
Ex: Na, S, O2, H2, Cl2, O3

4. Rules for Assigning Oxidation Numbers:
2) The ox. state of an element in a simple ion is the charge of the ion.
Mg2+  oxidation of Mg is +2

5. Rules for Assigning Oxidation Numbers:
3) the ox. # for hydrogen is +1
(unless combined with a metal, then it has an ox. # of –1)
Ex: NaOH (H bonded to O) v. NaH (H bonded to Na)
H = +1
H = -1

6. Rules for Assigning Oxidation Numbers:
4) the ox. # of fluorine is always –1.

7. Rules for Assigning Oxidation Numbers:
5) the ox. # of oxygen is usually –2.
Why USUALLY? Not -2 when it’s in a peroxide, such as hydrogen peroxide:
H2O2

8. Rules for Assigning Oxidation Numbers:
6) in any neutral compound, the sum of the oxidation #’s = zero.

9. Rules for Assigning Oxidation Numbers:
7) in a polyatomic ion, the sum of the oxidation #’s = the overall charge of the ion.

10. Rules for Assigning Oxidation Numbers:
**use these rules to assign oxidation #’s; assign known #’s first, then fill in the #’s for the remaining elements:

11. Examples: Assign oxidation #’s to each element:
a) NaNO3
Na = +1 O = -2
Therefore, N = +5

12. Examples: Assign oxidation #’s to each element:
b) SO32- +4 -2
O = -2, therefore S must = +4 to balance the charges and have an overall charge of 2-

13. Examples: Assign oxidation #’s to each element:
c) HCO3- +1 +4 -2

14. Examples: Assign oxidation #’s to each element: Do on your own:
H3PO4
Cr2O72-
K2Sn(OH)6

15. Definitions
Oxidation: the process of losing electrons (ox # increases)
Reduction: the process of gaining electrons (ox # decreases)
Oxidizing agents: species that cause oxidation (they are reduced)
Reducing agents: species that cause reduction (they are oxidized)

16. To help you remember…
OIL RIG
Oxidation Is Loss
Reduction Is Gain

17. OR…
LEO GER
Losing Electrons → Oxidation
Gaining Electrons → Reduction

18. Are all rxns REDOX rxns? You must determine this…
a reaction is “redox” if a change in oxidation # happens; if no change in oxidation # occurs, the reaction is nonredox.

19. Examples
MgCO3 is an ionic compound, so what is Mg’s charge in an ionic compound?
MgCO3  MgO + CO2 +2 -2 +4 -2 +4 +2 -2
The carbonate ion CO32- is the other ion, let’s figure out C because we already know O.
Is this a redox or nonredox reaction?
NONREDOX (no change in oxidation numbers)

20. Examples Zn + CuSO4  ZnSO4 + Cu
Which oxidation numbers do we already know?
Zn + CuSO4  ZnSO4 + Cu +6 +2 -2 +2 +6 -2
0, free element
Break down this ionic compound into its ions
Is this a redox or nonredox reaction?
Cu and SO42-
So, Cu must be Cu2+
REDOX reaction
O = -2 in SO42-, so S must be…?

21. Examples
NaCl + AgNO3  AgCl + NaNO3
Redox or nonredox?

22. Examples CO2 + H2O  C6H12O6 + O2 +4 -2 +1 -2 +1 -2+1 -2
What happened to Carbon? It went from +4 oxidation # to 0.
Was Carbon oxidized or reduced?
REDUCED
OIL RIG (oxidation is losing electrons so oxidation number increases, where as reduction is gaining electrons so oxidation number decreases)