1. Oxidation & Reduction Reactions
Section 1

2. Oxidation-reduction reactions involve a transfer of electrons
Oxidation involves the loss of electrons
Reduction involves the gain of electrons

3. Oxidation States
An oxidation number is assigned to an element in a molecule based on the distribution of electrons in that molecule
Rules for assigning oxidation numbers
Rule
Example
The oxidation number of a lone element is 0
Na = 0
N2 = 0
The oxidation number of an lone ion is the charge of the ion
Cl- = -1
The more electronegative element in a binary compound is assigned the number equal to the charge if it were an ion
The O in NO = -2
The oxidation number of fluorine is always -1
F in LiF = -1

4. Oxidation States (cont.)
Rule
Example
Oxygen has an oxidation number of -2 unless combined with F, the it is +1 or +2, or in a peroxide, then it is -1
O in NO2 = -2
O in H2O2 = -1
Hydrogen’s oxidation number is +1 unless it is combined with a metal, then it is -1
H in LiH = -1
In compounds group 1, 2 elements, and aluminum have oxidation numbers of +1, +2, and +3 respectively
Ca in CaCO3 = +2
The sum of the oxidation numbers of all atoms in a neutral compound is 0
C in CaCO3 = +4
The sum of the oxidation numbers in a polyatomic ion equals the charge of the ion
P in H2PO4- = +5

5. Oxidation
Processes in which the atoms or ions of an element experience an increase in oxidation state are oxidizing processes
2Na + Cl2  2NaCl
Each sodium atom loses an electron changing the oxidation state from 0 to +1
An element whose oxidation number increases is oxidized
Na is oxidized

6. Reduction
Processes in which the atoms or ions of an element experience an increase in oxidation state are oxidizing processes
2Na + Cl2  2NaCl
Each chlorine atom accepts an electron changing its oxidation state from 0 to -1
An element that undergoes a decrease in oxidation state is reduced
Cl is reduced

7. Redox as a Process
Just like mass has to be conserved in reactions, so does the number of electrons
This means that for oxidation to occur, reduction must also occur
Any chemical process in which elements undergo changes in oxidation number is an oxidation-reduction reaction
Or redox reaction for short

8. Redox as a Process (cont.)
The redox reactions can be written showing either just the oxidation, or just the reduction alone
These are called half-reactions
3Na  3Na+ + 3e- (oxidation)
Al3+ + 3e-  Al (reduction)
3Na + Al3+  3Na+ + Al (redox reaction)

9. If none of the atoms in a reaction change oxidation state, the reaction is not a redox reaction
SO2 + H2O  H2SO4
Na+ + Cl- + Ag+ + NO3-  Na+ + NO3- + AgCl

10. Redox Reactions & Covalent Bonds
Oxidation numbers are assigned based on the electronegativity of an atom relative to the other atoms to which it is bonded in a molecule
It is not based on a real charge

11. Redox in Covalent Bonds (cont.)
When hydrogen burns with chlorine, a covalent bond forms from the sharing of two electrons
The electrons in the bond are not shared equally and are more strongly attracted to the chlorine. Why?
H2 + Cl2  2HCl
Use the oxidation rules to assign the oxidation numbers