1. Oxidation-Reduction (Redox) Reactions

2. Oxidation-Reduction Reactions
Electron transfer between ionic compounds, change in oxidation numbers
One compound wants to GIVE electrons, other compound wants to TAKE electrons.
Reduction—
GAIN of electrons in element/compound
Compound becomes more NEGATIVE
DECREASE in oxidation number
Oxidation
LOSS of electrons in element/compound
Compound becomes more POSITIVE
INCREASE in oxidation number

3. Oxidation CANNOT happen without Reduction.
Reduction CANNOT happen without Oxidation
Both have to happen in a redox reaction ! ! !

6. Example 1:
Mg (s) + Cu+2 (aq) Mg+2(aq) Cu (s)

7. Ex. 2: 2Mg + O2  2MgO Where does reduction happen?
Which does oxidation happen?
Oxidation between Magnesium
Reduction between oxygen
Assign Oxidation Numbers

8. Example 3: 2 Na + Cl2  2NaCl Assign oxidation numbers
Identify where oxidation occurs, where reduction occurs.

9. Determine whether each of these reactions is a oxidation-reduction reaction. If so, identify what is oxidized and what is reduced.
Mg + 2HCl  MgCl H2
I Cl2  ICl3
NaOH + HCl  NaCl H2O
2Na H2O  2NaOH H2
Magnesium oxidized, hydrogen reduced
Iodine oxidized, chlorine reduced
Not a redox reaction
Sodium oxidized, hydrogen reduced

10. Half-Reaction
Describes the oxidation or reduction process going on in a redox reaction
2 half-reactions make ONE redox reaction
Contains “half” of a redox reaction

11. Half-Reactions
A method of separating the OXIDATION part of a chemical reaction from the REDUCTION part.
Oxidation process---one equation
Reduction process---one equation
Then we put it all together.

12. Half-Reaction Example
Cu+2(aq) e Cu (s) Reduction
Mg (s) Mg+2 (aq) e Oxidation
Total: Mg(s) Cu+2(aq) Mg+2(aq) Cu(s)

13. Example 4:
Oxidation: Cu (s) Cu+2(aq) + 2e -1 Reduction: Ag+1 (aq) + 1e -1 Ag (s) *When electrons do not balance, multiply one or both half-reactions when needed to balance the amount of electrons gained and lost in the reaction. **

14. How do we write half-reactions?
Original Equation: 2 Na Cl2  2NaCl
Oxidation Half-Reaction:
2Na  2Na e-
Show electrons “lost” through process, 2 because you have 2 Na
Reduction Half-Reaction:
Cl2 + 2e-  2Cl-
Show electrons “gained” through process, 2 because you have 2 Cl
**Electrons MUST equal in number for BOTH half-reactions!!!

15. Putting Redox Equation back together……
Original Equation:
2 Na Cl2  2NaCl
Assign Oxidation Numbers
Identify where oxidation/reduction happening
Half-Reactions:
Oxidation: 2Na  2Na e-
Reduction: Cl2 + 2e-  2Cl-
Cancel elements and electrons existing on both sides, Add half-reactions together.

16. RESULT ! !
2 Na Cl2  2NaCl

17. Example 5:
2Mg + O2  2MgO

18. Example 6:
Cu(s) + NO3  Cu NO

19. Example 7:
Zn + 2HCl  ZnCl H2

20. Practice! Write half-reactions and balance if the reaction is redox !
Mg + 2HCl  MgCl H2
I Cl2  ICl3
NaOH + HCl  NaCl H2O
2Na H2O  2NaOH H2