1. Oxidation-Reduction Reactions
Oxidation-reduction reaction – a chemical reaction involving the transfer of electrons
Oxidation – loss of electrons
Reduction – gain of electrons

2. Oxidation occurs when:
An atom or molecule gains oxygen atoms
2Mg + O2  2MgO
An atom or molecule loses electrons
Mg  Mg e-
An atom or molecule loses hydrogen
CH3OH  CH2O + H2

3. Reduction occurs when:
An atom or molecule loses oxygen atoms
2KClO3  2KCl + 3O2
An atom or molecule gains electrons
Cu e-  Cu(s)
An atom or molecule gains hydrogen
CO + 2H2  CH3OH

4. Oxidation-Reduction Reactions
Which element is oxidized?
Which element is reduced?

5. Assignment:
Page 651/ section 18-2: 1-7

6. Balancing Oxidation-Reduction Reactions by the Half-Reaction Method
Half reaction – equation which has electrons as products or reactants

7. Balancing Oxidation-Reduction Reactions by the Half-Reaction Method

8. Electrochemistry: An Introduction
Electrochemistry – the study of the interchange of chemical and electrical energy
Two types of processes
Production of an electric current from a chemical reaction
The use of electric current to produce chemical change

9. Electrochemistry: An Introduction
Making an electrochemical cell *

10. Electrochemistry: An Introduction
If electrons flow through the wire charge builds up.
Solutions must be connected to permit ions to flow to balance the charge.

11. Electrochemistry: An Introduction
A salt bridge or porous disk connects the half cells and allows ions to flow, completing the circuit.

12. Electrochemistry: An Introduction
Electrochemical battery (galvanic cell) – device powered by an oxidation-reduction reaction where chemical energy is converted to electrical energy
Anode – electrode where oxidation occurs
Cathode – electrode where reduction occurs

13. Batteries Lead Storage Battery Anode reaction - oxidation
Pb + H2SO4  PbSO H e
Cathode reaction - reduction
PbO2 + H2SO e + 2H+  PbSO H2O

14. Batteries
Overall reaction
Pb + PbO2 + 2H2SO4  2PbSO H2O

15. Batteries
Electric Potential – the “pressure” on electrons to flow from anode to cathode in a battery

16. Batteries Dry Cell Batteries – do not contain a liquid electrolyte
Acid version
Anode reaction - oxidation
Zn  Zn e
Cathode reaction – reduction
2NH4+ + 2MnO e  Mn2O NH3 + 2H2O

17. Batteries Dry Cell Batteries – do not contain a liquid electrolyte
Alkaline version
Anode reaction - oxidation
Zn + 2OH  ZnO + H2O + 2e
Cathode reaction – reduction
2MnO2 + H2O + 2e  Mn2O OH

18. Batteries Dry Cell Batteries – do not contain a liquid electrolyte
Other types
Silver cell – Zn anode, Ag2O cathode
Mercury cell – Zn anode, HgO cathode
Nickel-cadmium – rechargeable

19. Corrosion
Corrosion is the oxidation of metals to form mainly oxides and sulfides.
Some metals, such as aluminum, protect themselves with their oxide coating.
Corrosion of iron can be prevented by coatings, by alloying and cathodic protection.
Cathodic protection of an underground pipe *

20. Electrolysis
Electrolysis – a process involving forcing a current through a cell to produce a chemical change that would not otherwise occur *