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

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

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

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

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

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

Balancing Oxidation-Reduction Reactions by the Half-Reaction Method

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

Electrochemistry: An Introduction Making an electrochemical cell *

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.

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

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

Batteries Lead Storage Battery Anode reaction - oxidation Pb + H2SO4  PbSO4 + 2H+ + 2e Cathode reaction - reduction PbO2 + H2SO4 + 2e + 2H+  PbSO4 + 2H2O

Batteries Overall reaction Pb + PbO2 + 2H2SO4  2PbSO4 + 2H2O

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

Batteries Dry Cell Batteries – do not contain a liquid electrolyte Acid version Anode reaction - oxidation Zn  Zn2+ + 2e Cathode reaction – reduction 2NH4+ + 2MnO2 + 2e  Mn2O3 + 2NH3 + 2H2O

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  Mn2O3 + 2OH

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

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 *

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