Electrochemistry

1. Electrochemistry “It is the study of the interchange of chemical and electrical energy”

2. Applications of Electrochemistry in our daily life • Batteries (car batteries – calculators-digital watches) • Corrosion of iron • Preparation of some industrial materials eg. Al, Cl2, NaOH, …… • In analytical chemistry e.g. the analysis of chemicals in blood to determine • the development of a certain diseases

3. Oxidation Reduction Reactions (REDOX) • Oxidation : Loss of electrons • Reduction : Gain of electrons M X Reduced Gains electrons Oxidizing agent Oxidized Loses electrons Reducing agent e M+ X-

4. Example • Mg →Mg++ + 2e- (Oxidation Reaction) • O2 + 4e-→2O--(Reduction Reaction) • When these half equations are paired and electrons balanced • (i) (Mg → Mg+++ 2e- ) x 2 loses electrons – oxidized – reducing agent • (ii) O2 + 4e-→2O--gains electrons – reduced – oxidizing agent • Adding (i) and (ii) • 2Mg + O2+4e→2Mg++ +4e + 2O-- • 2Mg + O2 → 2 MgO Redox Reaction A Redox reaction is a reaction in which electrons are transferred from a reducing to an oxidizing agent

5. When reducing and oxidizing agents are present in the same solution, electrons are directly transferred. • If we separate the oxidizing agent from the reducing agent transfer of electrons through a wire current production working of a motor useful work But current flows for an instant and then stops because of charge buildup

6. To solve the problem the solution must be connected either by a salt bridge or a porous disk (allows flow of ions without mixing of solutions) • Now electrons flow through the wire from reducing to oxidizing agent and ions flow from one compartment to the other to keep the net charge zero

7. Galvanic Cell A device in which chemical energy is changed into electrical energy

8. An Example of a Galvanic Cell The voltmeter measures the cell potential or electromotive force (emf) of the cell) The unit is the volt Cathode :Cu++ + 2e- Cu Anode :Zn  Zn++ + 2e- Cell reaction :Zn + Cu++ Zn++ + Cu

9. What is Cell Potential? • It is the “pull” or “driving force” on the electrons from reducing agent to oxidizing agent The reaction in a galvanic cell is always an oxidation-reduction reaction that can be broken into 2 half reactions. It would be convenient to assign a potential to each ½ reaction so that we can obtain the cell potential by summing the ½ cell potentials. Ecell = E(anode, oxid.)+ E(cathode, red.) Ecell = E(anode, oxid.) – E(cathode, red) But although we can measure the total potential of a cell there is no way to measure the potentials of the individual electrode processes

10. Standard Hydrogen Electrode The potential of cathode reaction 2H++2e-→H2 = 0 v ∴ the potential of the anode reaction Zn→Zn+++2e-=0.76 v

12. Example Conside a galvanic cell based on the reaction: Fe+++(aq)+Cu(s)→Cu++(aq) + Fe++(aq) What are the 2 half reactions? Give the balanced cell reaction and calculate E° of the cell. 2 [ Fe++++ e→ Fe++] E°cathode=0.77 Cu→Cu+++2e E°anode= -0.34 _______________________________ Cu + 2Fe+++ → Cu++ + 2Fe+ E°cell=0.43 N.B.: The E°cell (cell potential) is always +ve for a galvanic cell E°cell= E°cathode - E°anode

13. Example Describe completely the galvanic cell based on the following ½ reactions under standard conditions: Ag++e- Ag E°cell =0.8 v Fe+++ + e-  Fe++ E°cell = 0.77 v Since E°cell must be +ve , thus the ½ reactions are: Cathode (red.) Ag+ + e- → Ag E°=0.8 v Anode (oxid) Fe++→Fe+++ + e- E°= -0.77 v _____________________________________________ Cell reaction Ag+ + Fe++ → Ag + Fe+++ E°cell=0.03 v

14. Standard cell potential Cell potential at conc. C Ionic concentration (equilibrium constant) No. of transferred electrons Dependence of Cell Potential on Concentration Nernest Equation

15. Example If E°cell is 0.48 v for the galvanic cell based on the reaction 2Al(s) + 3 Mn++(aq)  2Al+++(aq) + 3 Mn(s) What is the cell potential if [Mn++] = 0.5 M and [Al+++]=1.5 M

16. Concentration Cells • Since cell potential depends on concentration, we can construct galvanic cells where both compartments contain same component but at different concentrations Batteries • A battery is a galvanic cell or a group of cells connected in series, where the potential of the individual cells add to give the total battery potential. • Eg. Lead storage battery (in automobiles) Dry cell Mercury battery

17. Chemical Impact Printed Batteries • Soon you may reach for a compact disc in a record store, and as you touch it, the package will start playing one of the songs on the disc. Or you may stop to look at a product because the package begins to glow as you pass it in the stores. These effects could happen soon due to the invention of a flexible, super thin battery that can actually be printed on the package. The battery consists of 5 thin layers of zinc (anode) and manganese diioxide (cathode) and is only 0.5 mm thick. The battery can be printed onto paper with a regular printing press. This battery intends to bring light, sound and other special effects to packaging to entice potential customers. Within a year or two, you might see talking, singing or glowing packages on the shelves.