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ElectrochemiElectrochemistrystry
“It is the study of the interchange of chemical and
electrical energy”
Electrochemistry
The generation of an electric currentfrom a spontaneous chemical reaction
The use of current to produce chemical change
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
Oxidation Reduction Oxidation Reduction Reactions (REDOX)Reactions (REDOX) Oxidation : Loss of electrons Reduction : Gain of electrons
M
M+
X-
X
eReduced
Gains electrons
Oxidizing agent
OxidizedLoses electronsReducing agent
ExampleExample
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
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
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
An Example of a Galvanic CellAn Example of a Galvanic Cell
Anode :Zn Zn++ + 2e-
Cathode :Cu++ + 2e- Cu
The voltmeter measures the cellpotential or electromotive force(emf) of the cell) The unit is the volt
Cell reaction :Zn + Cu++ Zn++ + Cu
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
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
++ZnZn2H+Hcell E+E=E→→
++ZnZncell E=E
→
ExampleExample
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
ExampleExample
Describe completely the galvanic cell based on the following ½ reactions under standard conditions:
Ag++e- Ag E°cell =0.8 vFe+++ + 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
Dependence of Cell Potential Dependence of Cell Potential on Concentrationon Concentration
Nernest Equation
)Qlog(n
0591.0 -E=E
Cell potentialat conc. C
Standard cell potential No. of
transferred electrons
Ionic concentration(equilibrium constant)
ExampleExample
If E°cell is 0.48 v for the galvanic cell based on the reaction2Al(s) + 3 Mn++(aq) 2Al+++(aq) + 3 Mn(s)
What is the cell potential if [Mn++] = 0.5 M and [Al+++]=1.5 M
v47.0=]5.0[
]5.1[log
60591.0
-48.0=E
]Mn[
]AL[log
60591.0
-48.0=E
)Qlog(n
0591.0 -E=E
3
2
cell
3++
2+++
cell
cell
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
Chemical Impact
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.
Printed Batteries