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Minh Pham

on 28 April 2011

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Transcript of Electrochemistry

Electrochemistry The branch of chemistry that is focused on the studies of the interconversions of chemical and electrical energy that takes place in a solution with the interface of electronic and ionic conductors. Electrochemistry occurs in redox reactions that take place in devices called electrochemical cells. Suzanne Abdelazim
Nam Kim
Adam Miller
Minh Pham Luigi Galvani Attached brass hooks and an iron railing to the spinal cord of the dissected frog to discover that the frog's legs to twitch during atmospheric electrical discharge
Discovered the concept of "Galvanism" which refers to the production of electrical current from the contact of two metals in a humid environment (1737-1798) Alessandro Giuseppe Volta (1745-1827) Repeated Galvani's experiments of atmospheric electrical discharge as a Physics Professor at the University of Pavia
Develop the first device that chemically produced an electric current
Arrangement of a pile of metal discs (zinc with copper or silver) separated from one another with paper discs soaked in saline solution. First Battery Produced Humphry Davy (1778-1829) Sucessfully isolated potassium, sodium, barium, calcium, strontium, magnesium, boron, and silicon using the recently discovered voltaic cell Michael Faraday (1791-1867) 1. The mass of a given substance that is produced or consumed at an electrode is proportional to the quantity of electric charge passed through the cell.
2. Equivalent masses of different substances are produced or consumed at an electrode by the passage of a given quantity of electric charge through the cell. Faraday constant, F = 96,485.34 C mol-1 Electrochemical Cells Draws electrical energy from oxidation-reduction reactions or facilitating such reactions through electrical energy
Voltaic vs. electrolytic cell Galvanic Cell Vs Electrolytic Cell Electrolytic Cells
Anode Vs Cathode

Common Representation of Cell
Cu|Cu ||Ag |Ag 2+ + Half Cell Voltage Half reaction
How is the half cell voltage found?
Cell Voltage Calculation Example
Zn2+ +2e- -> Zn E0=-0.76V
Cu2+ +2e- -> Cu E0= 0.34V

So what is Keq? Gibbs Free Energy G = non-PV work
ΔG = ΔG° +RTlnQ
ΔG°= -nFΔE°
Nernst Equation
lnKeq = (nF/RT)ΔE° ? ? ? ? Batteries Battery Types Primary Cell
- Leclanche cell
- Alkaline dry cell
- Zinc-mercuric oxide cell
Secondary Cell (Rechargeable)
- Nickel-Cadmium cell
- Lead-acid storage battery
Primary Cell Most used primary cell is the Leclanche Cell (Zinc-Carbon dry Cell)
Zinc shell as anode
Graphite rod as cathode
Salt bridge mixture of Ammonium Chloride, Zinc Chloride, graphite, and Manganese Dioxide
Secondary Cell (Rechargable) To recharge greater potential then what the cell originally produce and has to be in opposite charge. By recharging, the batteries are being brought back to its original state before depletion of energy. At anode Zinc is oxidized by:
Zn(s) →Zn2+(aq) +2e-
At cathode Manganese Dioxide is reduced to Manganese (III) oxide by:
2 MnO2(s)  + 2NH4+(aq) + 2e- →Mn2O3(s) +2 NH3(aq) + H2O (l)
By adding Oxidation and Reduction together, 1.5V is created by:
2 MnO2(s)  + 2NH4+(aq) + Zn(s) →Mn2O3(s) +2 NH3(aq) + H2O (l) + Zn2+(aq) Primary Cell- Leclanché cell example Since reaction results in aqueous solution, over time the reactants and volume of solution decrease, thus decreasing the voltage over time
Other types of primary cells
- Alkaline dry cell (KOH instead of Ammonium Chloride)
- Zinc-mercuric oxide cell (Cameras, hearing aids, calculators, watches) Primery Cell (Cont'd) Nickel-cadmium Cell
Used in portable shavers, cell phone batteries, and other rechargeable electronic devices
Lead-acid storage battery
Used mainy in cars, boats, etc. Produces 12-V potential, from 6 2-V cells connected in a series
Metallic lead used as anode by:
Pb(s) +SO42-(aq) → PbSO4(s) + 2 e-
Lead dioxide is used as cathode by:
PbO2(s) +SO42-(aq) + 4 H3O+(aq) + 2 e-  → PbSO4(s) + 6 H2O (l)
When combining the overall reaction is shown by:
Pb(s)+PbO2(s) +2SO42-(aq) + 4 H3O+(aq) → 2PbSO4(s) + 6 H2O (l) Secondary cell-Lead-acid battery example Fuel Cells Opposite of batteries where reactants are supplied, and products are removed
Energy that is released power electrical equipment
Hydrogen-oxygen fuel cell example 2 H2(g) + O2(g) → 2 H2O (l)
Reduction potential of 1.229V
Hopes to replace traditional internal combustion engine in car with Hydrogen fuel cell
Efficiency of internal combustion engine with fuel is ~30-35%
Fuel cells can become ~60-70%
Produces water as bi-product, while traditional engine produces non-eco friendly bi-products Corrosion When Iron turns to rust
Chemically, when a metal becomes a metal oxide
Can be thought of as an electrochemical cell having both an anode and cathode To prevent corrosion, a sacrificial anode is used
By using a metal that is more easily oxidized in surrounding areas, it becomes oxidized instead of the iron
Must be periodically replaced
Electrometallurgy The process of purifying metals by using electrochemical reduction/oxidation processes Aluminum Ores (Bauxite)
50%-60% Al2O3; 1%-20% Fe2O3; 1%-20% silica and minor concentrations of transition metals
Purification via Hall-Heroult Process
Hall-Heroult Process Magnesium Ores Chem277 Relevant Experiments Experiment #6: Redox Potential of Cytochrome c
EMF (Electromotive Force) Series Lab: Identifying the solution
Electrochemistry is Essential to Life Electrochemistry used to make alcoholic beverages THE END
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