Post on 27-Dec-2015
ElectrochemistryLesson 6
Electrochemical Cells
Electrochemical cells are Batteries
Alkaline Batteries “dry cells” (Zn/MnO2)
KOH
Car Batteries
Pb-Acid H2SO4
Mitsubishi iMiEV - Pure Electric Car
Powered by a 330 v Li-Ion Rechargeable battery
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$ 50,000 Can
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Cell Phone batteries
Lithium Ion Rechargeable battery/secondary cells
Lithium ion or Ni-Cadmium cells
Space Ship Batteries
Powered by Radioisotopes
Ni-Metal Hydride
Standard Electrochemical Cells Definitions
Electrochemical cell – A device which converts chemical energy into electrical energy through a spontaneous redox reaction; also known as galvanic or voltaic cells.Electrode – A conductor (usually a metal) at which a half-cell reaction(oxidation or reduction) occurs. The metal electrode is immersed in a solution that contains ions of that same metal.Anode - The electrode at which oxidation occurs (A & O are both vowels) LEO-A Cathode – The electrode at which reduction occurs (R & C are both consonants) GER-C
1 M Zn(NO3)2 1 M Cu(NO3)2
Cu
Anode – Oxidation half-rxn Zn(s) Zn2+
(aq) + 2e-
Cathode – Reduction half-rxn
Cu2+ + 2e- Cu
Notes on Electrochemical Cells
An electrochemical cell – a system of electrodes, electrolytes, and salt bridge that allow oxidation and reduction reactions to occur and electrons to flow through an external circuit.
1. Spontaneous redox reaction 2. Produces electricity from chemicals
3. Is commonly called a battery
Analyzing Electrochemical Cells
The reaction that is higher on the reduction chart is the reduction and the lower is oxidation and is written in reverse.
In other words, it is a spontaneous redox reaction between the strongest oxidizing agent and the strongest reducing agent.
Oxidation always occurs at the anode and reduction at the cathode
For any cell
Electrons flow through the wire and go from anode to cathodeAnions (- ions) migrate to the anode and cations (+ions) migrate towards the cathode through the salt bridge
The salt bridge allows ions to migrate from one half-cell to the other without allowing the solutions to mix.
The salt bridge- where ion migration occurs to prevent a charge build-up. How? the reduction half cell would become more negative, while the oxidation half-cell would become more positive…this leads to polarization that would stop electron flow…a salt bridge prevents that!
Cations + flow (migrate) toward the cathode In the salt bridge And Anions – flow toward the anode
voltmeter
1. Draw and completely analyze a Cu+2/Sn electrochemical cell.
Higher
Greatest Electron Affinity
Reduction
Cathode
Cu2+ + 2e- → Cu(s)
0.34 v
Gains mass(called plating)
+
CuSn
Cu2+
NO3-
Sn2+
NO3-Na-+
NO3-
0.48 v
Lower
Oxidation
Anode
Sn(s) → Sn2+ + 2e-
+0.14 v
Loses mass
-
Overall Reaction: Cu2+ + Sn → Sn2+ + Cu(s) 0.48 v
e-e-
Salt bridge
Final notes…Be able to tell which two half-reactions would occur ( which one is the reduction one, which is the oxidation one)
Where do electrons flow from-to? Through?
Where do ions migrate from-to? Through?
Immerse the electrodes in solutions that contain their ions.
A salt bridge can be any soluble salt such as NaNO3, KNO3, etc…
Final notes…A salt bridge can be any soluble salt such as NaNO3, KNO3, etc…
Spectator ions
Na+, K+, Cl-, Ca+2, Mg+2, SO4-2(in neutral solutions)
They have such low tendencies to either reduce or oxidize that they are spectators under most circumstances…
Homework
Page 217
#34,35