Electrochemical Cells (Batteries)
Chemical reaction produces electricity.
Called “voltaic cells”
Happens SPONTANEOUSLY.
Moving Electrons = Electricity
Electrons from oxidized substance
travel towards substance reduced.
Traveling electrons move
through “external circuit”
where they do work.
Parts of a Simple Battery
(Voltaic Cell)
Made of Two “Half Cells” containing:
2 Metal Electrodes
2 Solutions of Ions
External Wire
Salt Bridge
“Charge” flows in a “circuit” that
is connected.
External Wire:
allows e- to flow
between metal electrodes
Salt Bridge:
allows ions to flow
between solutions
What is Ox/Red?
Start with Table J
Electrons travel from more
“Active” metal toward less
active metal.
Metal above = oxidized
Ion on Metal below = reduced
Ex:
Ox: Zn
Red: Cu+2
Which way do electrons
flow in the external wire?
Electrons flow
“Down Table J”
From metal
above to ion of
metal below
from Zn to Cu+2
e-
Which electrode is
negative or positive?
Electrons flow from negative to
positive electrode.
Negative electrode: Zn
Positive electrode: Cu
e-
Which electrode is the
anode or cathode?
Anode: metal electrode
where oxidation occurs
Ex: Zn
Cathode: metal electrode
where reduction occurs
Ex: Cu
Remember
AN OX
RED CAT
Anode is where oxidation happens
Cathode is where reduction happens
What are the Half Reactions?
Ox:
Zn0
Zn+2 + 2e-
Red:
Cu+2 + 2e-
Cu0
What is the Net Equation?
Net: (add ½ reactions)
Zn0 + Cu+2
Zn+2 + Cu0
Make sure final net equation is
balanced for electrons and atoms!
e-
Which electrode gains/loses weight?
Look at half reactions!!
Which forms solid metal?
Which forms dissolved ions?
Ox:
Zn0
Zn+2 + 2eRed:
Cu+2 + 2eCu0
Which way do the ions in the salt
bridge “migrate” or move?
Remember:
“The negative ions complete the circuit”
(The ions actually end up moving towards
the solution of opposite charge that forms.)
Dead Battery
Voltage = 0
Means the reaction in the battery has
reached EQUILIBRIUM.
You try it…
Mg/Mg+2//Al+3/Al
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Draw and label Battery
What is oxidized/reduced?
What are the half reactions and net(balanced)?
What is the neg/pos electrode?
What is the anode/cathode?
Which way do e- flow in wire?
Which way do -/+ ions flow in salt bridge?
Which electrode gains/loses mass?
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Lemon Battery Demo
http://youtu.be/AY9qcDCFeVI
Batteries
http://www.mhhe.com/physsci/chemistry/ess
entialchemistry/flash/galvan5.swf
Battery Simulation
http://www.chem.iastate.edu/group/Greenbo
we/sections/projectfolder/flashfiles/electroCh
em/volticCell.htmlN
Nerst Equation Simulation (HONORS)
http://www.chem.iastate.edu/group/Greenbo
we/sections/projectfolder/flashfiles/electroCh
em/voltaicCellEMF.html
Finding Voltage of a Battery
(Honors)
Use Voltage Table
Find your half reactions and record voltage
Note:
All ½ reactions shown are reductions.
For oxidation, reverse the sign of the voltage
Nerntz Equation (Honors)
Find voltage of a battery when the conc. of dissolved
ions is not 1 Molar (as on “standard voltage” table)
Ecell = E0 –
0.0592 log [product ion]x
n
[reactant ion]y
n = total # of moles electrons being transferred
The concentration of dissolved ions can affect voltage.
Greater concentration of reactant ions (see net)
increases the overall voltage.