Transcript Electrochemical Cells

Topic: Electrochemical Cells
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Electrochemistry – the study of the
relationship between chemical
potential energy and electrical energy
• Chemical rxns can produce electricity
OR
• Electrical energy can be used carry out
chemical reactions
All this occurs Via flow of electrons
A Chemical reactions can generate electrical
energy if it occurs spontaneously
How does a spontaneous reaction generate
electricity?
Electrochemical Cells
AKA voltaic cell or galvanic cell
• Used when the chemical rxn is spontaneous
• It produces electricity (energy) via flow of
electrons through a wire therefore is exothermic
• Oxidation and Reduction Reaction have to be
separated (half cells)
Remember the Al foil lab
+2
-1
0
0
+3
-1
AlCl3
CuCl2 + Al  _______
Cu +_______
Was this spontaneous? yes
Was this a redox? Yes,
Cu was Reduced
& Al was Oxidized
So why didn’t we produce electricity?
Half reactions weren’t separated
Electrochemical Cell Requirements
• 2 half-cells containing
aqueous solution & an
electrode
• Need to be connected by
a wire for the electrons to
flow through.
• Need to be connected by
a salt bridge to maintain
electrical neutrality. (if
electrons are moving ions
need to move also = +
ions will flow in same
direction as electrons)
e-  e- e- 
Al
Cu
Al+3 Cl-1
AlCl3
Cl-1 Cl-1
Cu+2
CuCl2
Cl-1 Cl-1
Voltmeter = measures electrical
current
When the cells reach equilibrium
voltage = 0 =no more electrical
current
Electrode
Surface at which oxidation or
reduction half-reaction
occurs.
Anode – Oxidation
The anode = location for the
oxidation half-reaction.
Reduction – Cathode
The cathode = location for the
reduction half-reaction.
MOVIE CLIP
• MEMORIZE
–Fat Red Cat ate An Ox
–Anode oxidation
–Fat (getting bigger) Reduction
ouch 
Cathode
yummy
• Since Cu is the cathode – a fat red cat = gain mass
– Where does the mass come from?
– Reduction of Cu+2 is gaining 2 e- to make Cu
– So the concentration of Cu+2 ions decreases
• Since Al is the anode – An ox was eaten = lost e- = getting smaller
– Oxidation = Lose electrons, who is going to lose Al or Al+3
– Al  Al+3(aq) + 3e– So the concentration of Al+3 increases
Al
Al+3
Al  Al+3(aq) + 3e-
Cu+2(aq) + 2e- Cu
Is this reaction at equilibrium? How can you tell? No – volts not a 0
What is happening to the concentration of Zn+2 ions? increasing
What is happening to the concentration of Cu+2 ions? decreasing
How do you know which electrode is
which?
• Use Table J to predict which electrode is
the anode / cathode.
– higher in Table J. gets oxidized
– Anode = Oxidation = Electron Donor
– lower in Table J. gets reduced
– Cathode = Reduction = Electron Acceptor
Label anode and cathode
anode
cathode
• Electrons flow from Anode to Cathode
e-

e-  e-

e-  e-
+  +  +
anode
Anode = site of oxidation
=getting smaller


e-
+
cathode
cathode = site of
reduction
=getting larger
• Cathode is (+) Anode is (-)
• Positive Ions flow in same direction as electrons
via Salt bridge (too offset the negative electrons)
• Which direction would the negative ions flow?
What’s wrong with this picture?
No salt bridge = needed to
complete the circuit so ions can
flow
Now you try
Create an galvanic (electrochemical) cell with Al and Pb and
AlNO3 and Pb(NO3)2
Label anode and cathode(Use Table J), direction of electron
flow in wire, direction of positive ion flow in salt bridge,
positive electrode, negative electrode.
Pb
Al
Create an galvanic (electrochemical) cell with Al and Pb and
AlNO3 and Pb(NO3)2
Label anode and cathode(Use Table J), direction of electron
flow in wire, direction of positive ion flow in salt bridge,
positive electrode, negative electrode.
e-

e-  e-
Pb
& NO3
e-  e-

e-
Al
Al
cathode
Pb+2

anode
-1
Al+3 & NO3-1
What half-reactions occurred?
REMEMBER Al got smaller and Pb got
bigger
Al  Al+3 + 3eAl was the anode, it was oxidized, it lost e-
Pb+2 + 2e-  Pb
Pb got bigger. How? By gaining
electrons. The Pb+2 ions gained 2 e- to
make Pb.
Overall Spontaneous Rxn
2(Al  Al+3 + 3e-)
+
+2 + 2e-  Pb)
3(Pb
_______________________
2Al + 3Pb+2  2Al+3 + 3Pb
2Al + 3Pb+2  2Al+3 + 3Pb
• Which electrode is losing mass?
Al because now Al+3 ions in solution
• Which electrode is gaining mass?
Pb, Pb+2 gains 2e- an makes solid Pb
• What’s happening to the [Al+3]?
Increasing
• What’s happening to the [Pb+2]?
Decreasing
Notation for Cells
Application: Batteries
Dry Cell
Mercury battery