Electrochemistry
Part II: The Galvanic Cell
Jespersen Chap. 20 Sec 1
Dr. C. Yau
Fall 2014
1
What is a Galvanic Cell?
• A galvanic cell is a spontaneous
electrochemical cell in which electricity is
produced by a spontaneous redox
reaction.
• The resulting electron transfer is forced to
take place through a wire.
• It is also known as a voltaic cell.
• Do not confuse it with the electrolytic cell
(discussed later) where an electrochemical
reaction is forced to take place by passing
electricity through the cell.
2
Comparison of Types of Cell
Galvanic or Voltaic Cell:
Spontaneous electrochemical rxn
Electricity produced
Electrolytic Cell:
Non-spontaneous electrochemical rxn
Electric current is passed thru a wire to
force the reaction to take place.
3
Anatomy Of A
Galvanic Cell
e-
e-
Note: Textbk is
inconsistent with
cathode
which is on the left anode
(cathode or anode).
There is no
convention which is
on the left side in a
sketch of this sort.
Zn  Zn2+ + 2 e−
Cu2+ (aq) + 2e− Cu (s)
• Half-cells (compartments containing reactants for each halfreaction)
• Electrodes to conduct current through the solution.
• Salt bridge to allow ion movement to keep solns neutral.
• Supporting electrolyte (spectator ions: NO3-)
4
• Connecting external circuit (wire and voltmeter)
Electrochemical Cells
• In all cells, electrons transfer between the
cathode (the reduction half-cell) and the
anode (the oxidation half-cell)
REMEMBER! "Red-Cat and An-Ox”
Reduction at the Cathode
& Oxidation at the Anode
5
A closer look at the electrodes:
Cu
cathode
Zn anode
Zn2+
Zn2+
Cu2+
Zn
Zn2+
Cu2+
Zn2+
Zn2+
Oxidation of Zn to
Leaves e- behind on the
electrode (soln becomes
more positive)
Cu
Cu2+
Cu2+
Zn2+
Cu2+
Cu2+
Cu2+
Reduction of Cu2+ to Cu
e- extracted from electrode
(soln becomes
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more negative)
Electrochemical Cells
• Electrical current is conducted via the
movement of electrons and ions.
• To prevent charge buildup, a salt bridge
allows ions to move between the cells.
REMEMBER!
• Electrons flow from anode to cathode
through the wire.
a to c
• Cations move towards the cathode.
• Anions move towards the anode.
• Red-Cat and An-Ox.
7
KCl or KNO3
Zn
Cu
Zn2+
There is a buildup of what charges at each cell?
The salt bridge often made of KCl or KNO3
(unreactive ions – spectator ions)
What ions in the salt bridge move to which cell?
8
Towards which compartment will electrons flow in
an electrochemical cell?
A. Toward the cathode
anode to cathode
B. Toward the anode
C. It depends on the reaction
Through which components of the cell will ions not
flow?
A. The electrodes
B. The solution
C. The salt bridge
http://www.mhhe.com/physsci/chemistry/essentialc
hemistry/flash/galvan5.swf
9
Standard Cell Notation (Line Cell Notation)
Rxn at anode salt bridge Rxn at cathode
Zn (s) | Zn2+ (aq) || Cu2+ (aq) | Cu (s)
anode
anode
electrode electrolyte
cathode
electrolyte
cathode
electrode
• Cell reactions separated by || that represents the salt
bridge with ANODE on left, CATHODE on right.
• Electrodes appear at the outsides
• Reaction electrolytes in inner section
• Phases (phys. States) separated with |
• Species in the same state separated with ;
• Concentrations shown in ( )
10
Standard Cell Notation (Line Cell Notation)
Rxn at
Rxn at
Write the half reactions for the galvanic cell
shown above.
Cu (s)
Cu2+ (aq) + 2eAg+ (aq) + eAg (s)
Make a sketch of the galvanic cell and label
it fully.
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Now, consider the reaction of
Al3+(aq) + Zn (s)
Al(s) + Zn2+(aq)
Write the half-reactions.
Balance the electrons and write the
balanced net ionic equation.
Sketch the galvanic cell (electrochemical
cell). Label it fully.
Write the standard cell notation.
Do Prac Exer 1 & 2 on p. 924
12
Given:
Mg(s) | Mg2+(aq) || Sn2+(aq) | Sn(s)
Sketch the galvanic cell corresponding to
this standard cell notation. Label it fully.
Practice with p. 969 #20.50, 20.51
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Now, consider the reaction of
Fe3+ + Zn
Fe2+ + Zn2+
Write the half-reactions.
Balance the electrons and write the
balanced net ionic equation.
Write the standard cell notation.
Sketch the galvanic cell (electrochemical
cell). Label it fully.
How can you have an electrode that is an
ion (such as Fe3+)?
14
Where there are no conductive metals
involved in a process, an inert electrode is
used. C(gr) and Pt are often used.
2Fe3+ + Zn
Zn (s)
2Fe2+ + Zn2+
Zn2+(aq)
Fe3+(aq)
Fe2+(aq)
Zn(s) |Zn2+(aq) || Fe3+(aq); Fe2+(aq)|Pt(s)
Zn
anode
inert
cathode
(where Fe3+ reduces to Fe2+
at the surface of the Pt electrode) 15
Balance and identify the cathode and anode
H2O2(aq) + CO2(g) → H2C2O4(aq) + O2(g) (acidic)
H2O2(aq)
→ O2(g) +2H+ + 2eoxid
2H+ + 2e- + 2CO2(g) → H2C2O4(aq) + (acidic)reduc
H2O2(aq) + 2CO2(g) → H2C2O4(aq) + O2(g) (acidic)
Which is at the cathode? At the anode?
16
Write Line Notation for the cell:
H2O2(aq) + CO2(g) → H2C2O4(aq) + O2(g) (acidic)
oxidation
reduction
Standard Cell notation for the reaction:
C(gr)| H2O2(aq) ;H+|O2(g)||CO2(g)|H2C2O4(aq); H+|C(gr)
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Balance and identify the cathode and anode
CrO3(s) + MnO2(s)→MnO4-(aq) + Cr3+(aq) (basic)
CrO3(s) + 3H2O(l) + 3e- → Cr3+(aq) +6OH-(aq)
MnO2(s) + 4OH- → MnO4-(aq) + 2H2O + 3e-
CrO3(s) + MnO2(s) + H2O(l) →MnO4-(aq) + Cr3+(aq) + 2OH-(aq)
Balancing redox equations by half-reaction method
is given in Sec 6.2 (p. 222)
18
Galvanic Cells without Metal
Electrodes
Equation from previous slide:
CrO3(s)+ MnO2(s) + H2O(l) →MnO4-(aq)+Cr3+(aq) + 2OH-(aq)
Write the Standard Cell Notation:
C(gr);MnO2(s)|MnO4-(aq)||CrO3(s)|Cr3+(aq);OH-|C(gr)
p. 970 #20.52
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