or Galvanic Cells
A galvanic/voltaic cell is the same as the electric cells we
have looked at so far, but the two electrodes are in two
different electrolytes.
They split up an electric cell into two ½ reactions so they
can examine each reaction (oxidation/reduction) more
closely.
The two “½ cells” are connected together by a “salt
bridge”.
There are still two metal electrodes which need to be
connected by an external (wire) for the cell to function
Copper-Zinc Voltaic Cell
Cu(s) Cu(NO3)2(aq
)
Zn(NO3)2(aq Zn(s)
)
”Shorthand Notation”
First Electrode(s)
= Phase Change
(s)-(aq)
Electrolyte(aq)
Electrolyte(aq)
Second Electrode(s)
= Salt Bridge
Describing Voltaic Cells
So far we can describe how a voltaic cell has:
2 Electrodes (solid poles).
2 Electrolytes….related to the electrode.
Two electrodes are connect together with a wire.
The two beakers (solutions) are connected together by a
“salt bridge”.
But how do we know which pole is the anode and which
is the cathode?
Describing Voltaic Cells
Well, SOA undergoes Reduction at the Cathode.
The SRA undergoes Oxidation at the Anode.
Well that’s useless information….how can we tell where
oxidation and reduction are happening?
What’s the SRA?
What’s the SOA?
To determine SOA and SRA the process is the same as
when you predicted redox reactions last unit.
“The Ox and The Cat”
Easiest way to remember this stuff is the following
analogy.
n
SRA
“ An Ox”
n x
o i
d d
e a
t
i
o
n
o e
it d
c o
u h
d t
e a
“Red Cat”
SOA
Determining The SRA/SOA
Lets Try One
***Remember! The Na+ and NO3- are part of the salt bridge and are
spectators……so don’t list them as species present.
Determining The SRA/SOA
Determining The SRA/SOA
Anode
Cathode
SOA undergoes Reduction at the cathode (+V).
The SRA undergoes Oxidation at the anode (-V).
Cathode
Lets Try One
Anode
Voltaic Cell Summary
Inert Electrodes
Usually the electrodes (solids) used in a voltaic cell are
related to the electrolytes used.
Exp:
Cu(s) for Cu2+(aq) Electrolyte
Ag(s) for Ag+(aq) Electrolyte
Cu(s) for Cu(NO3)2(aq) Electrolyte
Zn(s) for Zn(NO3)2(aq) Electrolyte
But there are powerful oxidizing and Reducing agents
that aren’t related to any solid form…what electrode do
you use then?
Inert Electrodes
Strong oxidizing agents such as Manganate (MnO4-(aq)) or
Chromate (Cr2O72-(aq ))which react in acidic environments
(with H+) cannot form permanently solid
(insoluble/Do not conduct electricity) electrodes.
The reason why so often the electrolyte and electrode are
related is because the electrolyte ions reduce at the same
rate as the solid electrode oxidizes.
1:1
2:2
3:3
Ratio between the electrons transferred.
The most common inert electrode is C(s).
C(s) = Carbon….also called Graphite
Inert Electrodes C(s).
Inert electrodes DO NOT REACT with the electrolyte.
Inert electrodes are 100% spectator species.
Inert electrodes only purpose is to provide an attachment for an wire to be
connected to each pole so e- can flow between them.
Standard
Cell
Potentials
Voltaic Cell
Standard Cell Potential (E°) is the maximum electric
potential difference between the cathode and the anode.
E° specifies that the cell is under specific SATP
conditions and the electrolytes each have an EXACT
concentration of 1.0 mol/L.
Electrodes
Electrolytes
Calculating Standard Reduction
Potentials
We know the Standard Cell potentials of all the possible
oxidation and reduction reactions.
Hint: All those numbers down the side of the
Reduction table in your data booklet, yah, that’s them.
Standard REDUCTION potentials are the difference
between the cathode’s Standard Cell Potential and the anode’s
SCP.
FORMULA:
Calculating Standard Reduction
Potentials
So we know the formula for calculating the standard
reduction potential of a cell:
But how do they get these
+ and – numbers on the
table in the book???
There isn’t some magical machine that measures these
values. So where do they come from?
***THEY COMPARE ALL OTHER OA’s AND RA’s TO
HYDROGEN AS A ZERO MARKER.***
They just use Hydrogen=Zero as a reference point and
rate their E° values relative (stronger/weaker) to it.
SAME AS WHEN YOU MADE A REDOX TABLE EARLIER.
Calculating Standard Cell
Potentials (Easy)
You have to write
the standard cell
notation.
2. Find the
SRA(anode1/2) and
SOA(cathode1/2).
3. Use the Standard
Reduction
Potential Formula
to calculate the
number.
1.
**Inert SRA
Electrode**
Do we have to
write it?
NO.
SOA
Calculating Standard Cell
Potentials (Hard)
You have to write
the standard cell
notation.
2. Find the
SRA(anode1/2) and
SOA(cathode1/2).
3. Use the Standard
Reduction
Potential Formula
to calculate the
number.
1.
SOA
SRA
Zn2+(aq)
Cu2+(aq)
0.34 V – (-0.76 V)
+ 1.1 V
Lets Practice What We Know
A Combined Question
SRA
SOA
In order to write the equations of what reaction is happening at the
cathode and anode we need to find out the SRA and SOA.
So……..?
Page 7: “Reduction/Standard Electrode Potentials” Table.
Lets Practice What We Know
A Combined Question
SRA
SOA
As soon as you have found the SOA and SRA, you list
their half-reactions Exactly as they appear in the table.
Sn2+(aq) + 2e-  Sn(s)
SRA: Cr(S)  Cr2+(aq) + 2eSOA:
Cathode
Anode
Sn2+(aq) + Cr(S) Sn(s) + Cr2+(aq)
-0.14 V – (-0.91 V)
+ 0.77 V
Corrosion Case Study
(P.634-636)