Chapter A2
2.4 – Voltaic Cells
Voltaic cells
• the focus on metals so far been been what
happening on the surface of the metal
– tarnishing/ rusting, (aka color change), precipitate
formation or bubbles/ gas formation
O The rest of the chapter focuses on WHY
this occurs.
O a focus on the movement of electrons
between the two metals
Voltaic cells
• though commonly a voltaic
cell is referred to as a
“battery”, technically cells
are only referred to as a
battery when several are
together
– when an electronic device
is operating, voltaic cells
provide a continuous flow
(current) of electrons,
which is converted into
current to power the device
Voltaic cells
• the voltaic cell that you know
looks like this:
• this is the version of the voltaic
cell we will make in the lab.
Voltaic cells
• an electrode is a solid piece of metal that
is suspended in a solution (of the same
metal ions as the electrode) and connected
to an external circuit.
Voltaic cells
• the electrode zinc, is immersed into an
electrolyte solution, where the zinc
electrode acquires an excess of electrons,
becoming negatively charged
Voltaic cells
• the other electrode is usually composed of a
different material (copper) and will become
positively charged
Voltaic cells
• once a circuit is closed between the two
electrodes, the electrons will repel from the
negative zinc electrode, pass through the
circuit and flow through to the positive
electrode
Voltaic cells
• the reaction will continue until the negative
electrode can no longer be supply electrons.
Voltaic cells
Voltaic cells
• a salt bridge is a glass U-shaped tube that is
filled with an ionic solution
– this is to allow for free flow of electrons from one
solution to the other
How the cell works:
• because it is the more
reactive of the two
metals, the zinc
electrode will become
oxidized, (lose electrons)
• these electrons will
travel from the
electrode, through a
metal wire, and then
into an electronic device
– the device; a voltmeter,
measures the quantity of
electrons passing through
it (= amount of
electricity)
How the cell works:
• the electrons will pass
through the device,
back through another
wire, into the copper
electrode
– these electrons will be
attracted to the Cu2+(aq)
ions in the solution and
they will be reduced.
• over time, the zinc
electrode shrinks in size
(as Zn  Zn2+) and the
copper electrode grows
(Cu2+  Cu)
How the cell works:
– if the two solutions
were NOT connected,
the zinc would run out
of electrons and the
cell would stop working
• the solution in the
salt bridge allows a
continuous flow of
electrons back into
the zinc solution
How the cell works:
Analyzing a
voltaic cell:
Step #1: identify the electrode
where oxidation occurs
• locate the two metals on the
activity series (right side)
– the metal closer to the
BOTTOM will be OXIDIZED =
reducing agent
– the electrode that is oxidized
is called the anode
– the other electrode is
reduced, and is called the
cathode
Analyzing a
voltaic cell:
Step #2: describe the oxidation process
in the anode
• write the oxidation half-reaction
– Eg. Pb(s)  Pb2+(aq) +2e-
• electrons leave the anode and
travel to the external circuit
running the electronic device
– the voltmeter measures the quantity
of electrons (amount of electricity)
being produced
• because the anode is the
electrode where the electrons
originate, it is considered the
negative electrode
Analyzing a
voltaic cell:
Step #3: describe the reduction
process in the anode
• the electrons travel through the
voltmeter and into the cathode
• the electrons are attracted to the
positively-charge metal ions in the
cathode solution
• the cathode ions will unite with
the electrons and form a solid
metal, which is deposited on the
electrode
– Eg. Ag+ (aq) + e-  Ag (s)
Analyzing a voltaic cell:
Step #4: describe how the salt bridge completes the circuit
• all electrical circuits require a complete circuit in
order to function.
• the salt bridge connects the cathode back to the
anode to replenish the electrons on the anode side
– the salt bridge contains a third ionic solution
– the positive ions from the salt bridge solution will be
attracted to the cathode, while the negative ions from
the salt bridge solution will migrate toward the anode.
Voltaic Cell- Example
• in this voltaic cell:
anode
– zinc is the
_________ – it is
oxidized
cathode
– copper is the
__________– it
is reduced
– the solution in the salt bridge
KCl
is _____________ (aq)
– chloride ions are a spectator
ion – their job is to replenish
the electron supply at the
anode
Cell Notation
•
voltaic cells can also be represented using short hand cell notation
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
anode
salt bridge
cathode
 oxidation
reduction
• the anode is listed on the left, the cathode on the right (think
alphabetical order)
– the vertical line | represents a boundary between a metal and its solution
– the double line || represents the salt bridge
Practice Problem #1:
a) Draw a voltaic cell using the following supplies:
–
–
–
–
–
two beakers
U-tube & cotton balls
wire & voltmeter
tin and magnesium strips
solutions of SnSO4(aq), MgSO4(aq), and NaNO3(aq)
b) Label the direction of e- flow, the anode, cathode, OA, RA, - and +
electrodes, voltmeter and salt bridge
c) Write the short hand cell notation
Practice Problem #1 (Solution):
• Practice problem
(page 87)
– 34
• Practice problems
(page 91)
– 37 & 39
• Practice problem (page
92)
Zn(s) / Zn2+(aq) // Ni2+ (aq) / Ni(s)
a)Identify which
metal will be
oxidized and
which would be
reduced (1 mark)
a) According to the activity series,
zinc is the more reactive metalso Zn(s) is oxidized and the
Ni2+(aq) are reduced.
a) Anode = Oxidation= zinc metal
Cathode = Reduction = nickel
metal
a)Identify the anode a) Oxidation: Zn(s) Zn2+(aq) +
2e–
and the cathode (1
Reduction: Ni2+(aq) + 2e– 
mark)
Ni(s)
Zn(s) / Zn2+(aq) // Ni2+ (aq) / Ni(s)
d) Draw the voltaic cell (2 marks)
Label the direction of the electron flow (1 mark)
and the anions of the salt bridge. (1 mark)
Cell must have 2 beakers, (one
with Zn(s) and Zn2+(aq), one
with Ni(s) and Ni 2+ (aq)),
connected by wires to a
voltmeter, and a salt bridge
 Electrons must leave the Zn
anode and flow towards the
Ni cathode
 The negative NO3- anions are
attracted to the anode. The
positive K+ cations are
attracted to the cathode
(think alphabetical…AnionAnode. Cation= Cathode)
?
?
• Complete the
pre-lab
assignment for
the Voltaic Cells
Lab
– Your pre-lab
MUST BE done,
in order to
Chapter A2
2.5 – Electrolytic Cells
Electrolytic vs. Voltaic
• an electrolytic cell is a system where a nonspontaneous redox reaction is forced to occur
– a reaction that is non-spontaneous will only occur if
energy is added
– in an electrolytic cell, energy is added in the form of
electricity
spontaneous?
requires energy?
produces voltage?
use
change in energy
Voltaic
yes
no
yes
energy source
exothermic
Electrolytic
no
yes
no
electroplating
endothermic
Electroplating
• metals, like gold and silver,
that are the most stable and
corrosion-resistant are also
the most expensive
– to manufacture a metal object
that is resistant to corrosion it
would NOT be cost-effective to
make the whole thing out of
gold
– instead, a thin coating of gold is
applied to the surface of a more
affordable metal
Electroplating
•
the object to be coated is submerged
in a solution of the metal ions (e.g.
silver ions for objects that are to be
coated in silver metal)
• an external energy source (a battery)
supplies energy forcing electrons to
flow into the object
– the negatively charged electrons
will attract the positively
charged metal ions from the
solution, and turn them back
into metal atoms, which will
Electrolytic cells
• Step #1:
– electrons from the
plating (the
expensive) metal
cathode are
attracted to the +
electrode of the
power source
– by removing
electrons from the
metal atoms, ions
Electrolytic cells
• Step #2:
– once removed from
the metal, the free
electrons flow
through the power
source.
• Step #3:
– electrons are forced
out of the - end of
Electrolytic cells
• Step #4:
– positive Au+(aq) from
the solution are
attracted to the
negative electrons in
the object to be
plated
– the Au+(aq)ions gain
the electrons, and
turn back into solid
gold coating the
Electroplating
• electroplating is a good way to protect
metals that are easily oxidized, like iron
– metals that work as good electroplaters
(coatings) are chromium (aka chrome),
platinum, silver and gold
Gold jewelry
 two
types of gold jewelry exist - that which is made
out of solid gold, and that which is gold plated
• solid gold
• gold plated
– karats - pure gold is 24K – if you have a piece
– gold is a soft metal, so it
of gold plated
is often combined with
jewelry, care must
other metals like brass
be taken to avoid
(copper and zinc) and
any deep scratches
nickel to make it more
• deep scratches will
durable
expose the
– the number of karats in
oxidizable metal
the gold refers to how
Other uses for electrolytic cells
• refining metals
– a sample of impure metal (anode), pure metal
(cathode)
– ions of the pure metal will travel from the anode to
the cathode to build up the atoms of pure metal
• electrolysis
– decomposition of a compound by means of an electric
current
– e.g. electrolysis of water makes it decompose into O2
and H2
Other uses for electrolytic cells
• producing non-metals
– non-metals, especially the halogens, are difficult to
obtain in pure form because they are so reactive
– non-metal atoms will accumulate around the anode of
an electrolytic cell
• recharging voltaic cells
– when you use a battery recharger, you are using an
electrolytic cell to reverse the process that occurs
normally in the voltaic cell
– you are literally re-charging the voltaic cell with a new supply of
electrons
• Complete the Voltaic
& Electrolytic cells
Worksheet