Transcript Electrolytic Cell and Electroplating

Electrolytic Cell and
Electroplating
Chapter 19 Page 776-781
Chem 12
• An electrolytic cell is a device in which
an external source of electrons (electrical
circuit) is used to make a nonspontaneous redox reaction take place.
• Molten (liquid) state of a pure substance is
the medium which the cell is made. Ex:
molten NaCl
• Oxidation at anode
2Cl- Cl2 + 2e• Reduction at
cathode
Na+ + 1e- Na
NOTE:
The cathode is still under going reduction, but now
is negative.
The anode is still under going oxidation, but now is
positive.
REASON:
• The electrodes are connected to a power source
that pump e- from negative to positive.
• The electrode that is connected to the power
source is the cathode.
• Since electrons are being forced into the
cathode it becomes negative.
• An example of an
electrolytic cell is
when molten NaCl is
electrolyzed to form
liquid sodium and
chlorine gas. The
sodium ions migrate
toward the cathode,
where they are
reduced to sodium
metal. Similarly,
chloride ions migrate
to the anode and are
oxided to form
chlorine gas.
• E0net
= E0ox + E0red
= (-1.36 V) + (-2.71 V)
= -4.07 V
• The net cell voltage of -4.07
• Because it is a non-spontaneous reaction the
E0net is always less than zero.
• In the electrolytic cells using molten ionic
compounds, the negative ions will be
oxidized.
• The two electrodes used in an electrolytic
cell do not take part in any reaction. They
are non-reactive. Ex: carbon and platinum.
Electroplating
• Electroplating is a procedure that uses
electrolysis to apply a thin layer of a metal
over the surface of another metal.
• Electrolysis is the process of making a
non-spontaneous chemical redox reaction
occur by passing electricity through a
substance. (electrolytic cell)
• In electroplating, the anode is made up of
the metal you want to coat the surface of
another metal with.
• There is also a salt solution present of the
anode metal.
• While electrolysis is taking place, the
anode metal is oxidized and goes into
solution as positive ions.
• These positive ions are then reduced on
the surface of the cathode (the metal you
wish to coat).
Examples of electroplating
• Coating jewlery with
thin layer of
expensive metal.
• Coating chromium
over steel to make
rust resistant.
SAMPLE PROBLEM 1
A spoon is to be plated
with silver, Ag.
1. Identify the anode
and cathode.
2. Write an equation for
the reaction taking
place at the anode
and at the cathode
and indicate whether
it is oxidation or
reduction.
3. What electrolyte is
used?
ANSWER
1. Cathode = spoon (metal object to be coated),
Anode = silver electrode
2. As electrolysis takes place, the silver anode is
oxidized,
Ag(s) → Ag+(aq) + 1e• The Ag+ (aq) ions in solution travel to the spoon
cathode and are reduced to form neutral Ag(s)
on the surface of the spoon (cathode):
Ag+ (aq) + 1e- → Ag(s)
3. Electrolyte solution is made of AgNO3
SAMPLE PROBLEM 2
A knife to be used as an eating utensil is to be
plated with gold, Au.
a) Sketch and label a diagram of the apparatus
used to electroplate the knife with gold.
b) Identify the anode and cathode.
c) Write an equation for the reaction taking place
at the anode and at the cathode and indicate
whether it is oxidation or reduction.
d) What electrolyte is used?