Download Electrolytic Cell and Electroplating

Unit 2: Electrochemistry
Electrolysis
Electrolytic Cells
• The term “electrochemical cell” is often used to refer to a:
• Electrolytic cell – one with a nonspontaneous reaction
SOA below SRA – i.e. zinc sulfate and lead solid cell
Eocell less than zero= nonspontaneous
vs
• Galvanic Cell – one with a spontaneous reaction
SOA over SRA on the activity series
Eocell greater than zero = spontaneous
• Why would anyone be interested in a cell that is not spontaneous?
• This would certainly not a good battery choice, but by supplying electrical energy to a
nonspontaneous cell, we can force this reaction to occur.
• This is especially useful for producing substances, particularly elements. I.e. the zinc
sulfate cell discussed above is similar to the cell used in the
industrial
production of zinc metal.
• An electrolytic cell is a device in which an external source of
electrons (electrical circuit) is used to make a non-spontaneous redox
reaction take place.
Electrolysis of water
• Anode:
2H2O→ O2 + 4H+ + 4 e-
• Cathode: 4H2O
+ 4e- → 2H2 + 4OH-
• 6H2O → 2H2 + O2 + [4 H+ + 4OH-]
2H2O → 2H2 + O2
Electrolytic Cells
• Electrolytic Cell –.
• The external power source acts as an “electron pump”; the
electric energy is used to do work on the electrons to cause
an electron transfer
Electrons are pulled from the
anode and pushed to the
cathode by the battery or power
supply
Molten (liquid) state of a pure substance can be the medium of the cell
Eg: molten NaCl
• Oxidation at anode
2Cl- Cl2 + 2e• Reduction at cathode
Na+ + 1e- Na
• 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.
Electrolysis of Molten NaCl (Down’s cell)
• (anode):
2 Cl- → Cl2 + 2 e• (cathode):
2Na+ + 2e- → 2Na
2
NaCl
(l)
• 2 NaCl(l) → 2 Na(l) + Cl2(g)
Molten NaCl Electrolytic Cell
cathode half-cell (-)
REDUCTION
Na+ + e-  Na
anode half-cell (+)
OXIDATION
2Cl-  Cl2 + 2e-
overall cell reaction
2Na+ + 2Cl-  2Na + Cl2
Non-spontaneous reaction!
X2
What chemical species would be present in
a vessel of aqueous sodium chloride, NaCl
(aq)?
Na+
Cl-
H2O
Will the half-cell reactions be the same or different?
Aqueous NaCl Electrolytic Cell
possible cathode half-cells (-)
REDUCTION
Na+ + e-  Na
2H20 + 2e-  H2 + 2OHpossible anode half-cells (+)
OXIDATION
2Cl-  Cl2 + 2e2H2O  O2 + 4H+ + 4eoverall cell reaction
2H2O → 2H2 + O2
Procedure for Analyzing Electrolytic Cells
• Use the redox table to identify the SOA and SRA
• Don’t forget to consider water for aqueous electrolytes.
• Write equations for the reduction (cathode) and oxidation (anode) halfreactions. Include the reduction potentials if required.
• Balance the electrons and write the net cell reaction including the cell
potential. E0 cell = E0r cathode - E0r anode
• If required, state the minimum electric potential (voltage)
force the reaction to occur. (The minimum voltage is
absolute value of E0 cell)
• If a diagram is requested, use the general outline in
and add specific labels for chemical entities.
to
the
Figure 6,
Analyzing Electrolytic Cells #1
• Example: What are the cell reactions and the cell potential of the aqueous
potassium iodide electrolytic cell?
• Identify major entities and identify the SOA and SRA.
• Write the half-reaction equations and calculate the cell potential.
• State the minimum electric potential (voltage) to force the reaction to occur.
Electrons must by supplied with a
minimum of +1.37 V from an external
battery or other power supply to
force the cell reactions.
Potassium-Iodide Electrolytic Cell
• In the potassium iodide electrolytic cell, litmus
paper does not change colour in the initial solution
and turns blue only near the electrode from which
gas bubbles. Why?
• At the other electrode, a yellow-brown colour and a
dark precipitate forms. The yellow brown substance
produces a purplish-red colour in the halogen test
(pg. 805). Why?
Analyzing Electrolytic Cells #2
• Example: An electrolytic cell containing cobalt(II) chloride solution and lead
electrodes is assembled. The notation for the cell is as follows:
a)Predict the reactions at the cathode and anode, and in the overall cell.
b)Draw and label a cell diagram for this electrolytic cell, including the power supply.
c)What minimum voltage must be applied to make this cell work?
Analyzing Electrolytic Cells #3
• Example: An electrolytic cell is set up with a power supply connected to two nickel
electrodes immersed in an aqueous solution containing cadmium nitrate and zinc nitrate.
• Predict the equations for the initial reaction at each electrode and the net cell reaction.
Calculate the minimum voltage that must be applied to make the reaction occur.
Electrolytic Cells
• Summary:
• An electrolytic cell is based upon a reaction that is nonspontaneous; the
Eocell for the reaction is negative.
An applied voltage of at least the absolute value of Eocell is required to
force the reactions to occur.
• The SOA undergoes reduction at the cathode (- electrode)
• The SRA undergoes oxidation at the anode (+ electrode)
• Electrons are forced by a power supply to travel from the anode to the
cathode through the external circuit.
• Internally, anions move toward the anode and cations move toward the
cathode
Electroplating
• Electroplating is a procedure that uses electrolysis to apply a thin
layer of a metal over the surface of another metal.
• 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)