Electrolytic Cell and Electroplating

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Electrolytic Cell and Electroplating

• Chapter 19 Page 776-781
• Chem 12

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• 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.
• Molten (liquid) state of a pure substance is the
  medium which the cell is made. Ex molten NaCl

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• Oxidation at anode
• 2Cl- ?Cl2 2e-
• Reduction at cathode
• Na 1e- ?Na

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• 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.

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• 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.

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• 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.

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• 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.

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Electroplating
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• 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)

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• 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).

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Examples of electroplating

• Coating jewlery with thin layer of expensive
  metal.
• Coating chromium over steel to make rust
  resistant.

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SAMPLE PROBLEM 1

• A spoon is to be plated with silver, Ag.
• Identify the anode and cathode.
• Write an equation for the reaction taking place
  at the anode and at the cathode and indicate
  whether it is oxidation or reduction.
• What electrolyte is used?

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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)

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• 3. Electrolyte solution is made of AgNO3

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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?