Electroplating, or electrodeposition, is the process of applying a layer
of metal to the surface of another metal. It is a variation on the theme
of electrolysis. There are many applications of this process in use in
the modern world.
Syllabus reference R3.2.16
Reactivity 3.2.16 - Electroplating involves the electrolytic coating of an object with a metallic thin layer. (HL)
- Deduce equations for the electrode reactions during electroplating.
Tools and links
- Tool 1 - How is an electrolytic cell used for electroplating?
The purification of copper
The participating electrode effect discussed in the previous section can be put to good use in the purification
of copper. Impure copper can be used as the anode in an electrolysis cell and
pure copper gets deposited at the cathode. The concentration of copper ions
in the electrolyte remains unchanged.
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The reaction at the anode:
Cu(s) - 2e → Cu2+(aq)
Reaction at the cathode
Cu2+(aq) + 2e → Cu(s)
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Electroplating
Any metal lower than hydrogen in the electrochemical series, i.e. with a positive
electrode potential, can be deposited at the cathode during electrolysis. This
means that these metals can be used to electroplate other substances. The requirements
for successful electroplating are quite stringent in that the deposit of metal
often fails to adhere successfully to the material of the cathode, unless the
conditions are optimised.
Suitable electroplating metals include:
- Copper
- Chromium
- Nickel
- Silver
Chrome plating
Chromium is used in many areas as a decorative coating due to its bright silver
colour and shine. There are two electrolytic processes used for chrome plating,
which differ only in the electrolyte used. There are advantages and disadvantages
in both processes.
In the hexavalent chromium process, the electrolyte used is chromium(VI) oxide
dissolved in sulfuric acid. This produces a highly acidic and toxic mixture.
In the trivalent chromium process the electrolyte is chromium(III) sulfate or
chromium(III) chloride.
Both processes involve three phases:
- 1 Activation
- 2 Chromium electroplating
- 3 Rinsing
The activation stage involves etching and cleaning the surface of the metal
to be electroplated, so that the layer of chromium adheres tightly to the surface.
It typically takes place in a bath of chromic acid, using reverse current to
remove surface impurities.
The electrolytic
stage the current and temperature are carefully controlled to give the coating
characteristics required. In the chromium(III) process additives must be used
to prevent oxidation taking place at the anode.
The main disadvantages of the hexavalent process is its cost and toxicity (The
film Erin Brockovich is a true story about a woman pursuing compensation from
a company that polluted the land with hexavalent chromium causing health problems
and morbidity in the local population). The final product is not toxic, as the
rinsing procedure leaves only pure chromium.
The disadvantage of the trivalent process is that the colour of the chrome
plate is not always up to customer expectations. However, this problem has been
resolved by the use of certain additives. The plating is generally more even
than the hexavalent process.
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Worked examples
Q973-01 Electroplating processes
usually involve depositing a metal from a solution of some salt, or ion of the
metal. This deposition occurs
- at the cathode of the cell.
- at the anode of the cell.
- when the metal is oxidized.
- when electrons are lost by the metal.
Answer
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Metal ions always have a positive charge and deposit on the negative
electrode, the cathode in electrolysis.
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Q973-02 In the electrolytic
refining of copper, an electric current is passed through a cell containing a
pure copper electrode, an electrode of impure copper, and an aqueous solution
containing copper cations. As metallic copper is removed from the impure electrode
and redeposited on the pure copper electrode other metallic impurities are left
behind, if the potential of the cell is adjusted properly. In this process, the
impure copper electrode is called the..?
- anode.
- cathode.
- reference electrode.
- indicating electrode.
Answer
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The process occurring at the impure copper electrode is:
This occurs at the anode.
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Q973-03 A metallic object is
electroplated with copper using a solution of copper(II) sulfate. Which statement
is correct?
- The positive electrode increases in mass
- The concentration of Cu2+ ions in the solution decreases
- Reduction occurs at the positive electrode
- The reaction occurring at the negative electrode is Cu2+ + 2e
→ Cu
Answer
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The positive electrode decreases in mass as copper ions leave into
the solution. The concentration of copper ions in solutionremains the
same as ions are being added and removed at the same rate. The positive
electrode pulls electrons off the atoms according to the equation: Cu(s)
- 2e → Cu2+(aq)
The reaction occurring at the negative electrode is
Cu2+ + 2e → Cu
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Q973-04 Calculate the mass of
pure copper deposited at the cathode during the purification of an impure copper
anode if 10A are passed through a solution of copper(II) sulfate(aq) for 1 hour.
[Cu = 63.5]
Answer
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Charge passed = It = 10 x 60 x 60 = 36,000 Coulombs
Faradays of charge = 36,000/96,500 = 0.373 F = 0.373 moles of electrons
Equation for the deposition: Cu2+(aq) + 2e → Cu(s)
Hence 0.373 moles of electrons deposits 0.373/2 moles of copper = 0.187
moles Cu
Therefore mass of Cu = 0.187 x 63.5 = 11.8
g
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Q973-05 Two copper strips X
and Y are placed in a solution of copper(II) sulfate and electrolysed for a certain
time. X is attached to the negative terminal of the battery and Y to the positive
terminal. X is then removed, dried and weighed. State and explain what would happen
to the mass of X. [3]
Give two ways in which the change in mass of X could be increased. [2]
Answer
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At the negative electrode copper is deposited according to the equation:
so the mass of the cathode increases.
The mass could be increased by electrolysing for longer, or by using
a higher current.
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Q973-06 Electroplating may be
used to coat one metal with another metal.
a) Identify the three factors affecting the amount of metal discharged during
electroplating [3]
b) Explain why electrolysis of zinc sulfate solution is not used for coating
with zinc metal. [2]
Answer
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a) The three factors are current, time and charge on the metal ion.
b) zinc has a redox potential of -0.76 V. The electrode potential of
hydrogen is 0 V, therefore hydrogen gas is preferentially released instead
of zinc in aqueous solution.
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Q973-07 Electrolysis of an aqueous
solution of copper(II) sulfate can be carried out using copper electrodes instead
of inert electrodes. Describe the difference in the observations at each electrode
and the electrolyte in the two cases.
Answer
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The anode undergoes the following reaction:
This means that the mass of the anode decreases as copper metal turns
to copper ions in solution
The cathode undergoes the following reaction:
This means that the mass of the cathode increases as copper metal is
deposited.
In the solution, the copper ions are created at the anode, but they
are used up at the cathode. Overall there is no change in concentration
of copper ions in solution.
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Q973-08 A current of 0.2 amperes
passing for 5 hours through a solution of gold ions deposits a mass of 1.45g of
gold at the cathode. Calculate the charge on the gold ions. [A
r of
gold = 197]
Answer
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Faradays of charge = Coulombs/96500 = 0.0373 moles of electrons.
Moles of gold deposited = mass/RAM = 1.45/197 = 0.0124 moles
Thus 0.0373 moles of electrons deposit 0.0124 moles of gold, i.e. 1
mole of gold requires 3 moles of electrons.
Therefore the charge on a gold ion = 3+
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Q973-09 Calculate the mass of
copper deposited on a copper cathode by electrolysis of copper(II) sulfate solution,
if the same current deposited 0.9g of silver from silver nitrate solution in the
same period of time. [Ag = 108 Cu = 63.5]
Answer
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0.9g of silver represents 0.9/108 moles = 0.00833 moles
Silver gets deposited according to the equation:
However, copper requires twice as many moles of electrons for the same
moles of copper:
Hence, moles of copper produced = 0.00833/2 = 0.00417 moles
Mass = moles x RAM = 0.00417 x 63.5 = 0.265g
(3 sig figs)
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Q973-10 A current of 0.75 amperes
passes through 250cm
3 of 0.25 mol dm
-3 copper(II) sulfate
solution. Calculate how long it will take for all of the copper to be deposited
at the cathode.
Answer
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250cm3 of 0.25 mol dm-3 copper(II) sulfate solution
contains 0.25 x 0.25 moles of copper = 0.0625 moles
Copper deposition occurs according to the equation:
1 mole of copper is produced per 2 moles of electrons.
Hence 0.0625 moles of copper requires 0.0625 x 2 moles of electrons
(Faradays) = 0.125 F
0.125 Faradays = 12063 Coulombs = current x time (seconds)
Thus time required = 12061.5/0.75 = 16083 s = 268 mins 33s =
4 hours 28 mins 33 seconds
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