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elecrolytic cells IEB grade 12 summary
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Grade 12 Physical Science Electrochemistry Notes 3The Electrolytic Cell – converting electrical energy into chemical energy e.g. electroplating
Electrolysis is the process whereby a chemical reaction is caused by passing an electric current through
an electrolyte.
An electrolyte is a molten or dissolved substance that contains ions that are free to move.
Ions are charged particles, and moving ions constitute an electric current. In an electrolyte, there are
positive ions that move towards the negative electrode, and negative ions that move towards the
positive electrode.
• The greater the concentration of ions in the electrolyte, the lower the resistance of the solution
and the greater the current (with V constant)
• The greater the potential difference, the greater the current (with concentration constant)
• Ions need to be free to move in the electrolyte and this can be attained by
i. Melting an ionic solid: PbBr2 → Pb2+ + 2 Br –
ii. Dissolving an ionic solid in water: KCl (s) → K+ (aq) + Cl –
iii. Dissolving certain polar covalent substances in water:
HCl (g) + H2O → H3O+ (aq) + Cl- (aq)
iv. Water contains a very low concentration of H+ (aq) and OH- (aq) ions
Electrolysis
• causes the decomposition of a chemical (the electrolyte) into simpler substances.
• requires the input of electrical energy so it is a non-
spontaneous, endothermic process.
• requires an electrolyte into which two electrodes are
placed, a source of DC and the completion of the
circuit by conductors joining the electrodes outside
the cell.
• when the source is switched on, the electrode
connected to the positive terminal of the source
becomes positive, and the electrode connected to the
negative terminal becomes negative.
• positive ions are attracted to the negative electrode,
and negative ions are attracted to the positive
electrode . http://www.meritnation.com
• at the negative electrode, positive ions are discharged. They receive electrons and are reduced.
This electrode is the cathode.
• at the positive electrode, negative ions are discharged. They give up electrons and are oxidised.
This electrode is the anode. [Note that the charge on the cathode and anode of an electrolytic cell
is opposite to that of a galvanic cell].
• at the electrodes, the following may occur
i. a gas may be given off
ii. a solid may be deposited
iii. the electrodes may go into solution as ions
1
, Grade 12 Physical Science Electrochemistry Notes 3
Reactions occurring at the electrodes.
What happens at the electrode depends on:
i. the position of the ion on the table of standard reduction potentials
ii. the concentration of the ion in the electrolyte
iii. the nature of the electrode
i. The position of the ion on the table of standard reduction potentials
If two positive ions arrive at the negative electrode, there is a competition for accepting electrons
(i.e. to be reduced). The ion/species that is most easily reduced (i.e. it is the strongest oxidising
agent with the largest E0 value lower on the table) will be reduced.
e.g. If both Cu2+(aq) and H+(aq) ions arrive at the negative electrode (the cathode)
2 H+(aq) + 2 e- ⇌ H2 (Eo = 0.00 V)
Cu2+(aq) + 2 e- ⇌ Cu (Eo = 0.34 V)
The copper ion is more likely to accept electrons (i.e. is the stronger oxidising agent) so Cu will
be preferentially deposited on the electrode.
If two negative ions arrive at the positive electrode, there is a competition for losing electrons (i.e.
to be oxidised). The ion/species that is most easily oxidised (i.e it is the strongest reducing agent
with the smallest E0 value higher on the table) will be oxidised.
e.g. If both Cl-(aq) and OH-(aq) ions arrive at the positive electrode (anode), one of these will need to
be oxidised or give up electrons. The one that will be preferentially discharged will be the one
that is most likely to give up electrons (i.e. is the stronger reducing agent).
O2 + 2 H2O + 4 e- ⇌ 4 OH- (Eo = 0.40 V)
Cl2 (aq) + 2 e- ⇌ 2 Cl- (aq) (Eo = 1.36 V)
The chlorine is most likely to gain electrons (larger E0) , so the chloride ions will be least likely
to lose electrons. The OH- will be preferentially discharged, in the reaction
4 OH- → O2 + 2 H2O + 4 e-
For an aqueous solution, H2O molecules compete with ions from dissolved salts to receive or give
up electrons at the electrode. Water molecules are:
• Oxidised at the anode : 2 H2O (l) → O2 (g) + 4 H+ (aq) + 4 e- Eo = 1.23 V
• Reduced at the cathode : 2 H2O (l) + 2e- → H2 (g) + 2 OH- (aq) Eo = - 0.83 V
ii. The concentration of the electrolyte
If the concentration of the electrolyte is high, the ion discharged may be altered. This is because
the Eo reported on the table of standard reduction potentials is obtained at standard conditions
(i.e. at 1 mol.m-3 concentration).
e.g. if sodium chloride is electrolysed
• Low concentration of NaCl will result in H2 (g) and O2 (g) being produced (H2O is oxidised at
the anode and reduced at the cathode).
• High concentration of NaCl will result in the Cl- ions being discharged instead so that Cl2 (g)
will be produced.
iii. The nature of the electrode
Usually inert electrodes (platinum or graphite) are used. These do not react. If, however, more
reactive metal electrodes are used, these may go into solution as metal ions at the anode. This is
called electrode participation. e.g. If a copper electrode is used, it may be involved in the oxidation
reaction at the anode: Cu (s) → Cu 2+ (aq) + 2 e-
2
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