1. Introduction to electrolysis - electrolytes and non-electrolytes
Electrolysis is the process of electrically inducing chemical changes in a conducting
melton or solution e.g. splitting an ionic compound into the metal and non-metal.
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SUMMARY OF COMMON ELECTRICAL CONDUCTORS and what makes up the circuit?
o These materials carry an electric current via freely moving electrically charged
particles, when a potential difference (voltage!) is applied across them, and they
include:
o All metals (molten or solid) and the non-metal carbon (graphite).
 This conduction involves the movement of free or delocalised electrons (e charged particles) and does not involve any chemical change.
o Any molten or dissolved material in which the liquid contains free moving ions
is called the electrolyte and can conduct an electrical current. (see nonelectrolyte)
 Ions are charged particles e.g. Na+ sodium ion, or Cl- chloride ion, and their
movement or flow constitutes an electric current, in other words the
electrolyte consists of a stream of moving charged particles.
 What does the complete electrical circuit for electrolysis consist of?
 There are two ion currents in the electrolyte flowing in opposite
directions
 positive cations e.g. sodium Na+ are attracted to the negative
cathode electrode,
 and negative anions e.g. chloride Cl- are attracted to the
positive anode electrode,
The chemical changes occur at the electrodes which connect the electrolyte liquid containing
ions with the external d.c. electrical supply.
o If the current is switched off, the electrolysis process stops.
Non-electrolytes are liquids or solutions that do not contain ions, do not conduct
electricity readily and cannot undergo the process of electrolysis e.g. ethanol (alcohol),
sugar solution etc. and are usually covalent molecules.
Liquids that conduct must contain freely moving ions to carry the current and
complete the circuit.
o You can't do electrolysis with an ionic solid!, the ions are too tightly held by
chemical bonds and can't flow from their ordered situation!
o When an ionically bonded substances are melted or dissolved in water the ions
are free to move about.
 However some covalent substances dissolve in water and form ions.
 e.g. hydrogen chloride HCl, dissolves in water to form 'ionic' hydrochloric acid
H+Cl-(aq)
The solution or melt of ions is called the electrolyte which forms part of the circuit. The circuit
is completed by e.g. the external copper wiring and the (usually) inert electrodes like graphite
(form of carbon) or platinum AND electrolysis can only happen when the current is switched
on and the circuit complete.
 ELECTROLYSIS SPLITS a
COMPOUND:
o When substances which
are made of ions are dissolved in water,
or melted material, they can be broken
down (decomposed) into simpler
substances by passing an electric current
through them.
o This process is called
electrolysis.
o Since it requires an
'input' of energy, it is an endothermic
process.
During electrolysis in the electrolyte (solution or melt of free moving ions) ...
o
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positive metal or hydrogen ions move to the negative electrode (cations attracted to
cathode), e.g. in the diagram, sodium ions Na+ , move to the -ve electrode,
o and negatively charged ions move to the positive electrode (anions attracted to
anode), e.g. in the diagram, chloride ions Cl-, move to the +ve electrode.
The diagram shows the industrial electrolysis process (in a Down's Process Cell) to extract
sodium metal from sodium chloride (common salt).
During electrolysis, gases may be given off, or metals dissolve or are deposited at the
electrodes.
o Metals and hydrogen are formed at the negative electrode from positive ions by
electron gain (reduction), e.g. in molten sodium chloride
 sodium ions change to silvery grey liquid sodium, Na+ + e- ==> Na
o and non-metals e.g. oxygen, chlorine, bromine etc. are formed from negative ions
changing on the positive electrode by electron loss (oxidation), e.g. in molten sodium
chloride
 chloride ions change to green chlorine gas, 2Cl- - 2e- ==> Cl2
o The electrons released by the oxidation at the positive anode, flow round through the
anode and wire to the positive cathode and so bring about the reduction i.e. of the
sodium ion.
In a chemical reaction, if an oxidation occurs, a reduction must also occur too (and vice versa)
so these reactions 'overall' are called redox changes.
o You need to be able to complete and balance electrode equations or recognise them
and derive an overall equation for the electrolysis.
2b. Summary of ELECTRODE REACTIONS - half-cell electrode
equations
Eq.
no.
(-) negative cathode electrode
where reduction of the attracted
positive cations is by electron
gain to form metal atoms or
hydrogen [from Mn+ or H+, n =
numerical positive charge]. The
electrons come from the positive
anode (see below).
(+) positive anode electrode
where the oxidation of the atom or
anion is by electron loss. Nonmetallic negative anions are
attracted and may be oxidised to
the free element. Metal atoms of a
metal electrode can also be
oxidised to form positive metal
ions which pass into the liquid
electrolyte. The released electrons
move round in the external part of
the circuit to produce the negative
charge on the cathode electrode.
1
(-) Na
+
(l)
+
e-
==> Na(l) (sodium
metal)
2
(+) 2Cl-(l/aq) - 2e- ==> Cl2(g)
3
The electrode equations are
shown on the left with examples
of industrial processes where
this electrode reaction happens
below on the right. Unless
otherwise stated, the electrodes
are inert i.e. they do not
chemically change e.g. platinum or
carbon-graphite.
PLEASE NOTE - all electrode
equations are a summarysimplification of what happens on
an electrode surface in
electrolysis. There may be e.g.
two equations which are totally
equivalent to each other to
describe WHAT IS ACTUALLY
FORMED e.g. the formation of
hydrogen or oxygen and in some
cases other products may be
formed too.
sodium ion reduced to sodium
metal atoms: typical of
electrolysis of molten chloride
salts to make chlorine and the
metal
chloride ion oxidised to chlorine
gas molecules: electrolysis of
molten chloride salts(l) or their
concentrated aqueous solution(aq)
or conc. hydrochloric acid(aq) to
make chlorine
(-) 2H+(aq) + 2e- ==> H2(g)
(hydrogen gas)
or 2H3O+(aq) + 2e- ==> H2(g) +
2H2O(l)
or 2H2O(l) + 2e- ==> H2(g) + 2OH(aq)
hydrogen ion or water reduced
to hydrogen gas molecules:
electrolysis of many salt or acid
solutions to make hydrogen
All three equations amount to
the same overall change i.e. the
formation of hydrogen gas
molecules and as far as I know
any is acceptable in an exam?
4
copper(II) ion reduced to copper
atoms: deposition of copper in its
(-) Cu (aq) + 2e ==> Cu(s) (copper
electrolytic purification or
deposit)
electroplating using copper(II)
sulphate solution, electrode can
2+
-
be copper or other metal to be
plated
5
6
copper atoms oxidised to
copper(II) ions: dissolving of
(+) Cu(s) - 2e- ==> Cu2+(aq) (copper
copper in its electrolytic
dissolves)
purification or electroplating (must
have positive copper anode)
(-) Al3+(l) + 3e- ==> Al(l)
(aluminium)
7
(+) 2O
8
2-
(l)
-
- 4e ==> O2(g) (oxygen
gas)
(+) 4OH-(aq) - 4e- ==> 2H2O(l) +
O2(g) (oxygen gas)
or (+) 2H2O(l) - 4e- ==> 4H+(l) +
O2(g) (oxygen gas)
Both equations amount to the
same overall change i.e. the
formation of hydrogen gas
molecules and as far as I know
either is acceptable in an exam?
9
(-) Pb2+(l) + 2e- ==> Pb(l) (lead
deposit)
10
(+) 2Br-(l/aq) - 2e- ==> Br2(g/l)
(bromine)
hydroxide ions or water
molecules are oxidised to
oxygen gas molecules:
electrolysis of many salt solutions
such as sulphates, sulphuric acid
etc. gives oxygen (chlorides ==>
chlorine in concentrated solution,
but can also give oxygen in diluted
solution)
lead(II) ions reduced to lead
atoms: electrolysis of molten
lead(II) bromide(l)
bromide ions oxidised to
gas/liquid bromine molecules:
electrolysis of molten bromide
salts(l) or their concentrated
aqueous solution(aq) or conc.
hydrobromic acid(aq) to make
bromine
+ 2e ==> Zn(s) (zinc
deposit)
(-) Ag+(aq) + e- ==> Ag(s) (silver
deposit)
silver ions reduced to silver
atoms: silver electroplating from
silver salt solution(aq), electrode
can be other metal
(-) Zn
13
oxide ion oxidised to oxygen
gas molecules: electrolysis of
molten oxides e.g. anode reaction
in the extraction of aluminium from
molten bauxite.
zinc ions reduced to zinc
atoms: galvanising steel (the
electrode) by electroplating from
aqueous zinc sulphate solution,
(or from molten zinc chloride?)
11
12
aluminium ions reduced to
aluminium atoms: extraction of
aluminium in the electrolysis of its
molten oxide ore(l)
2+
(aq)
-
calcium ions reduced to
(-) Ca2+(l) + 2e- ==> Ca(s) (calcium
calcium atoms e.g. in molten
metal)
calcium chloride or bromide etc.