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Electrolysis
Francine
Taylor-Campbell, Contributor
YOU
SHOULD BE ABLE TO:
- Identify
ions present in electrolytes
- Predict
the electrode to which an ion will
drift
- Identify
the products appearing during electrolysis
and write relevant ionic equations
- Describe
the electrolysis of certain substances;
concentrated hydrochloric and diluted
sulphuric acid, concentrated sodium
chloride and diluted sodium chloride,
copper (II) sulphate using carbon
or platinum and copper electrodes,
and fused halide eg lead bromide.
POINTS
TO NOTE
- In
the electrolysis of molten substances
only two kinds of ions are present
and both are discharged. The cation
is discharged at the cathode where
it gains electrons (reduced) and
form atoms. The molten metal generally
coats the cathode. The anion is
discharged at the anode by losing
electrons (oxidized) and form atoms.
- When
electrolysing aqueous solutions,
the ions of the substance being
electrolysed and water are present.
The discharge of ions at the electrodes
will, therefore, be dependent on
position of the ion in the electrochemical
series, concentration and the type
of electrode being used. (Please
review last week's lesson to understand
this concept).
Question
1
Write
electrode reactions for the electrolysis
of molten sodium chloride.
In
a molten substance such as NaCl (l),
two ions are present; a positive ion
(cation) Na+ and a negative
ion (anion) Cl-.
At
the cathode: Na+ migrates
towards the cathode (negative electrode)
and gains electrons to form Na which
is deposited on the electrode.
Reaction:
Na+(l) + e === Na (s)
At
the anode: Cl- ions
migrate towards the anode (positive
electrode) and lose electrons to form
chlorine gas.
Reaction:
2Cl-(l) === Cl2(g)
+ 2e
Electrolysis
of dilute sulphuric acid.
Ions
present: H+ and OH- from water
and H+ and SO42-
from sulphuric acid
At
the cathode: H+ ions migrate towards
the cathode and are discharged as
hydrogen gas. 2H+(aq) +
2e == H2 (g)
At
the anode: OH- and
SO42- migrate
towards the anode but the OH- ion
is discharged in preference to the
SO42- ion.
4OH-
(aq) === 2H2O (l) + O2(g)
+ 4e
NOTE:
Since four mol of electrons are produced
at the anode, then these four mol
of electrons must be used up at the
cathode. So, in this reaction, for
every 1 mol of oxygen gas produced,
two mol of hydrogen gas are formed.
Electrolysis
of aqueous sodium chloride using Pt/C
electrode.
Ions
present: Na+, H+,
OH- and Cl-
At
the cathode: Na+ and
H+ migrate towards the
cathode but H+ is discharged
in preference to Na+ (lower
in electrochemical series).
2H+(aq)
+ 2e == H2(g)
At
the anode: OH- and
Cl- ions migrate towards
the anode. If the sodium chloride
solution is dilute then OH- ions are
discharged in preference to Cl-.
4OH-(aq)
=== 2H2O (l) + O2(g)
+ 4e
If
the solution is concentrated, then
Cl- ions are discharged preferentially,
and in this case a carbon electrode
is used instead of platinum, as they
are resistant to attack by chlorine.
2Cl- (aq) == Cl2
(g) + 2e
Electrolysis
of aqueous copper (II) sulphate solution.
Ions
present: H+, Cu2+,
OH- and SO42-
At
the cathode: Cu2+ and
H+ ions migrate towards
the cathode where the Cu2+ ions are
discharged in preference to H+ ions
as it is lower in the series.
Cu2+(aq)
+ 2e === Cu (s).
This
copper is deposited on the cathode
which gets thicker. The blue colour
of the copper sulphate solution gradually
fades.
At
the anode: Two reactions can occur
at the anode depending on the type
of electrode used. Using Pt/C electrodes,
OH- ions are discharged in preference
to
SO42-.
4OH- (aq) == 2H2O(l)
+ O2(g) + 4e. The solution
becomes acidic as Cu2+
and OH- ions are removed
leaving H+ and SO42-
ions.
Using
copper electrode as the anode results
in the copper from the electrode being
converted to copper ions (a process
which requires less energy) and the
anode dissolves and gets thinner.
Cu(s)
=== Cu2+ (aq) + 2e. The
concentration of the solution is unchanged
(the
blue colour remains the same).
Francine
Taylor-Campbell teaches at Jamaica
College. Send questions and comments
to kerry-ann.hepburn@gleanerjm.com
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