5(a).
The electrolysis of molten anhydrous lead(II) bromide
The products of electrolysing lead bromide are lead metal and bromine gas
You can electrolyse molten compounds
as long as they are ionic compounds, so that on melting, there free ions to move
to carry the current to facilitate the electrolysis process of splitting the
compound into its constituent elements.
Electrolysing molten salts with carbon electrodes
Inert carbon
(graphite)
electrodes are dipped into molten salt which has been strongly heated in a
crucible. It is difficult to collect the gases at
the electrodes! The salts may be very high melting, so sometimes a small
amount of another salt impurity is added to lower the melting point.
The electrolyte molten lead(II) bromide
PbBr2(l),
provides a high concentration of lead(II) ions Pb2+
and bromide ions Br– to carry the current during the
electrolysis process.
Remember that melting an ionic compound breaks
down the strong ionic bonding sufficiently to allow the ions to freely
move around and carry the electric current.
The electrolysis will only take place when
electricity is passed through the molten lead bromide.
This is a good teacher demonstration in the school
laboratory – brown vapour and silvery lump provide good evidence of
what's happened.
At the end of the experiment its best to pour
the molten salt onto cold ceramic surface. Let the residue cool and
break it up to find the silvery lump of lead - to prove you do get lead
from the electrolysis of lead bromide, the orange-brown vapour of
bromine is pretty obvious and pretty obnoxious! PLEASE do in a fume
cupboard!
The electrode reactions and products of the
electrolysis of the molten ionic compound lead bromide (the electrolyte) are illustrated by the
theory diagram above.
This is quite a simpler electrolysis situation
where the ionic compound lead bromide on melting provides a highly concentrated
mixture of positive lead ions and negative bromide ions.
The half-equations for the electrolysis of lead(II)
bromide.
(a) The negative cathode electrode reaction for the electrolysis
of molten lead(II) bromide
The positive lead(II) ions are attracted to
the negative electrode and are discharged to form molten
lead
Cathode (-):
Pb2+(l)
+ 2e– ==> Pb(l)
half-equation: positive ion reduction by
electron gain, lead ions to lead atoms
This is a reduction reaction
because the positive lead ions gain electrons to form neutral lead atoms.
(b) The positive anode electrode
reaction for the electrolysis of molten lead bromide
The negative bromide ions are
attracted to the positive anode electrode and discharged to form bromine
vapour, bromide ions to bromine molecules.
Anode (+):
2Br–(l)
– 2e– ==> Br2(g)
or
2Br–(l)
==> Br2(g) + 2e–
half-equation: negative ion oxidation by
electron loss
This is an oxidation reaction
because the negative bromide ions lose electrons to form neutral
bromine molecules.
Extra comments on the
electrolysis of lead bromide and other molten ionic compounds
1. Overall equation for the electrolysis of
molten lead bromide:
PbBr2(l) ==> Pb(l) + Br2(g)
2. Electrolysis of molten bromide salts(l)
or their concentrated aqueous solution(aq) or conc. hydrobromic acid(aq) to make bromine.
3. It takes 2 electrons to reduce on lead(II) ion to a lead atom
(reduction) and it takes the removal of 1 electron from each of 2 bromide
ions (oxidation) to form a bromine molecule. Therefore for the same quantity
of electric current the mole ratio of
Pb : Br2
is 1 : 1.
SUMMARY OF PRODUCTS FROM THE ELECTROLYSIS
OF LEAD(II) BROMIDE
with inert carbon electrodes |
Electrolyte |
negative cathode
product |
negative electrode
cathode half-equation |
positive anode
product |
positive electrode
anode
half-equation |
molten
lead(II) bromide
PbBr2(l) |
molten
lead |
Pb2+(l)
+ 2e– ==> Pb(l)
reduction,
electron gain |
bromine vapour |
2Br–(l)
– 2e– ==> Br2(g)
or
2Br–(l)
==> Br2(g) + 2e–
oxidation, electron loss |
TOP OF PAGE
and INDEXES
(b) to (d) Electrolysis of other molten ionic
compounds
Each provides a positive ion and a negative
ion, and this molten mixture of ions constitutes the electrolyte.
(b)
The electrolysis of molten
anhydrous calcium chloride CaCl2(l)
Electrode equations:
(i) solid/molten
calcium formed at the cathode
Cathode (-):
Ca2+(l)
+ 2e– ==> Ca(s)
half-equation: a reduction
electrode reaction - electron gain at cathode, positive calcium ions to
neutral calcium atoms
(ii) chlorine gas formed at the anode
Anode (+):
2Cl–(aq)
– 2e– ==> Cl2(g)
half-equation: an oxidation electrode reaction - electron loss at anode,
negative chloride ions to neutral chlorine molecules
or
2Cl–(aq)
==> Cl2(g) + 2e–
This is the basis for the industrial
production of calcium metal
(c) The
electrolysis of molten
anhydrous zinc chloride
This is a safer salt to use than lead
chloride, less toxic, but chlorine is a very harmful gas.
You should end up with a small lump of zinc, but chlorine gas is given
off, so this electrolysis needs to be done in a fume cupboard where you can
safely see the evolution of the green gas which bleaches litmus paper white.
(If you use damp blue litmus, chlorine is an
acidic gas and turns the litmus red (slightly acidic gas) before bleaching it white because it is
a powerful oxidising agent).
Electrode equations:
cathode (-):
Zn2+
+ 2e ==> Znhalf-equation: a reduction
electrode reaction - electron gain, zinc ions to neutral zinc atoms
anode (+):
2Cl ==> Cl2 +
2e–
or
2Cl–(aq)
– 2e– ==> Cl2(g)
half-equation: an oxidation electrode reaction - electron
loss, negative chloride ions to neutral chlorine molecules
(d) The electrolysis of molten
aluminium oxide Al2O3(l)
You can't do this in a school
laboratory, it is an industrial process!
Electrode equations:
(i) molten
aluminium is formed at the negative cathode
Cathode (-):
Al3+(l)
+ 3e– ==> Al(l)
half-equation: a reduction
electrode reaction - electron gain at cathode, positive aluminium ions
to neutral aluminium atoms
(ii) oxygen
gas is formed at the positive electrode
Anode (+):
2O2–(l)
– 4e– ==>
O2(g)
half-equation: an
oxidation electrode reaction
- electron loss at anode, negative oxide ions to neutral oxygen molecules
or
2O2–(l)
==>
O2(g) + 4e–
The
industrial method for the extraction of aluminium from its ore uses
electrolysis.
See
The extraction of
aluminium from purified molten bauxite ore
The mole ratio of Al : O2 is 4 : 3 since the
overall electrolysis equation for the industrial process is:
2Al2O3(l)
==> 4Al(l) + 3O2(g)
(e)
Learning objectives for the electrolysis of
molten lead bromide, calcium
chloride, zinc chloride and aluminium oxide
Know that electrolysis requires a conducting solution of ions (electrolyte
of molten lead bromide, calcium chloride, zinc chloride)
and two inert solid conducting electrodes e.g. graphite (carbon) or platinum
(expensive!).
Know that the electrolyte here contains free moving metal ions and
non-metal ions from the melted salt.
Know that electrolysis will only happen if a d.c. electrical current is
passed through the molten salt and reduction and oxidation reactions occur
on passage of the electric current.
Where practicable in a school or college laboratory, be able to describe
the apparatus required to electrolyse molten salts and be able to explain and understand the formation of the electrolysis
products by:
knowing that the positive ions are reduced by electron gain and
discharged at the negative cathode as metal atoms (lead, zinc, calcium),
knowing that the negative chloride or bromide ions are oxidised by electron loss and
discharged at the positive anode as chlorine or bromine molecules,
and be able to write out the electrode equations (half equations) for the formation of
neutral metal atoms by electron gain reduction and chlorine or bromine gas molecules
from the oxidation of bromide or chloride ions by electron loss.
From the electrode equations, be able to explain why the mole ratio e.g.
of lead/zinc/calcium atoms to bromine/chlorine molecules is
theoretically 1
: 1
Know how to test for chlorine gas formed from the electrolysis of a chloride
salt using inert electrodes and recognise brown vapour formed at the anode is an
indication that bromine was formed in the electrolysis of molten bromide salts.
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