|
8. Summary of electrode
reactions (half-equations)
and products
|
|
Equation
reference
number |
() 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.
So, before each electrode
equation is a () for a negative cathode electrode = a reduction
reaction equation or a (+) for a positive anode electrode = an
oxidation reaction equation
|
The electrode half-equations
are shown on the left with examples of
industrial processes where this electrode reaction happens on the right.
Unless otherwise
stated, the electrodes are inert i.e. they do not chemically change e.g. platinum or carbongraphite.
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. |
|
1 |
a reduction
electrode reaction () Na+(l)
+ e ==> Na(l) (sodium metal) |
sodium
ion reduced to sodium metal atoms: typical of
electrolysis
of molten chloride salts to make chlorine and the metal |
|
2 |
an oxidation
electrode reaction (+) 2Cl(l/aq)
2e ==> Cl2(g)
or 2Cl(l/aq)
==> Cl2(g) + 2e
Note that you can write
these anode oxidation reactions either way round.
|
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 |
|
3 |
a reduction
electrode reaction () 2H+(aq)
+ 2e ==> H2(g) (hydrogen gas)
or 2H3O+(aq)
+ 2e ==> H2(g) + 2H2O(l)
or
2H2O(l)
+ 2e ==> H2(g) + 2OH(aq)
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? |
hydrogen
ion or water reduced to hydrogen gas molecules: electrolysis
of many salt or acid solutions to make hydrogen |
|
4 |
a reduction
electrode reaction () Cu2+(aq)
+ 2e ==> Cu(s) (copper deposit) |
copper(II)
ion reduced to copper atoms: deposition
of copper in its electrolytic purification or electroplating using
copper(II) sulphate solution, electrode can be copper or other metal to
be plated |
|
5 |
an oxidation
electrode reaction (+) Cu(s)
2e ==> Cu2+(aq)
(copper dissolves)
or Cu(s)
==> Cu(s) + 2e |
copper
atoms oxidised to copper(II) ions: dissolving
of copper in its electrolytic purification or electroplating (must have
positive copper anode) |
|
6 |
a reduction
electrode reaction () Al3+(l)
+ 3e ==> Al(l)
(aluminium) |
aluminium
ions reduced to aluminium atoms: extraction
of aluminium in the electrolysis of its molten oxide ore(l) |
|
7 |
an oxidation
electrode reaction (+) 2O2(l)
4e ==>
O2(g) (oxygen gas)
or 2O2(l)
==>
O2(g) + 4e |
oxide
ion oxidised to oxygen gas molecules: electrolysis
of molten oxides e.g. anode reaction in the extraction of aluminium from
molten bauxite. |
|
8 |
an oxidation
electrode reaction (i) (+) 4OH(aq)
4e ==> 2H2O(l)
+ O2(g) (oxygen gas)
or 4OH(aq) ==> 2H2O(l)
+ O2(g) + 4e
(ii) (+) 2H2O(l)
4e ==> 4H+(aq)
+ O2(g) (oxygen gas)
or 2H2O(l) ==> 4H+(aq)
+ O2(g) + 4e
Both equations amount to the same
overall change i.e. the formation of oxygen gas molecules and as
far as I know either is acceptable in an exam? |
There are two
equations that describe the formation of oxygen in the electrolysis of
water. 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) |
|
9 |
a reduction
electrode reaction ()
Pb2+(l)
+ 2e ==> Pb(l) (lead deposit)
|
lead(II)
ions reduced to lead atoms: electrolysis
of molten lead(II) bromide(l) |
|
10 |
an oxidation
electrode reaction (+) 2Br(l/aq)
2e ==> Br2(g/l) (bromine)
or 2Br ==> Br2
+ 2e |
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 |
|
11 |
a reduction
electrode reaction () Zn2+(aq)
+ 2e ==> Zn(s) (zinc deposit)
|
zinc
ions reduced to zinc atoms: galvanising steel (the electrode) by electroplating from
aqueous zinc sulphate solution, (or from molten zinc chloride?) |
|
12 |
a reduction
electrode reaction () 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 |
|
13 |
a reduction
electrode reaction () Ca2+(l)
+ 2e ==> Ca(s) (calcium metal)
|
calcium
ions reduced to calcium atoms e.g. in molten calcium chloride or
bromide etc. |
|
***** |
******************************************************************************* |
*************************************************************** |
|
TOP OF PAGE
ELECTROCHEMISTRY INDEX: 1.
INTRODUCTION to electrolysis - electrolytes,
non-electrolytes, electrode equations, apparatus 2. Electrolysis of acidified water (dilute sulfuric acid)
and some sulfate salts and alkalis 3. Electrolysis of sodium chloride solution (brine)
and bromides and iodides 4. Electrolysis of copper(II) sulfate solution
and electroplating with other metals e.g. silver 5. Electrolysis of molten lead(II) bromide
(and other molten ionic compounds) 6. Electrolysis of copper(II)
chloride solution 7. Electrolysis of hydrochloric acid 8.
Summary of electrode equations and products 9.
Summary of electrolysis products from various
electrolytes 10.
Simple cells (batteries) 11.
Fuel Cells e.g. the hydrogen - oxygen fuel cell 12.
The electrolysis of molten aluminium oxide
- extraction of
aluminium from bauxite ore & anodising aluminium to
thicken and strengthen the protective oxide layer 13.
The extraction of sodium
from molten sodium chloride using the 'Down's Cell' 14.
The purification of
copper by electrolysis 15.
The purification of
zinc by electrolysis 16. Electroplating
coating
conducting surfaces with a metal layer 17.
Electrolysis
of brine (NaCl) for the production of
chlorine,
hydrogen & sodium hydroxide
AND 18.
Electrolysis
calculations
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