Factors affecting products
of Electrolysis
There are TWO considerations:
A. IF the electrolyte
is MOLTEN
[ MX
]
(and carbon/graphite INERT electrodes are
used)
the electrolytic reactions are simple and
straightforward.
During
electrolysis,
M+ positive ions migrate to Cathode: M
+ (l) + e ==> M (l)
X -
negative ions migrate to Anode: 2X - (l)
- 2e ==> X2 (g)
B. IF the electrolyte
is AQUEOUS
the electrolytic reactions become more
complicated.
Water comes into the picture as water ionises to form H
+ and OH - ions. Thus,
selection of ions for preferential discharge is based
on
the
following three factors:
1. POSITION of the ION in
the Electrochemical Series
Cations
K
Na Ca Mg
Al Zn Fe
Sn Pb H Cu
Ag
Anions
SO4
NO3
CO3
Cl
Br
I
OH
Increasing ease of
discharge
Note:
*If two or more
positive ions migrate to the cathode, it is the ion lowest in the
ECS
(i.e. of the least electropositive metal) that is discharged
preferentially.
Examples Copper ions are discharged in preference to
Hydrogen ions; Hydrogen ions are discharged in preference to Sodium
ions.
It will be recalled that the reactivity series
gives the order of the tendency of metals to form ions. Metals at
the top of the series have the greatest tendency to form ions and
are therefore the most difficult to discharge at the cathode.
* If two or more
negative ions migrate to the anode, the ion lower in the ECS is
discharged preferentially.
2.
CONCENTRATION of the ions in the electrolyte
IMPORTANT NOTE: A
very
dilutesolution of any electrolyte,
on electrolysis, liberates oxygen at the
anode.
However, if the concentration of a
particular ion is high, then this can alter the preferential
discharge.
For example, if dilute hydrochloric acid is electrolysed,
hydrogen gas is produced at the cathode and oxygen gas at the
anode. However, when concentrated hydrochloric acid is
electrolysed, hydrogen gas is still liberated at the cathode, but
chlorine gas (and NOT oxygen gas) is now produced at the anode
instead. The higher concentration of the chloride ions favor their
discharge over the hydroxide ions.
NOTE: Ions of Sodium, Potassium,
Calcium, Sulfate and Nitrate ions are NOT discharged even if they
are in high concentrations.
3. ELECTRODE material
used
Inert electrodes
(made of graphite or platinum) do not affect the products of
electrolysis ; only Reactive / Active electrodes do
affect.
Example
|
Electrodes
|
Electrolyte
|
At Cathode
|
At Anode
|
|
Carbon (inert)
|
aqueous copper(II) sulfate
|
copper metal deposited
|
oxygen liberated
|
|
Copper (active)
|
aqueous copper(II) sulfate
|
copper metal deposited
|
copper electrode
dissolves
|
|
Created: 1 Dec 2006 / 1.25 pm
|
