Electrochemistry
This involves electrochemical reactions.
There are three types of conductors:
1. Electrical conductors: These are substances which conduct
an electric current.
2. Non-conductors: These are substances which do not
conduct an electric current.
3. Semi-conductors: These are substances whose conducting
properties are in between those of a conductor and a nonconductor.
Types of conduction
1. Metallic conduction- This is where electricity is conducted
because there is a “sea” of mobile electrons and the
substance remains unchanged. Some electrons pass out of
the metal and into the cell at its positive electrode. They are
replaced by an equal number of electrons which flow into
the metal from the negative electrode of the cell.
2. Electrolytic conduction- This has free ions and these ions
can only be free when molten or dissolved in water. In this
process, there are chemical changes in the composition of
the electrolyte.
Electrolyte
An electrolyte is a molten ionic compound or a solution
containing free ions that conduct electricity. There are two types
of electrolytes which are:
1. Strong electrolytes- These contain a high concentration of
ions which are completely ionized. Strong electrolytes
include molten ionic compounds, aqueous solutions of
ionic compounds, strong acids and strong alkalis.
2. Weak electrolytes- These contain a low concentration of
ions which are only partially ionized. Weak electrolytes
include weak acids and weak alkalis.
Electrolysis
This is the process by which the passage of an electric current
through a molten or aqueous compound causing it to
decompose. During electrolysis, electrodes, an electrolyte and a
power supply are needed for the process.
The electrodes are rods that conduct the electric current to and
from the electrolyte. The electrodes are made from either
graphite or a metal. There are two types of electrodes which are:
- Anode: This is the positive electrode and is connected to
the positive terminal of the power supply or battery.
Anions move towards the anode during electrolysis.
- Cathode: This is the negative electrode and is connected
to the negative terminal of the power supply or battery.
Cations move towards the cathode during electrolysis.
Diagram illustrating electrolysis
Processes occurring during electrolysis
At the anode:
1. Oxidation occurs.
2. Loss of electrons occurs.
3. Anions lose electrons to form neutral atoms which are
discharged.
Half equation for the reaction at the anode:
A - ne à A
O - 2e à O / O à O + 2e
At the cathode:
1. Reduction occurs.
2. Gain of electrons occurs.
3. Cations gain electrons to form neutral atoms which are
discharged.
n-
-
2-
-
2-
2 (g)
-
2 (g)
Half equation for the reaction at the cathode:
C + ne à C
n+
-
Cu + 2e à Cu
2+
-
(s)
The electrons lost at the anode, enter the external circuit
and re-enter the electrolytic cell at the cathode. This
ensures that the number of electrons lost at the anode is
the same as the number of electrons gained at the
cathode.
The electric current can produce the following changes:
- Deposition of metals at the cathode.
- Evolution of hydrogen gas at the cathode.
- Discharge of non-metals at the anode.
- Metal atoms leaving the anode and entering the solution
as metal ions if active metals are used as the anode.
Preferential Discharge
During electrolysis, only one type of cation and anion
can be discharged. This is called preferential discharge of
ions. The factors which determine which ions are
discharged are:
1. Position in the electrochemical series: The
electrochemical series list ions in descending order of
their reactivity. Ions lower down in the series are
preferential discharged.
2. Concentration of the electrolyte: The electrolyte can
either be diluted or concentrated. If the electrolyte is
concentrated, the anions in the concentrated
electrolyte are preferential discharged over the anion
in water.
3. State of the electrolyte: The electrolyte can be either
molten or aqueous. If it is in the molten state, only the
ions in the electrolyte are present. If it is in the
aqueous state, the ions of the electrolyte as well as the
ions of water are present.
4. Nature of the electrode: The electrodes can either be
inert or active. Inert electrodes such as graphite and
platinum do not take part in the chemical changes
occurring in electrolysis. Active electrodes such as
copper take part in the chemical changes occurring
during electrolysis.
Table showing common electrolytes
Name
Formula Cation Anion
Hydrochloric acid HCl
H+
ClSulfuric acid
H2SO4
H+
SO4 2Sodium chloride
Water
NaCl
H 2O
Na+
H+
ClOH-
Copper sulfate
CuSO4
Cu2+
SO4 2-
Lead Bromide
PbBr2
Pb2+
Br-
The electrochemical series
This list metal and non-metal ions according to their reactivity
as reducing and oxidizing agents respectively.
In order to predict the products of electrolysis, you need to
know:
- The electrolyte
- The type of electrodes
- The product at each electrode
- Each electrode equation
Example:
1. The electrolysis of aqueous copper sulfate solution using
inert electrodes
Ions present: H , Cu , SO , OH
At the cathode; Cu is preferentially discharged at the
cathode because it is lower than H in the electrochemical
series.
Half equation: Cu + 2e à Cu
At the anode; OH is preferentially discharged at the anode
because it is lower than SO in the electrochemical series.
+
2+
2-
-
4
2+
+
2+
-
(aq)
(s)
-
2-
4
Half equation: 4OH -4e à O + 2H O / 4OH à O +
2H O + 4e
Observations: Copper sulfate solution becomes pale as
concentration decreases. Solution becomes acidic due to H
and SO ions being left in the solution.
-
-
-
(aq)
2(g)
2
(l)
(aq)
2(g)
-
2
(l)
+
2-
4
2. The electrolysis of aqueous copper sulfate solution using
copper electrodes
Ions present: H , Cu , SO , OH
At the cathode; Cu is preferentially discharged at the
cathode because it is lower than H in the electrochemical
series.
Half equation: Cu + 2e à Cu
At the anode; Copper metal becomes oxidized to form
copper ions and enter the solution because the electrodes
take part in electrolysis.
Half equation: Cu -2e àCu / Cu àCu + 2e
Observations: The anode becomes thinner as copper is
removed while the cathode gets thicker as copper is
deposited. The concentration of copper sulfate remains the
same.
3. Electrolysis of concentrated aqueous sodium chloride using
graphite electrodes
Ions present: H , Na , Cl , OH
At the cathode: H is preferentially discharged at the
cathode because it is lower than Na in the electrochemical
series.
Half equation: H + 2e à H
At anode: Cl is preferentially discharged at the anode
because it is in a higher concentration than OH . For
anions, concentration has a greater influence than the
position in the electrochemical series.
Half equation: 2Cl -2e à Cl / 2Cl à Cl + 2e
4. Electrolysis of dilute sodium chloride using graphite
electrodes
Ions present: H+, Na+ , Cl-, OHAt the cathode: H is preferentially discharged at the
cathode because it is lower than Na in the electrochemical
series.
+
2+
2-
-
4
2+
+
2+
-
(aq)
(s)
-
2+
(s
+
2+
(aq)
+
-
(s)
-
(aq)
-
+
+
+
-
(aq)
2 (g)
-
-
-
-
(aq)
-
2(g)
-
(aq)
+
+
2(g)
Half equation: 2H + 2e à H
At the anode; OH is preferentially discharged at the anode
because it is lower than Cl in the electrochemical series.
Half equation: 4OH -4e à O + 2H O / 4OH à O +
2H O + 4e
Observation: The sodium chloride solution becomes more
concentrated as water is used up.
+
-
(aq)
2 (g)
-
-
-
-
-
(aq)
2(g)
2
(l)
(aq)
2(g)
-
2
(l)
Electrolysis calculations
The quantity of electricity transferred in electrolysis is
given by:
Q= I x t
Where Q= quantity of electricity in coulombs (C)
I= the amount of current in amperes (A)
T= time in seconds (s)
Faraday’s law of electrolysis
Faraday’s first law of electrolysis
This states that the mass of a substance discharged at an
electrode during electrolysis is directly proportional to the
quantity of electricity passing through the electrolytic cell.
One mole of an electron has a change of 96500C which is
equal to one faraday (F)
Faraday’s second law of electrolysis
This states that the number of Faraday’s required to
discharge one mole of an ion at an electrode is equal to the
size of the charge on that ion.
Therefore;
1 Faraday is required to discharge one mole of an ion with
a single charge, eg. Na and Cl
2 Faraday is required to discharge one mole of an ion with
a double charge, eg. Cu and O
3 Faraday is required to discharge one mole of an ion with
a triple charge, eg. Al and P
+
-
2+
3+
2-
3-
Applications of electrolysis
1. Extraction of metals from their ores.
2. Anodizing Aluminium- Anodizing is a method of
producing corrosion-resistant objects. Aluminium readily
forms a protective oxide coating of Al O and when
anodized, it becomes thicker and tougher. Aluminium
acts as the anode while the cathode is inert. Dilute
sulfuric acid or dilute chromic acid acts as the
electrolyte.
The reaction for the anode is: 4OH à O + 2H O + 4e
. The liberated oxygen leads to the formation of a thick
oxide coating. 3O + 4Al à Al O The coating can then
absorb dyes which permanently fixed by the treatment
with boiling water.
2
3
-
-
(aq)
2(g)
(s)
2
2(g)
2
(l)
3(s)
3. Electroplating/ electro-deposition-This is the deposition
of a metal on a cathode when an electric current is
passed providing that the right combination of electrolyte
and electrodes is used. In electroplating, active
electrodes are used. The anode is the pure metal while
the cathode is the object to be plated. The electrolyte is a
soluble salt of the pure metal at the anode.
Electroplating works best when:
- The surface to be plated is clean.
- The surface is given an undercoat of another metal.
- Small currents are used.
- Temperature and pH are controlled.
Materials are plated to:
- Enhance the appeal of the plated object.
- Obtain an object which is more resistant to corrosion.
- Avoid using expensive metals for the object.
Example:
Chrome plating: The plating solution electrolyte is usually
chromic acid and chromium (III) sulfate and fluorosilicate
are added to the plating solution. The anode is pure
chromium and the cathode is the metal to be plated. In
decorative chrome plating, the item is first given an
undercoating copper and nickel.
Nickel plating: The plating solution, the additives and
actual conditions of nickel plating vary with the purpose of
the plated items. A common plating solution consists of a
nickel (III) sulfate or nickel (II) chloride solution. Boric
acid and a chemical wetting agent are added to the plating
solution.
4. Electro-refining/ Purification of metals- Metals can be
purified using electrolysis. The impure metal is used as
the anode while the pure metal is used as the cathode.
The electrolyte is a soluble salt of the metal to be
purified.
Example:
Purification of copper: Copper obtained by chemical
reduction methods has impurities. The purity can be
improved by electro-refining which is electrolysis using
active electrodes. Impure copper is used as the anode
while a strip of pure copper is used as the cathode. The
electrolyte is a mixture of copper (II) sulfate and sulfuric
acid. A large current is used during the process. During
this process, copper atoms leave the anode and enter the
solution as copper ions, while copper ions are discharged
and deposited on the cathode.
At the cathode: Cu + 2e à Cu
At the anode: Cu àCu + 2e
2+
-
(aq)
(s)
2+
(s)
-
(aq)