WEEK 4

TOPIC: ELECTROLYSIS: IONIC THEORY

CONTENT:
1. MEANING OF ELECTROLYSIS AND TERMINOLOGIES – ELECTRODES, ELECTROLYTES,
ELECTROLYTIC CELL
2. COMPARISON OF ELECTROLYTIC CELLS AND ELECTROCHEMICAL CELLS AND
PRINCIPLES OF ELECTROLYSIS
3. FACTORS AFFECTING THE DISCHARGES OF IONS/ PRODUCT OF ELECTROLYSIS,
4. EXAMPLES OF ELECTROLYSIS – ELECTROLYSIS OF ACIDIFIED WATER, COPPER (II)
TETRAOXOSULPHATE (VI) AND BRINE.

IONIC THEORY

Ionic theory was used to explain the behaviour of electrolytes when electric current
is passed through their solution.

A Swedish chemist, Swante Arrherius (1887), was the first to present the ionic theory
to describe electrolysis. The theory proposed that when an electrolyte is melted or
dissolved in water, some if not all of the molecules of the substance dissociate into
freely-moving charge ions i.e. positive ions (called cations) and negative ions
(anions). The process of dissociation into ions is called ionization.

When an electric current is passed through an electrolyte, the free ions lose their
random movement. The positive ion becomes attracted to the negative electrode
(called the cathode) while the negative ion s moves toward the positive electrode
(called anode). It is this ionization and mobility of ions that constitute current flow.

Arrhenius version of the ionic theory has been modified since the X-ray diffraction
studies show that salts and strong alkalis consists of oppositely charged ions even in
solid state. The modern theory of the proposes that ion in such solid state are pulled
away from one another either as a result of heat energy applied when the solid
melts or with the help of the solvent molecules when the solid dissolves. Thus,

−
+¿+Cl(aq ) ¿
Arrℎenius tℎeory ; NaC l ( s ) → N a (aq )
 −
+¿+ Cl( aq ) ¿
+¿ C l −→ Na (aq ) ¿
Moderntℎeory ; N a

PERIOD 1: MEANING OF ELECTROLYSIS

Electrolysis is the process whereby chemical decomposition occurs when an electric
current is passed through an electrolyte.

TERMINOLOGIES

1. Electrode: Electrodes are conductors in the form of wires rods or plates through
which an electric current enters or leaves the electrolyte. We have the cathode and
the anode
Note: In an electrolytic cell, the negative electrode is known as the cathode while the
positive electrode is known as the anode.
In an electrochemical cell, the negative electrode is the anode while the positive
electrode is the cathode.
2. ELECTROLYTES
An electrolyte is a compound, in a molten or aqueous state which can conduct
electricity and is decomposed in the process.
Electrolytes conduct electric currents by movement of ions. Examples of electrolytes
include NaCl, CuCl2, CUSO4, HCl, H2SO4 etc.
Electrolytes may be weak or strong:
(a) Strong electrolyte ionizes completely and can conduct large electric current since
they are composed entirely of free moving ions. Examples are strong acids , strong
bases and salts
(b) Weak electrolytes are partially ionized and conduct electricity poorly because
the contain low concentration of ions. Examples are weak acids and weak bases
3. Non-electrolytes do not conduct electricity since they do not ionize. Examples are
urea ethane, benzene, trichloromethane, cane sugar, ether, tetra chloromethane.
4. An electrolytic cell is an assembly of two electrodes in an electrolyte and is used
for the electrolysis of a substance.
EVALUATION
Write short note on the following terms:
1. Electrolysis
2. Electrolyte
3. Electrodes
4. Weak electrolyte
5. Strong electrolyte
6. Non-electrolyte
PERIOD 2: COMPARISON OF ELECTROLYTIC CELLS AND ELECTROCHEMICAL CELLS
AND PRINCIPLES OF ELECTROLYSIS

COMPARISON OF ELECTROLYTIC CELL AND ELECTROCHEMICAL CELL

ELETROLYTIC CELL ELETROCHEICAL CELL
1. Convert electrical energy to chemical Convert chemical energy to electrical
energy i.e. electric current is required energy i.e. chemical reaction produces
for the reaction to occur. electrical energy.
2. Electrons are forced or pushed by an Electrons are generated by oxidation at
outside source such as a battery. The an electrode (anode). The process is
process is non-spontaneous spontaneous
3. Cathode is the negative electrode Cathode is the positive electrode while
while anode is the positive electrode anode is the negative electrode
4. Electrodes are in the same Electrodes are in separate
compartment compartments
5. Salt bridge is not needed Salt bridge is required
6. Only one electrode is used Two electrodes are used

The table above gives the salient differences between an electrolytic cell and
electrochemical cell.
PRINCIPLES OF ELECTROLYSIS
Cations and anions
An electrolyte composed of positive and negative ions.
During electrolysis, electrons move from the negative pole of the battery to the
cathode, making it to be electron-rich and negatively charged. Hence, positive ions
migrate to the cathode; and are called cations. The anode becomes electron-poor and
positively charged. Hence, negatively charged ions migrate to the anode, and are
called anions.

PERIOD 3: FACTORS AFFECTING THE DISCHARGES OF IONS/ PRODUCT OF

ELECTROLYSIS
During electrolysis the ions in the electrolyte are positively and negatively charged.
The negatively charge ion will migrate to the negative charged cathode. In order to
know which of the ions will be preferentially discharged at the electrodes, the
following factors must be taken into consideration.
1. The position of ions in the electrochemical series

CATIONS ANIONES

K+ If two cations are present in If two anions are present in OH-

an electrolyte the cation electrolyte, the anions that is
Na+ below the other one in the above the other one in the I-
electrochemical series will be electrochemical series will be
Ca2+ preferentially discharged. preferentially discharged Br-

Mg2+ Cl-

Al3+

Zn2+

Fe2+ NO3

Sn2+

SO42-

Pb2+

H+
F-
Cu2+

Hg2+

Ag+

AU+

Pt+
ELECTROLYSIS OF SODIUM CHLORIDE SOLUTION USING INERT ELECTRODES
Note: Inert electrodes are the electrodes that will tamper with the electrolysis.
Examples are carbon (graphite) and platinum.

DIAGRAM

At the anode: Both OH- and Cl- will migrate to the anode where OH - ions will be
preferentially discharged because they are above Cl- in the electrochemical.
Cathodic half reaction: 4OH-(aq) → 2H2O(l) + O2(g) + 4e-

At the cathode: Both Na+ ions and H+ ions migrate to the cathode where H+ ions are
preferentially discharged because they are below Na+ ions in the electrochemical
series.
Anodic half reaction: 4H+(aq) + 4e- → 2H2(g)
Overall reaction: 4OH-(aq) + 4H+(aq) → 2H2(l) + O2(g) + 2H2(g).
Products of electrolysis: Hydrogen gas is produced at the cathode and oxygen gas is
produced at the anode.

Effect of electrolyte on the electrolysis: At the end of the electrolysis, the electrolyte
will more concentrated since OH- and H+ have been removed from the electrolyte.

2. CONCENTRATION OF IONS IN THE ELECTROLYTE e.g. electrolysis of concentrated

NaCl (aq) using inert electrodes. The electrolytic cell is the same as in number factor
above, but the electrolyte is concentrated NaCl(aq)
Ions in the electrolyte:
CATIONS ANIONS
Na+ OH-
H+ Cl-

At the anode: Both, Cl- and OH- will migrate to the anode where Cl - ions will be
preferentially discharged because they are more concentrated. (i.e they are more in
concentration inside the electrolyte).
Anodic half reaction: 2Cl-(aq) → Cl2(g) + 2e-
At the cathode: Both the Na+ and H+ will migrate to the cathode where H + will be
preferentially discharged. This is because even though Na + ions are more in
concentration in the electrolyte, the distance between Na + and H+ in the
electrochemical series is very far.
Cathodic half reaction: 2H+(aq) + 2e- → H2(g)

Overall reaction of electrolysis: Chlorine gas is produced at the anode and hydrogen
gas at the cathode.

Effect of electrolysis on the electrolyte

At the end of the electrolysis, the electrolyte will become alkaline. H+ and Cl - have
been removed from the electrolyte remaining Na+ and OH- which forms NaOH
solution.
3. EFFECT OF THE NATURE OF ELECTRODE e.g. Electrolysis of CuSO4 solution using
copper anode. The cathode can be any electrode e.g. platinum, graphite or even
copper.
The electrolysis cell is the same as above but the anode is copper and cathode is
graphite
Ions in the electrolyte:
CATIONS ANIONS
Cu2+ SO42-
H+ OH-
At the anode: Both SO42- and OH- will migrate to the anode but none of them will be
discharged. The reason is that, the anode (which is a copper metal) has the nature as
the metallic ions (copper (II) ions) in the electrolyte.
Cu (s) → Cu2+ (aq) + 2e-
(From anode)
At the cathode: Both the Cu2+ ions and H+ ions will migrate to the cathode where
Cu2+ ions will be preferentially discharged.
Cathode half reaction
Cu2+ (aq) + 2e- → Cu (s)
Product of electrolysis: Since the normal electrolysis did not take place, the anode
ionizes into the electrolyte. Copper metal get deposited on the surface of the
cathode and the cathode becomes bigger in size.
EVALUATION
1. Differentiate the following terms; Electrolysis, electrodes, anode, cathode,
ions, electrolytic cell, electrochemical cells.

PERIOD 4: ELECTROLYSIS OF ACIDIFIED WATER (DILUTE TETRAOXOSULPHATE (VI)

ACID).
The equation of the reaction that is involved in this process is given as shown below:

H2SO4 2H+ + SO42 - (1)

⇌ H+ + OH-
→

H 2O (2)
At the cathode: H+ ions migrate to the cathode and are reduced by gaining electrons
to become neutral hydrogen atoms.

H+(aq) + e- →
H(g) reduction.

The hydrogen atoms then combine in pairs to form diatomic hydrogen gas molecules

Hg + H(g) →
H2(g).

At the Anode: SO42- and OH- anode. The ions migrate to the anode. The OH- ions
being lower than SO42- in the electrochemical series are preferentially discharged.

OH-(aq) →
OH (g) + e-.

The OH groups interact to form water and oxygen molecules;

OH + OH →
H2O + O

O + O → O2

Overall half-equation

4OH- →
O2 + 2H20 + 4e-

The net result of the electrolysis is that two volumes of hydrogen are produced at
the cathode and one volumes of oxygen is produced at the anode. The migration of
SO42- ions to the anode and the discharge of the H + ions cause a decrease in the
concentration of H2SO4 acid around the anode disturbs the ionic equilibrium of
water. To reverse this, more water ionizes
H2O(L) ⇌ H+ + OH-(aq).
This produces an access of H + ions and with the incoming SO 42- ions, an increase in
the concentration of H2SO4 acid is obtained at the anode. The overall result of the
electrolysis is that the total amounts of the acid in the solution remain unchanged at
the end of the electrolysis. Since two volumes of hydrogen are obtained at the
cathode and one volume of oxygen is obtained at the anode, the entire process is
equivalent to the electrolysis of water.

ELECTROLYSIS OF COPPER (II) CHLORIDE USING COPPER – CARBON OR PLATINUM

ELECTRODE
A copper rod is used as the cathode and carbon rod as the anode. The equations of
the reaction involved were given as shown below:
CuCl2 (aq →
Cu2+ (aq) + 2Cl-(aq)

H2O(l) →
H+(aq) + OH-(aq)

When electric currents is passed through the electrolytic solution, Cu 2+ and H+ ions
migrate to the cathode, and Cl - with OH- migrate to the anode. At the cathode: Cu 2+
being lower than H+ in the electrochemical series is preferentially discharged. The
Cu2+ gains two electrons from the cathode; which are in turn deposited as neural
metallic copper at the cathode.

Cu2+(aq) + 2e- →
Cu (s)

At the Anode: Cl- for its higher concentration than OH - is preferentially discharged as
it loses an electron to the anode and becoming chlorine atoms.

Cl- (aq) →
Cl (g) + e-

The chlorine atoms then combine to form chlorine molecules. The discharge of
copper and chlorine causes the solution to become progressively dilute as evidenced
by the fading away of the light green colour of the electrolyte. If electric current is
passed continuously without adding mire electrolyte, at a stage, the hydroxyl ions
begin to discharge, because at this stage, the influence of concentration of Cl ion is
no longer the overall controlling factor. In the end, a mixture of chlorine and oxygen
is obtained at the anode.

ELECTROLYSIS OF CUSO4 USING COPPER –COPPER ELECTRODES (I.E COPPER AS

CATHODE AND COPPER AS ANODE)
The equation of the reaction of the electrolysis is given as shown below.

CuSO4 aq) →
Cu2+ + SO42-(aq)

H2O (l) →
H+ (aq) + OH-(aq)

In this case, because both the cathode and anode are made up of copper metal, at
anode three possible mechanisms occurs.
1. Discharge of SO42- ion by loss of electrons

SO42-(aq) →
SO4 (g) + 2e-

2. Discharge of OH- ions by loss of electrons

OH-(aq) →
OH(g) + e-

3. Copper metal loses electrons become Cu2+ (aq)

Anode : Cu (s) →
Cu2+ (aq) + 2e-

Cathode: Cu2+(aq + 2e- → Cu(s)

Out of these possibilities, (3) occurs most readily because it requires the least
energy. as a result, no ions are discharge at the anode. For each copper atom
deposited, at the cathode, one atom of copper is dissolved from the anode to form
Cu2+ ion in solution. Therefore, there is no change in the concentration of the
electrolyte. The electrolysis merely produces copper ions at the anode and deposits
same at the cathode. The colour of the electrolyte does not change.

ELECTROLYSIS OF CUSO4 SOLUTION USING COPPER – CARBON ELECTRODE

The electrolyte ionizes as follows

CuSO4 (aq) →
Cu2+ + SO42-(aq)

H2O(l) ⇌ H+(aq) + OH-(aq)

When electric current is passed through the electrolyte Cu 2+ and H+ ion migrate to
the cathode, and SO42-(aq) and OH- ions migrate to the anode. At cathode, Cu 2+(aq) + 2e-

→
Cu(s)

At anode: The OH- ions is discharged preferentially because it is lower in the

electrochemical series. Hence, oxygen is liberated at the anode.

OH-(aq) →
OH (g) + e-

4OH- →
H2O (l) + O2 ( g)

The solution gradually becomes acidic due to the presence of H + and SO42- ions left
behind in the solution. This is evident from the gradual fading away of the blue
colour of the electrolyte.

ELECTROLYSIS OF BRINE: Concentrated sodium chloride solution (ionic NaCl

solution) Note that in this electrolysis, the cathode can be either platinum carbon to
the anode must be carbon to resist attack by chloride.
The electrolysis ionizes thus:
NaCl (aq) Na+(aq) + Cl-(aq)

⇌
→

H2O (l) H+(aq) + OH-(aq)

When electric current is passed through the electrolytic solution Na + and H+ ions
migrate to the cathode, while Cl- and OH- ions move to the anode.
At Cathode: Na+ being lower than H+ in the electrochemical series is preferentially
discharged. The H+ gains electrons from the cathode and is in turn liberated at the
cathode

2H+ (aq) + 2e- →

H2(g)

At Anode: Cl- ions is preferentially discharged because its concentration is much

higher than that of OH- ions. The Cl- ion loses an electron to the anode and becomes
chloride atoms. The chlorine atoms then combine to form chlorine molecules.

Cl- →
Cl + e-

Cl(g) + Cl(g) →
Cl2(g).

The discharge of hydrogen ions and chloride ions causes the electrolyte to become
basic.

ELECTROCHEMICAL SERIES

Electrochemical series also known as reactivity series is an arrangement of the

standard electrolyte potentials of metal ions/metal half cells and those of non-
metal/non-metal half cells in order of starting from the most negative to the most
positive. Strongly electropositive series while strongly electronegative elements
(usually non-metals and oxidizing agents) are at the bottom of the series.

Importance of electrochemical series

1. Predicting the preferential discharge of ions in electrolysis. Metal down the

series is preferentially discharged to the one above it and the non-metal higher in
the series is discharged in preference to the ones below it.
2. Chemical reactivity of elements: metals in the series are arranged in decreasing
order of chemical reactivity except for calcium which is less reactive than sodium
but is above sodium.
3. Natural occurrence: metals above hydrogen in the series do not occur in the free
state instead, they exist as compounds e.g. chlorides, sulphides, oxides etc.
4. Displacement and redox reactions: the series shows at a glance the order in
which element will displace another. Metals will displace metals which are below
them I the series. For redox reactions, reducing power of elements decrease
down the reactivity series while their oxidizing power increases down the series