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Ionic Theory and Electrolysis

Effect of Electricity on Matter
The observations that certain substances allow electric current to pass through them,
while others do not, have been used to classify elements into conductors and nonconductors (insulators), and compounds into electrolytes and non-electrolytes
...
They are
metals, such as iron nail, magnesium wire, zinc rod and copper plate, and graphite(an
allotrope of carbon)
...

Non-conductors are elements that do not allow electricity to pass through them
...

Non-metals are poor conductors of electricity, since their atoms do not
possess mobile electrons; no flow of electrons within the atoms
...
Examples are silicon and germanium
...

Generally, dilute mineral acids such as HCl, HNO3 and H2SO4; alkalis –
NaOH and KOH; and ionic salts – NaCl and CuSO4 are electrolytes
...

Generally, covalent solids such as sugar (sucrose and glucose) and
paraffin wax; and liquids such as kerosene, petrol and alcohols are non-electrolytes
...
strong electrolytes: These are compounds which dissociate (ionize) completely
in solution to produce ions
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Strong acids like dilute hydrochloric acid and dilute tetraoxosulphate(VI) acid
b
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Salts like zinc tetraoxosulphate(VI); al sodium and potassium salts
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Weak electrolytes :These are compounds that do not dissociate (ionize)
completely in solution
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Trioxocarbonate(IV) acid
b
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Ethanoic acid
d
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Table: Differences between conductors and electrolytes
1
2
3

4
5

Conductor
A metal or graphite
Conducts electricity in solid state

Electrolyte
An ionic or polar compound
Conducts electricity when molten
or in solution
Electric current is carried by the flow of Electric current is carried by the
electrons (electronic conduction)
flow
of
ions
(electrolytic
conduction)
Transmits heat by conduction
Transmits heat by convention
Not decomposed by electricity and uses Decomposed by electric current
mobile electrons
and uses mobile ions

Arrhenius Ionic Theory

The ionic theory was first presented by Arrhenius in 1887 to explain electrolysis
...
The process of dissociation into ions is known as
ionization
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The
number of electrical charges carried by an ion is equal to the valency of the
corresponding atom or group
...


When we pass an electric current through an electrolyte, the free ions lose
their random movement
...
e
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The negative
ions move towards the anode (positive electrode) and are called anions (i
...
anode
ions)
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Arrhenius’ version of the ionic theory has been modified since X – ray
diffraction studies show that salts and strong alkalis consist of appositively charged

ions even in the solid state
...
Thus,
Arrhenius theory
Modern theory

NaCl(s)  Na+(aq) + Cl-(aq)
Na+Cl-(s)  Na+(aq)

+ Cl-(aq)

Although the second equation is a more accurate representation of what
takes place, for simplicity, we still use the first type to show the dissociation of ionic
compounds in solution
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g
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They do so
by reacting with water or other solvents to form hydroxonium ions, H3O+, or other
ions
...


Before the passage of current, the ions move about randomly in the electrolyte
...
The negatively
charged cathode then attracts cations in the electrolyte to itself
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The positive
terminal (cathode) of the battery draws electrons from the anode of the electrolytic
cell
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Hence, an electric current passes through the complete circuit
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Oxidation occurs at the anode where the anions lose electrons
...
The cathodic half-reaction which
takes place simultaneously at the cathode is a reduction reaction since the cations

gain electrons here
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As a result of the chemical reactions that occur during electrolysis, certain
products are formed at each electrode
...


Preferential discharge of ions
During the electrolysis of a given electrolyte, the products formed at the electrodes
depend on the nature of the electrolyte
...
In such a case, the cations and anions of both the
electrolyte and the solvent will migrate to the cathode and anode respectively where
they will compete with one another to be discharged
...

When two or more ions of the same charge are present in a solution of an
electrolyte, under identical conditions, and are competing for discharge at the same
electrode, one of them gets preferentially discharged
...

Factors affecting Preferential/selective discharge of ions: These are
1
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Relative concentrations of the ions in the electrolyte or ionic concentration
3
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If all other factors are constant, a cation which is lower in the series (less
electropositive) will show a greater tendency to be discharged than another which is
higher up (more electropositive)
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For example, K+, Na+ and Ca2+ are

never discharged at all from aqueous solution since H + or other ions will these
strongly electropositive elements be discharged
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An anion which is higher in the series (less electronegative) is discharged in
preference to another which is lower down the series (more electronegative) as the
former loses electrons more readily
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2
...
The influence of
concentration, however, is effective only when the two competing ions are
closely positioned in the electrochemical series
...

We can illustrate this by considering what happens during the electrolysis of a
concentrated solution of sodium chloride
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At the cathode H+ are preferentially discharged although the concentrations of Na+
is far greater
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At the anode
Cl- are preferentially discharged
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3
...
Platinum and carbon (graphite) usually
behave as inert electrodes, although platinum is attacked by liberated chlorine
and carbon by liberated oxygen
...
For example,
in the electrolysis of aqueous sodium chloride using platinum electrodes, H +
are preferentially discharged
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Na+(aq) + Hg(l) + e-  Na/Hg(l)
H+(aq) + e-  H;
H + H
H2(g)
Other electrodes may also influence the ionic discharge because they possess
characteristics similar to those of the ion in the electrolyte
...
Electrolysis of acidified water: Acidified water is made by adding a few
drops of tetraoxosulphate(VI) acid to water
...

The electrolytic cell used is designed for the collection of gaseous
products at the two electrodes
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Acidified water contains the following ions:
H2SO4
H2O
+
Cations
2H (aq)
H+(aq)
Anions
SO42-(aq)
OH-(aq)
Concentration of ions
High
Low
Position in electrochemical
Series

OH- are higher in the series than
SO42-

At the cathode: The H+, which are also the only cations present, migrate to the
cathode where they acquire an electron each to form neutral hydrogen atoms
...
Each OH- loses its electron to the anode to become a neutral – OH group
...

Gaseous oxygen is produced when the oxygen atoms combine in pairs to form
diatomic molecules
...
e
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The total amount of acid in the solution remains unchanged at the end of the
electrolysis, but its concentration increases due to the removal of water molecules
...
Electrolysis of copper(II) tetraoxosulphate(VI) solution (using
different electrodes) The electrolytic cell used is designed to collect solid
copper deposits at the cathode and a gas at the anode
...

Series
OH- are higher in the series than SO42At the cathode: Both Cu2+ and H+ migrate to the cathode where Cu2+ are
discharged preferentially
...
This reaction is the same whether a platinum,
carbon or copper cathode is used
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Here, the positions of the ions in
the series are more than their concentrations
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As the Cu2+ and OH- are gradually discharged, the solution becomes acidic
with tetraoxosulphate(VI) acid due to the H+ and SO42-, which remains in the
solution
...
If we continue to pass a current through the
solution, subsequent electrolysis of this dilute acid will occur, yielding hydrogen at
the cathode and oxygen at the anode
...
Electrolysis of copper(II) tetraoxosulphate(VI) solution (using copper
anode)

At the anode(copper): if a copper rod is used as the anode, electrons can be supplied
through the anode to the external circuit in three ways:
 The SO42- ions in the solution could be deposited as salt, i
...
sulphation of the
anode
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4OH-(aq)  2H2O(l) + O2(g) + 4e-

 The metallic atoms on the anode surface might dissolve in the solution as ions,
leaving their electrons behind
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Instead, for each atom of copper
deposited at the cathode, one atom of copper is dissolved from the anode to
form Cu2+
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Anode
electrolyte
cathode
-2e-

Cu(s)

Cu2+(aq)

+2e-

Cu(s)

What are the products of the electrolysis of molten aluminium oxide, aqueous copper
sulphate solution, aqueous sodium chloride solution (brine), hydrochloric acid,
sulphuric acid, molten lead(II) bromide, molten calcium chloride? add overall
equations



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