Electrochemistry
5th Year Chemistry
Electrical Conduction
• In order for a substance to conduct an
electric current, it must contain charged
particles that are able to move through
the substance.
• These charged particles can be either
electrons or ions.
• Materials may be classified into two
groups based on their ability to conduct
an electric current.
Conductors
and NonConductors
Metallic Conduction
• This is conduction in a metal.
• In a metal, the valence electrons are delocalised (
free to move around the metal lattice). These
delocalised electrons, known as mobile electrons are
able to move throughout the metal.
• Metals are able to conduct an electric current
because these mobile electrons can move through
the metal.
• Graphite is a non-metal but it is able to conduct an
electric current because of the presence of
delocalised, mobile electrons.
Electrolytic Conduction
• When the ionic compound melts, the ions are no
longer held together by ionic bonds and they become
free to move throughout the lattice.
•
NaCl(s) → Na+ (l) + Cl –(l)
• Similarly, When an ionic compound dissolves in
water, the ionic bonds break and the ions move
throughout the solution.
•
NaCl (s) → Na+ (aq) + Cl- (aq)
• These electrolytes conduct an electric current
because of the presence of mobile ions which are
able to move throughout the liquid or aqueous
solution.
• During electrolytic conduction, the electrolyte is
decomposed. i.e it is chemically changed.
Metallic
Conduction vs
Electrolytic
Conduction
WHAT IS
ELECTROLYSIS?
ELECTROLYSIS
Electrolysis is the chemical change
which occurs when an electric current is
passed through an electrolyte.
During electrolysis, the electrolyte is
decomposed into simpler substances.
THE ELECTROLYTIC CELL
• An electrolytic cell has three main components
• A battery or a d.c power supply that provides
the electric current.
• Two electrodes connected via wires to the
battery or power supply. Electrodes take the
electric current into and out of the electrolyte.
Therefore, they must be able to conduct an
electric current and are usually made of inert
material such as graphite or platinum.
• The electrolyte – which is a molten ionic
compound or a solution containing mobile ions.
What happens during
Electrolysis?
• During electrolysis, both anions and cations are
discharged, i.e they lose or gain electrons to form
neutral atoms. This occurs as follows:
• The anions (negative ions) are attracted to the anode
(positive electrode). The anions lose electrons to the
anode and form atoms, i.e they are discharged.
A n- → A + ne –
Oxidation occurs at the anode (OIL).
The anode acts as the oxidising agent.
What happens during
Electrolysis?
The electrons lost by the anions
at the anode, travel through the
circuit to the positive terminal of
the battery. They then re-enter
the circuit from the negative
terminal of the battery and travel
to the cathode.
What happens during
Electrolysis?
• The cations (positive ions) are attracted to
the cathode (negative electrode). The
cations gain the electrons from the cathode
forming atoms i.e they are discharged.
• C n+ + ne - → C
• Reduction occurs at the cathode (RIG) .
The cathode acts as the reducing agent.
Electrolysis of Molten
Electrolytes
• Electrolysis of molten Sodium chloride, NaCl
At the anode:
Cl – (l) → Cl (g) + e2Cl – (l) → Cl2 (g) + 2eAt the cathode:
Na + (l) + e - → Na (l)
Observations: green yellow chlorine gas
evolved at the anode, Sodium metal
formed at the cathode.
Electrolysis of Molten
Electrolytes
• Electrolysis of molten lead (II) bromide, PbBr
At the anode:
Br – (l) → Br (g) + e2Br – (l) → Br2 (g) + 2eAt the cathode:
Pb 2+ (l) + 2e - → Pb(l)
Observations: brown bromine gas evolved at
the anode, Lead metal formed at the cathode.
ELECTROLYSIS OF
AQUEOUS
ELECTROLYTES
THE ELECTROCHEMICAL SERIES
• If an aqueous electrolyte is used during
electrolysis, then several ions will be
attracted to particular electrode.
• Only one type of ion can be discharged
at a time.
• The ion discharged depends on its
position in the electrochemical series.
THE ELECTROCHEMICAL SERIES
• The electrochemical series is a
listing that places ions in the
order of ease of electrical
discharge.
• The lower an element is in the
series, the easier it is to
discharge.
• When one ion is discharged in
favour to another, preferential
discharge is said to have occurred.
LEARNING THE SERIES
Predicting the
Products of
Electrolysis of
Aqueos
Electrolytes
ELECTROLYSIS
OF AQUEOUS
SOLUTIONS
USING INERT
ELECTRODES
Apparatus Used for the Electrolysis of
Aqueous Electrolytes
Electrolysis of Dilute Sulphuric Acid
Electrolysis of Dilute Sulphuric Acid
• At the cathode: 2H+ + 2e- → H2
• At the anode: 4OH- → O2 + 2H2O + 4e• Colourless gases are produced at each electrode. The volume of
hydrogen produced at the cathode is twice the volume of oxygen
produced at the anode.
• The electrolyte becomes more concentrated as water molecules are
removed during electrolysis
•
How does the
concentration of the
halide ions affect the
product of electrolysis of
their aqueous salts?
Electrolysis of Concentrated Sodium chloride
solution ( BRINE)
Electrolysis of Concentrated Sodium Chloride
• The concentration of the halide solution affect the reaction at the
anode only.
• Cl- is discharged in preference to OH- even though OH- is lower in the
electrochemical series.
• This occurs in cases where both anions are close together on the
electrochemical series
• The large number of Cl- ions give it the advantage over OH-.
• Concentration is an important factor to consider in the electrolysis of
aqueous solutions of halides e.g. HI, HCl, CuCl2 and NaBr.
ELECTROLYSIS OF AQUEOUS
SOLUTIONS USING ACTIVE
ELECTRODES
Electrolysis of Copper (II) sulphate
using copper (active) electrodes
• At the anode:
Cu (s) → Cu 2+ (aq) + 2eThe copper electrode loses electrons
(becomes oxidized) in preference to the OHand SO42- ions. They remain in solution.
• At the cathode:
Cu 2+ (aq) + 2e- → Cu (s)
The Cu2+ ions in solution become
preferentially discharged and Copper is
deposited on the cathode.
Electrolysis of Copper(II)
sulphate using copper
electrodes.
The cathode increases in mass while the anode decreases
• This occurs as copper atoms are oxidised at the anode
and form ions while copper ions are reduced at the
cathode, forming copper atoms
• The gain in mass by the negative electrode is the same as
the loss in mass by the positive electrode
• Therefore the copper deposited on the negative
electrode must be the same copper ions that are lost from
the positive electrode
• That implies that the concentration of the Cu2+ ions in the
solution remains constant
Electrolysis of
Copper (II) Sulphate