Chapter 6 electrochemistry

CHEMICAL
SUBSTANCES

ELECTROLYTES NON-ELECTROLYTES

Substances that can conduct Substances that cannot
electricity when they are in conduct electricity either in
molten state or aqueous and molten state or aqueous
undergo chemical changes
Example:
Example:
Covalent substances
Ionic substances
(iii) Naphthalene
(iii) Dilute acid solution
(iv) Sugar solution
(iv) Dilute alkaline solution
(v) Latex
(v) Molten salts
(vi) Hydrogen chloride in organic
(vi) Aqueous salt solution solvents

switch
switch

Carbon
Bulb Bulb
electrodes Carbon
electrodes

…. …. ….
….
….
…. ..
… ………..
….
…………. …. …. ….
…. …. ….
….
… ……….

Heat
Heat

Solid lead (II) iodide Molten lead (II) iodide

IN A SOLID STATE
•Do not conduct
electricity
Pb 2+ I- Pb 2+ I-
•This is because the
Pb 2+ I- Pb 2+ I- Pb 2+
ions are held in a
Pb 2+ I- Pb 2+ I- fixed crystal lattice.

Solid lead (II) iodide
•They are held with
the strong
electrostatic forces
of attraction
•Do not freely move

IN A MOLTEN STATE

Pb 2+ Pb 2+
-
•Can conduct
I
I - electricity
I-
Pb 2+ •This is because the
presence of moving
ions. The ions are
free to move in the
molten or aqueous
solution
Molten lead (II) iodide

ELECTROLYTES AND NON
ELECTROLYTES
Classify the following chemicals into electrolytes or non-
electrolytes

 Potassium chloride, KCl solution
 Molten lead (II) bromide, PbBr2
 Glucose solution
 Trichloromethane
 Molten zinc oxide, ZnO
 Sodium chloride, NaCl solution
 Ammonia in tetrachloromethane

ELECTROLYSIS

A process of breaking down the chemical
compounds into their constituent elements using
electric current
Example: Electric current
Molten aluminium oxide (I) aluminium (s) + oxygen (g)
Electric current
Copper (II) chloride (aq) copper (s) + chlorine (g)

ELECTROLYTIC CELL
e- e-

-
e e-
+ (anode) - (cathode)
Electrode Electrode
…… ……………… …..
…. ….. …… … ….
.. . - …….. …… …
+ …
Electrolyte
……………………………………...

Anion Cation

ELECTROLYTIC CELL
 The set of apparatus needed to conduct
electrolysis
 It consists of a battery, an electrolyte, and two
electrodes (anode and cathode)
 Anode - the electrode which is connected to the
positive terminal of an electric source in the
electrolytic cell
 Cathode - the electrode which is connected to
the negative terminal of an electric source in the
electrolytic cell

 During electrolysis:
ANODE CATHODE
(POSITIVE ELECTRODE) (NEGATIVE ELECTRODE)
Anions (negative ions) Cations (positive ions) will
will be pulled towards be pulled towards the
the anode (positive cathode (negative
electrode) electrode)

At the anode, anions At the cathode, cations will
will be discharged by be discharged by receiving
releasing the electrons the electrons from the
to the anode cathode

ELECTROLYSIS OF MOLTEN
COMPOUNDS

QUESTIONS
(a) Molten zinc chloride (I) Electric current
(b) Molten aluminium (III) bromide Electric current
(c) Molten zinc (II) iodide Electric current
(d) Molten lead (II) oxide Electric current

Example 1
e- e-

-
e e-
Carbon Carbon
electrode …… ……………… ….. electrode
…. ….. …… … ….
.. . …….. …… … …
Molten Potassium
……………………………………... Iodide, KI

The ions that presents in electrolyte are:
(ii) Anion:
(iii) Cation:

Example 2
e- e-

-
e e-
Carbon Carbon
…. ….. …… … ….
.. . …….. …… … …
……………………………………... Molten zinc oxide,
ZnO

(ii) Anion:
(iii) Cation:

Example 3
e- e-

-
e e-
Carbon Carbon
…. ….. …… … ….
.. . …….. …… … …
Molten lead (II)
……………………………………... bromide, PbBr2

(ii) Anion:
(iii) Cation:

Example 4
e- e-

-
e e-
Carbon Carbon
…. ….. …… … ….
.. . …….. …… … …
Molten lead (II) oxide,
……………………………………... PbO

(ii) Anion:
(iii) Cation:

Example 5
e- e-

-
e e-
Carbon Carbon
…. ….. …… … ….
.. . …….. …… … …
……………………………………... Molten zinc (II)
chloride, ZnCl2

(ii) Anion:
(iii) Cation:

COMPOUND
e- e-

-
e e-

Carbon Carbon
…. ….. …… … ….
.. . …….. …… … …
Molten lead (II)
……………………………………... bromide, PbBr2

(ii) Anion:
(iii) Cation:

COMPOUND

e- e-

e - e-

Carbon Carbon
Br- Pb2+
…. Br - ….. …… Pb
… 2+
….
-
Br…….. Pb2+
.. . ……Pb … 2+
… Molten lead (II)
Br-
……………………………………... bromide

ANODE CATHODE
(POSITIVE ELECTRODE) (NEGATIVE ELECTRODE)

Br- are attracted to anode Pb2+ are attracted to the cathode

Br- discharge by releasing electrons to the
anode Pb2+ discharge by receiving electrons
Br- Br + e-
(provided by the battery) at the cathode
to form neutral lead atoms

Neutral atoms form and combine to form
neutral bromine molecules
The half reaction at cathode can be
Br + Br Br2 (g) represented by the half equation

Pb2+(l) + 2e- Pb (s)

The half reaction at anode can be
represented by the half equation

2Br- (l) Br2 (g) + 2e-

The Half Equation:

2+ -
Cathode: Pb (l) + 2e Pb (s)
Anode: 2Br- (l) Br2 (g) + 2e-

Overall Equation:

Pb2+(l) + 2Br- (l) Pb(s) + Br2 (g)

Write the half equation and the overall
equation for the electrolysis of these
molten compounds :

(A) Molten potassium iodide, KI
(B) Molten zinc (II) chloride, ZnCl2
(C) Molten lead (II) oxide, PbO

ELECTROLYSIS OF AQUEOUS
SOLUTION
Water molecules in an aqueous solution can slightly dissociate to
produce H+ and OH-
H2O H+ (aq) + OH-(aq)

So, aqueous solution contains H+ and OH- and electrolyte ions
Examples:

Ions present
Aqueous Solution Anion Cation
NaCl solution Cl- , OH- Na+ , H+
CuSO4 solution
HNO3 solution

If they are more than one cations or anions
attracted, the choice of ion to be discharged
depends on :

Positions of the ions
in the electrochemical series

Concentration of ions
in the electrolyte

Types of
electrodes

Electrochemical Series (pg.101)
CATIONS ANIONS

K+
F-
Na+
Ca2+ SO42-
Mg2+ NO3-
Al 3+
Cl-
Zn2+
Fe2+ Br-
Sn2+ I-
Pb2+ OH-
H+
Tendency to
Cu2+ discharge
Ag+ increases


e- e-

e - e-

Carbon Carbon
electrode electrode
OH- Cu2+
Cl- H+

OH-
Cu2+
Cl - H+ Copper (II) chloride
solution, CuCl2

Anode Cathode

Ions that are OH- , Cl- H+ , Cu2+
attracted OH- are preferred to be Cu2+are preferred to be
discharged discharged
(OH- is below Cl- in the ES) (Cu2+ is below H+ in the ES)
Observation
Bubbles of gas, O2
and Brown solid, Cu is formed
Product is released

4OH- (aq) 2H2O (l) + O2 (g)
Half equation Cu2+ (aq) + 2e- Cu (s)
+ 4 e-

Concentration of ions in
electrolyte solutions
 Concentration of ions in electrolyte solution also
can affects the choice of ions to be discharged
 An ion that is more concentrated is preferably
discharged
 Anode – the ions to be discharged is
determined by the concentration of ions
 Cathode – the ions to be discharged is
determined by the position of the ion in the
E.S


e- e-

-
e e-

Carbon Carbon
electrode electrode

OH-
Cu2+
Cl - H+ Concentrated
copper (II) chloride
solution, CuCl2

Anode Cathode

Ions that are OH- , Cl- H+ , Cu2+
attracted
Cl- are preferred to be Cu2+are preferred to be
discharged discharged
(Cl- is more concentrated) (Cu2+ is below H+ in the ES)
Observation Brown solid, Cu is formed
Yellow bubbles gas, Cl2
and
Product is released
Half equation 2Cl- (aq) Cl2 (g) + 2 e- Cu2+ (aq) + 2e- Cu (s)

Types of Electrodes

e- e-

-
e e-

Silver plate Carbon
electrode

Ag+ Ag+ Ag Silver nitrate, AgNO3
solution

Types of Electrodes
Anode Cathode

Ions
Ag atom releases one Ag+ receives one electron to
electron to form Ag+ form metal atoms

Observation
and The silver plate corrodes Grey silver solid, Ag is
Product formed

Half equation Ag (s) Ag+ (aq) + e- Ag+ (aq) + e- Ag (s)

Exercises
 Electrolysis of copper (II) sulphate, CuSO4, solution
using carbon electrodes
Anode Cathode
Ions that are
attracted
Ion that are
prefer to be
discharged
Observation
Product
Half equation

Exercises
 Electrolysis of concentrated potassium chloride, KCl,
solution using carbon electrodes

Anode Cathode
Ions that are
attracted
Ion that are
prefer to be
discharged
Observation
Product
Half equation

Exercises
 Electrolysis of nickel (II) sulphate, NiSO4, solution
using nickel plate as anode and cathode

Anode Cathode
Half equation
Observation
Product

Factors that influence electrolysis of
aqueous solutions

POSITION OF IONS
IN THE ELECTROCHEMICAL
SERIES

CONCENTRATION OF IONS
IN THE ELECTROLYTE SOLUTION

TYPES OF ELECTRODES

EXTRACTION OF METALS

USES OF ELECTROLYSIS
IN INDUSTRIES

Reacts with a carbonate
metal to form carbon OF
PURIFICATION ELECTROPLATING
dioxide gas (CO2),water
METALS OF METALS
(H2O) and salt

Extraction of Metals
 Electrolysis can be used to extract metals from
their ores
 Reactive metals that are more reactive than C
such as (K, Na, Ca, Mg, Al) cannot be
extracted through heating of their metal
oxides with C
 They need to be extracted from their molten
ores using the electrolysis process
 In this process:
Electrolyte The molten ore
Anode Carbon electrode
Cathode Carbon electrode

Example:
 Extraction of aluminium from electrolysis
of molten aluminium oxide, Al2O3

Purification of Metals
 Metals that have been extracted from their
ores are normally not pure. They contains
impurities which need to be removed
 These metals can be purified by electrolysis
process
 In the purification process
Electrolyte The solution that contains
the metal ions
Anode The impure metal plate
Cathode The pure metal plate

Example:
 Purification of copper

Electroplating of Metals
 Many types of metals can be plated with other
metals through electrolysis.
 The aim of metal plating through electrolysis
includes:
(i) making the metal more resistant to corrosion
(ii) making the metal appear more attractive
 In the electroplating process

Electrolyte The solution which contains
ions of plating metal
Anode Plating metal
Cathode Metal to be plated

Example:
 Electroplating of a iron spoon with copper
metal

ELECTROLYTIC CELL
e- e-

e- e-
- (cathode) + (anode)
Carbon
Carbon
electrode
electrode

Copper (II)
sulphate
solution

VOLTAIC CELL
e- e-
V
e- e-
- (anode) + (cathode)
Zn

Cu

Copper (II)
sulphate

Zn (s) Zn2+ (aq) + 2e- Cu2+ (aq) + 2e- Cu (s)

VOLTAIC CELL
 Also known as galvanic cell
 Has two different metals which are immersed
into an electrolyte and connected by wire
 Produces electrical energy from the chemical
reactions occurring inside the cell
Chemical energy electrical energy

 Example : Daniell cell

DANIELL CELL
e- e-
V
e- e-
Zn (-) Cu (+)
(Anode) (Cathode)

Dilute H2SO4
(Salt bridge)

ZnSO4 solution
CuSO4
solution
Zn (s) Zn2+ (aq) + 2e- Cu2+ (aq) + 2e- Cu (s)

SALT BRIDGE
 Can be made from any electrolyte that does not
react with the electrodes in Daniell Cell
 Function :

(i) to allow the flow of the ions so the electric
current is completed
 Example:

(i) diluted H2SO4
(ii) sodium nitrate, NaNO3 solution
(iii) Potassium chloride solution, KCl

DANIELL CELL
e- e-
V
e- e-
Zn (-) Cu (+)
(Cathode)
(Anode)

CuSO4
solution
Porous
pot
contains
ZnSO4

Reactivity Series
METALS
MORE
ELECTROPOSITIVE
K
Na (negative terminal)
Ca
Mg
Al
Zn
Fe
Sn
Pb MORE
ELECTRONEGATIVE
H
Cu (positive terminal)
Ag

Anode and Cathode
 Anode – the electrode where the process of
donation of electrons takes place
 Cathode – the electrode where the process of
receiving of electrons takes place

ELECTROLYTIC CELL VOLTAIC CELL

Negative terminal – Cathode Negative terminal – Anode
Positive terminal - Anode Positive terminal - Cathode

ELECTROCHEMICAL
SERIES

An arrangement of metals, based on the
tendency of each metal atom to donate electrons

ELECTROCHEMICAL SERIES

The higher the position of a metal in the
electrochemical series:

More electropositive The greater the tendency
of the metal atoms to
donate electrons

PRINCIPLES DURING CONSTRUCTING
THE ELECTROCHEMICAL SERIES

Potential differences Ability of a metal to
between two metals in the displace another metal
voltaic cell from its salt solution

Potential differences between two
metals in the voltaic cell
When two different metals are immersed in an electrolyte
solution, a potential difference is generated

The value of the potential difference of a simple voltaic
cell is affected by the position of the metals in the
electrochemical series

The further the two metals in the electrochemical series

The larger the value of the cell voltage is produced


The metal that is more
electropositive

Located higher in the Act as the negative terminal

The metal that is less
electropositive

Located lower in the Act as the positive terminal

Mg

Zn
3.0 V
Pb
1.1 V
0.4 V
Cu
0.5 V
Ag

Predict the negative terminal, positive terminal and cell
voltage of the following voltaic cell
(b) Magnesium and silver (c) zinc and lead
(c) Magnesium and zinc (d) zinc and silver

Metal Displacement

A metal that is more electropositive can displace
another metal which is less electropositive from
its salt solution

A metal that is located higher in the electrochemical
series can displace another metal which is located
lower in the electrochemical series from its salt
solution

This reaction – displacement reaction

Metal Displacement

Example:
Zinc, Zn can displace copper, Cu, from copper (II) nitrate, Cu(NO3)2
solution
Zn (s) + Cu(NO3)2 (aq) Zn(NO3)2 (aq) + Cu (s)

Metal
displaced
• Zn is more electropositive than copper
• Zn can displace Cu from copper (II) nitrate, Cu(NO3)2 solution
• The zinc, Zn, will dissolve and brown copper solid, Cu, will form

It can determined the terminals of voltaic cells

IMPORTANCE OF THE
ELECTROCHEMICAL
SERIES

Reacts with a carbonate
It can determined the It can predicted the
metal voltage carbon
cell to form for a pair of potential of a metal to
dioxide gasmetals
(CO2),water displace another metal
(H2O) and salt from its solution

Chapter 6 electrochemistry

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