chapter 6 electrochemistry spm
TRANSCRIPT
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ELECTROCHEMISTRY
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CHEMICALSUBSTANCES
ELECTROLYTES NON-ELECTROLYTES
Substances that can conduct
electricity when they are inmolten state or aqueous and
undergo chemical changes
Substances that cannot
conduct electricity either inmolten state or aqueous
Example:
Ionic substances
(i) Dilute acid solution
(ii) Dilute alkaline solution
(iii) Molten salts
(iv) Aqueous salt solution
Example:
Covalent substances
(i) Naphthalene
(ii) Sugar solution
(iii) Latex
(iv) Hydrogen chloride in organic
solvents
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Heat
Molten lead (II) iodide
Bulb
Carbon
electrodes
Heat
Solid lead (II) iodide
Carbon
electrodes
Bulb
.. .. ... .
.... ... .
....
switchswitch
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Do not conductelectricity
This is because theions are held in afixed crystal lattice.
They are held withthe strong
electrostatic forcesof attraction
Do not freely move
I-
Pb2+
Pb2+
Pb2+
I-
I-
I-
I-
I-
Pb2+
Pb2+
Pb2+
Pb2+
Solid lead (II) iodide
IN A SOLID STATE
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Molten lead (II) iodide
I -Pb
2+
Pb2+ Pb
2+
I-
I-
Can conductelectricity
This is because thepresence of movingions. The ions are
free to move in themolten or aqueoussolution
IN A MOLTEN STATE
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ELECTROLYTES AND NONELECTROLYTES
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
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A process of breaking down the chemical
compounds into their constituent elements using
electric currentExample:
Molten aluminium oxide (I) aluminium (s) + oxygen (g)
Copper (II) chloride (aq) copper (s) + chlorine (g)
Electric current
Electric current
ELECTROLYSIS
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ELECTROLYTIC CELL
Electrolyte
e-
e- e-
e-
ElectrodeElectrode
- (cathode)+ (anode)
..
. .. .
.. . ..
...
- +
Anion Cation
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The set of apparatus needed to conductelectrolysis
It consists of a battery, an electrolyte, and twoelectrodes (anode and cathode)
Anode - the electrode which is connected to thepositive terminal of an electric source in theelectrolytic cell
Cathode - the electrode which is connected tothe negative terminal of an electric source in theelectrolytic cell
ELECTROLYTIC CELL
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ANODE
(POSITIVE ELECTRODE)
CATHODE
(NEGATIVE ELECTRODE)
Anions (negative ions)
will be pulled towards
the anode (positiveelectrode)
Cations (positive ions) will
be pulled towards the
cathode (negativeelectrode)
At the anode, anions
will be discharged by
releasing the electrons
to the anode
At the cathode, cations will
be discharged by
receiving the electrons
from the cathode
During electrolysis:
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ELECTROLYSIS OF MOLTENCOMPOUNDS
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QUESTIONS
(a) Molten zinc chloride (I)
(b) Molten aluminium (III) bromide
(c) Molten zinc (II) iodide
(d) Molten lead (II) oxide Electric current
Electric current
Electric current
Electric current
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Example 1
Molten Potassium
Iodide, KI
e-
e- e-
e-
Carbon
electrode Carbonelectrode
- (cathode)+ (anode)
..
. .. .
.. . ..
...
The ions that presents in electrolyte are:
(i) Anion:
(ii) Cation:
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Example 2
e-
e- e-
e-
Carbon
electrode Carbonelectrode
- (cathode)+ (anode)
..
. .. .
.. . ..
...
The ions that presents in electrolyte are:
(i) Anion:
(ii) Cation:
Molten zinc oxide,
ZnO
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Example 3
e-
e- e-
e-
Carbon
electrode Carbonelectrode
- (cathode)+ (anode)
..
. .. .
.. . ..
...
The ions that presents in electrolyte are:
(i) Anion:
(ii) Cation:
Molten lead (II)
bromide, PbBr2
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Example 4
e-
e- e-
e-
Carbon
electrode Carbonelectrode
- (cathode)+ (anode)
..
. .. .
.. . ..
...
The ions that presents in electrolyte are:
(i) Anion:
(ii) Cation:
Molten lead (II) oxide,
PbO
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Example 5
e-
e- e-
e-
Carbon
electrode Carbonelectrode
- (cathode)+ (anode)
..
. .. .
.. . ..
...
The ions that presents in electrolyte are:
(i) Anion:
(ii) Cation:
Molten zinc (II)
chloride, ZnCl2
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ELECTROLYSIS OF MOLTENCOMPOUND
e-
e- e-
e-
Carbonelectrode
Carbonelectrode
- (cathode)+ (anode)
..
. .. .
.. . ..
...
The ions that presents in electrolyte are:
(i) Anion:
(ii) Cation:
Molten lead (II)
bromide, PbBr2
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Molten lead (II)
bromide
e-
e-
e-
e-
Carbon
electrodeCarbon
electrode
- (cathode)+ (anode)
..
. .. .
.. . ..
...
Pb2+
Pb2+
Pb2+
Pb2+
Br-
Br-
Br-
Br-
ELECTROLYSIS OF MOLTENCOMPOUND
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Pb2+
are attracted to the cathodeBr-are attracted to anode
Br-discharge by releasing electrons to the
anode
CATHODE
(NEGATIVE ELECTRODE)
ANODE
(POSITIVE ELECTRODE)
Br-
Br + e-
Neutral atoms form and combine to formneutral bromine molecules
Br + Br Br2 (g)
The half reaction at anode can berepresented by the half equation
2Br-(l) Br2 (g) + 2e
-
Pb2+
discharge by receiving electrons
(provided by the battery) at the cathode
to form neutral lead atoms
Pb2+
(l) + 2e-
Pb (s)
The half reaction at cathode can be
represented by the half equation
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The Half Equation:
Cathode:
Anode:
Overall Equation:
2Br-(l) Br2 (g) + 2e
-
Pb2+(l) + 2e- Pb (s)
Pb2+
(l) + 2Br-(l) Pb(s) + Br2 (g)
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Write the half equation and the overallequation for the electrolysis of thesemolten compounds :
(A) Molten potassium iodide, KI(B) Molten zinc (II) chloride, ZnCl2
(C) Molten lead (II) oxide, PbO
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ELECTROLYSIS OF AQUEOUSSOLUTION
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:
Aqueous Solution
Ions present
Anion CationNaCl solution Cl- , OH
-Na
+, H
+
CuSO4 solution
HNO3 solution
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If they are more than one cations or anions
attracted, the choice of ion to be discharged
depends on :
Positions of the ionsin the electrochemical series
Types ofelectrodes
Concentration of ionsin the electrolyte
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Electrochemical Series (pg.101)
CATIONS
K+
Na+
Ca2+
Mg2+
Al3+
Zn2+
Fe2+
Sn
2+
Pb2+
H+
Cu2+
Ag+
Tendency to
discharge
increases
ANIONS
F-
SO42-
NO3-
Cl-
Br-
I-
OH-
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Positions of the ionsin the electrochemical series
Copper (II) chloride
solution, CuCl2
e-
e-
e-
e-
Carbon
electrodeCarbon
electrode
- (cathode)+ (anode)
H+
Cu2+
Cu2+
H+
Cl-
Cl-
OH-
OH-
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Positions of the ionsin the electrochemical series
Observation
and
Product
H+
, Cu2+
Cu2+
are preferred to be
discharged
(Cu2+
is below H+
in the ES)
OH-, Cl
-
OH-are preferred to be
discharged
(OH-is below Cl
-in the ES)
Ions that are
attracted
CathodeAnode
Bubbles of gas, O2
is releasedBrown solid, Cu is formed
Half equation4OH
-(aq) 2H2O (l) + O2 (g)
+ 4 e-
Cu2+
(aq) + 2e-
Cu (s)
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Concentration of ions inelectrolyte 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 isdetermined by the position of the ion in theE.S
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Concentration of ions inelectrolyte solutions
Concentrated
copper (II) chloride
solution, CuCl2
e-
e-
e-
e-
Carbon
electrodeCarbon
electrode
- (cathode)+ (anode)
Cu2+H+Cl
-OH
-
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Concentration of ions inelectrolyte solutions
Observation
and
Product
H+
, Cu2+
Cu2+
are preferred to bedischarged
(Cu2+
is below H+
in the ES)
OH-, Cl
-
Cl-
are preferred to bedischarged
(Cl-
is more concentrated)
Ions that are
attracted
CathodeAnode
Yellow bubbles gas, Cl2
is released
Brown solid, Cu is formed
Half equation 2Cl-(aq) Cl2 (g) + 2 e
-Cu
2+(aq) + 2e
-Cu (s)
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Types of Electrodes
Silver nitrate, AgNO3
solution
e-
e-
e-
e-
Silver plate Carbon
electrode
- (cathode)+ (anode)
Ag+Ag
Ag+
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Types of Electrodes
Ag+
(aq) + e-
Ag (s)
Observation
and
Product
Ag+
receives one electron to
form metal atoms
Ag atom releases one
electron to form Ag+
Ions
CathodeAnode
The silver plate corrodes Grey silver solid, Ag is
formed
Half equation Ag (s) Ag+
(aq) + e-
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Exercises
Electrolysis of copper (II) sulphate, CuSO4, solution
using carbon electrodes
Anode Cathode
Ions that areattracted
Ion that are
prefer to be
dischargedObservation
Product
Half equation
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Exercises
Electrolysis of concentrated potassium chloride, KCl,
solution using carbon electrodes
Anode Cathode
Ions that areattracted
Ion that are
prefer to be
discharged
Observation
Product
Half equation
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Exercises
Electrolysis of nickel (II) sulphate, NiSO4, solution
using nickel plate as anode and cathode
Anode CathodeHalf equation
Observation
Product
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Factors that influence electrolysis ofaqueous solutions
TYPES OF ELECTRODES
POSITION OF IONS
IN THE ELECTROCHEMICALSERIES
CONCENTRATION OF IONS
IN THE ELECTROLYTE SOLUTION
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USES OF ELECTROLYSIS
IN INDUSTRIES
Reacts with a carbonate
metal to form carbon
dioxide gas (CO2),water
(H2
O) and salt
ELECTROPLATINGOF METALS
EXTRACTION OF METALS
PURIFICATION OFMETALS
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Extraction of Metals
Electrolysis can be used to extract metals fromtheir ores
Reactive metals that are more reactive thanC such as (K, Na, Ca, Mg, Al) cannot be
extracted through heating of their metaloxides with C
They need to be extracted from their moltenores using the electrolysis process
In this process: Electrolyte The molten ore
Anode Carbon electrode
Cathode Carbon electrode
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Example:
Extraction of aluminium from electrolysis
of molten aluminium oxide, Al2O3
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Purification of Metals
Metals that have been extracted from theirores 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
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Example:
Purification of copper
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Electroplating of Metals
Many types of metals can be plated with othermetals 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
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Example:
Electroplating of a iron spoon with copper
metal
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Copper (II)sulphate
solution
ELECTROLYTIC CELL
e-
e- e-
e-
Carbon
electrode
Carbon
electrode
- (cathode) + (anode)
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VOLTAIC CELL
Cu
Copper (II)sulphate
V
e-
e-
e-
e-
- (anode) + (cathode)
Zn
Zn (s) Zn2+
(aq) + 2e-
Cu2+
(aq) + 2e-
Cu (s)
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VOLTAIC CELL
Also known as galvanic cell
Has two different metals which are immersedinto an electrolyte and connected by wire
Produces electrical energy from the chemicalreactions occurring inside the cell
Example : Daniell cell
Chemical energy electrical energy
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DANIELL CELL
V
DiluteH2SO4
(Salt bridge)
Zn (-) Cu (+)
CuSO4
solution
ZnSO4
solution
(Anode) (Cathode)
e-
e-
e-
e-
Zn (s) Zn2+
(aq) + 2e-
Cu2+
(aq) + 2e-
Cu (s)
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SALT BRIDGE
Can be made from any electrolyte that does notreact with the electrodes in Daniell Cell
Function :
(i) to allow the flow of the ions so the electriccurrent is completed
Example:
(i) diluted H2SO4
(ii) sodium nitrate, NaNO3 solution
(iii) Potassium chloride solution, KCl
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DANIELL CELL
V
PorouspotcontainsZnSO4
Zn (-) Cu (+)
CuSO4solution
(Anode)(Cathode)
e- e
-
e
- e-
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Reactivity Series
K
Na
Ca
Mg
AlZn
Fe
Sn
Pb
H
Cu
Ag
METALS MORE
ELECTROPOSITIVE
(negative terminal)
MORE
ELECTRONEGATIVE
(positive terminal)
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Anode and Cathode
Anode the electrode where the process ofdonation of electrons takes place
Cathode the electrode where the process of
receiving of electrons takes place
Negative terminal Cathode
Positive terminal - Anode
Negative terminal Anode
Positive terminal - Cathode
ELECTROLYTIC CELL VOLTAIC CELL
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ELECTROCHEMICAL SERIES
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Electrochemical Series (pg.101)
CATIONS
K+
Na+
Ca2+
Mg2+
Al3+
Zn2+
Fe2+
Sn2+
Pb2+
H+
Cu2+
Ag+
Tendency to
discharge
increases
ANIONS
F-
SO42-
NO3-
Cl-
Br-
I-
OH-
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ELECTROCHEMICALSERIES
An arrangement of metals, based on the
tendency of each metal atom to donate electrons
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The higher the position of a metal in theelectrochemical series:
ELECTROCHEMICAL SERIES
More electropositive The greater the tendencyof the metal atoms to
donate electrons
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PRINCIPLES DURING CONSTRUCTING
THE ELECTROCHEMICAL SERIES
Potential differences
between two metals in the
voltaic cell
Ability of a metal to
displace another metalfrom its salt solution
P t ti l diff b t t
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The value of the potential difference of a simple voltaic
cell is affected by the position of the metals in theelectrochemical series
When two different metals are immersed in an electrolytesolution, a potential difference is generated
The larger the value of the cell voltage is produced
The further the two metals in the electrochemical series
Potential differences between twometals in the voltaic cell
P t ti l diff b t t
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Potential differences between twometals in the voltaic cell
The metal that is moreelectropositive
Located higher in theelectrochemical series
Act as the negative terminal
The metal that is lesselectropositive
Located lower in theelectrochemical series
Act as the positive terminal
P t ti l diff b t t
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Potential differences between twometals in the voltaic cell
Mg
Zn
Pb
Cu
Ag
3.0 V
1.1 V 0.4 V
0.5 V
Predict the negative terminal, positive terminal and cellvoltage of the following voltaic cell
(a) Magnesium and silver (c) zinc and lead
(b) Magnesium and zinc (d) zinc and silver
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Metal Displacement
A metal that is more electropositive can displaceanother metal which is less electropositive fromits salt solution
This reactiondisplacement reaction
A metal that is located higher in the electrochemicalseries can displace another metal which is located
lower in the electrochemical series from its salt
solution
Metal Displacement
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Metal Displacement
Example:
Zinc, Zn can displace copper, Cu, from copper (II) nitrate, Cu(NO3)2solution
Zn (s) + Cu(NO3)2 (aq) Zn(NO3)2 (aq) + Cu (s)
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
Metaldisplaced
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IMPORTANCE OF THE
ELECTROCHEMICALSERIES
Reacts with a carbonate
metal to form carbon
dioxide gas (CO2),water
(H O) and salt
It can determined the terminals of voltaic cells
It can determined the
cell voltage for a pair of
metals
It can predicted the
potential of a metal to
displace another metal
from its solution
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