4541 CHEMISTRY Chapter 6 Chapter 6 : ELECTROCHEMISTRY A Understanding properties of Electrolytes and Non- electrolytes. Three classes of substance that can be made based on electrical conductivity : Type of substance Characteristic Example Conductors Substances that can conduct electrical current in solid or liquid state without any chemical changes. [normally metals and carbon ] Electrolytes Chemical compounds that can conduct electricity only in molten or aqueous solution state and undergo chemical changes , but cannot conduct electricity in solid state. [normally ionic compound , acid and alkali ] Non- electrolytes Chemical compounds that cannot conduct electricity in molten, aqueous solution and solid states; normally covalent compound . ☺What are electrolytes? Electrolytes are substances that …………………………………… electricity either in the ………………………… state or ………………… solution and undergo chemical changes. Activity 1:- 1 Classify substances given below into electrolytes and non- electrolytes. Tetrachloromethane, CCl 4 molten aluminium oxide, Al 2 O 3 sulphuric acid solution, H 2 SO 4 glucose solution, C 6 H 12 O 6 molten naphthalene, C 10 H 8 Chapter 6 Electrochemistry 1
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4541 CHEMISTRY Chapter 6
Chapter 6 : ELECTROCHEMISTRY
A Understanding properties of Electrolytes and Non-electrolytes.
Three classes of substance that can be made based on electrical conductivity :
Type of substance Characteristic Example
Conductors
Substances that can conduct electrical current in solid or liquid state without any chemical changes.[normally metals and carbon]
Electrolytes
Chemical compounds that can conduct electricity only in molten or aqueous solution state and undergo chemical changes, but cannot conduct electricity in solid state.[normally ionic compound, acid and alkali]
Non-electrolytesChemical compounds that cannot conduct electricity in molten, aqueous solution and solid states; normally covalent compound.
☺What are electrolytes?
Electrolytes are substances that …………………………………… electricity either in the
………………………… state or ………………… solution and undergo chemical changes.
Activity 1:-
1 Classify substances given below into electrolytes and non-electrolytes.
In solid copper(II) sulphate, these ions …… ……… move freely but are held in fixed
positions in a lattice.
In copper(II) sulphate solution, these ions are free to move.
3. During electrolysis of copper(II) sulphate solution, the ………………… ions move to the
………… whereas the ………………… ions move to the ………………..
At the cathode :
Cu2+ ions and H+ ions move to the cathode.
Cu2+ ions are selectively* discharged.
Each of Cu2+ ions accepts two electrons to form a neutral copper atom.
Half equation :
Cu2+ + 2e → Cu
………….. metal is formed.
At the anode :
SO42- ions and OH- ions move to the anode.
OH- ions are selectively* discharged by donating electrons.
Half equation :
4OH- → 2H2O + O2 + 4e
Oxygen gas is formed.
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4541 CHEMISTRY Chapter 6
Activity 5 :-
1. Write the formulae for the ions present in the following aqueous solution. Then identify which ions are selectively* discharged at the anode and cathode respectively during electrolysis. [ Inert electrodes such as carbon and platinum are used]
Aqueous solutionIons present in the
solution
Ions that are selectively discharge
Anode Cathode
Aqueous Zinc sulphate, ZnSO4(aq)
Dilute nitric acid, HNO3(aq)
Aqueous potassium hydroxide, KOH(aq)
Aqueous silver nitrate, AgNO3(aq)
0.1 mol dm-3 hydrochloric acid, HCl(aq)
2.0 mol dm-3 hydrochloric acid, HCl(aq)
2. Electrolysis of dilute sulphuric acid, H2SO4 using carbon electrodes :
Sulphuric acid, H2SO4 consist of ………………….. ions, …………………. ions and
………………….. ions.
During electrolysis, the ………………… ions move to the cathode whereas the
………………… ions and ………..……… ions move to the anode.
At the cathode :
……………………….. ions undergo discharged.
Each of ……………. ion accepts ………… electron to form a neutral
hydrogen atom.
Half equation :
H+ + e → H
……….. hydrogen atoms combine to form a hydrogen ………………...
Half equation :
2H+ + 2e → H2
…………………. gas is released.
At the anode :
SO42- ions and OH- ions move to the anode.
OH- ions are selectively discharged by donating electrons.
Half equation :
4OH- → 2H2O + O2 + 4e
Oxygen gas is formed.
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4541 CHEMISTRY Chapter 6
3. Electrolysis of 0.1 mol dm-3 silver nitrate solution, AgNO3 using carbon electrodes :
Silver nitrate solution, AgNO3 (aq) consist of ………………….. ions,
…………………. ions, …………………. ions and ………………….. ions.
Electrode Cathode (-) Anode (+)
Ions that are attracted to //Ions that are move to
Ions that are selectively discharge at
Reason
Half equation at
Substances formed at //Products at
Observation at
4. Write half equations to represent the discharge of ions at the anode and the cathode for each electrolyte and then, predict the products formed.
ElectrolysisHalf equations Products
Anode (+) Cathode (-) Anode Cathode
(a) Electrolysis of dilute zinc chloride, ZnCl2 aqueous solution using carbon electrodes
(b) Electrolysis of dilute lead(II) nitrate, Pb(NO3)2 aqueous solution using platinum electrodes
(c) Electrolysis of dilute magnesium sulphate, MgSO4 aqueous solution using carbon electrodes
Why all the products formed at the anodes and the cathodes for (a), (b) and (c) are the same?
……………………………………………………………………………………………………………
Chapter 6 Electrochemistry9
Dilute solution Inert electrode
4541 CHEMISTRY Chapter 6
B3 Factors That Affect The Electrolysis Of An Aqueous Solution
There are …………… factors that may influence the selective discharge of ions during the electrolysis of an aqueous solution :
……………………………………………………………………………………….
……………………………………………………………………………………….
……………………………………………………………………………………….
(a) When more than one type of ions move towards the electrode during electrolysis, only one type
of ion is selected to be discharged. Selective discharge only occur in aqueous solution because
it usually has more than one type of ions attracted to the anode or cathode.
(b) The selection of ion for discharge depends on three factors :
(I)The position of ions in the Electrochemical Series (normally in dilute solution and inert electrodes are used)
(II) The concentration of electrolyte / ions (normally in concentrated solution and inert electrodes are used)
(III)The types of electrode (when active metal electrode is used)
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(I) The position of ions in the Electrochemical Series
When electricity is conducted in dilute solution used inert electrodes, the cations or anions in
the lower position in the Electrochemical Series will be selectively discharged.
Electrochemical Series
Cation Anion
K+ F-
Na+ SO42-
Ca2+ NO3-
Mg2+ Cl-
Al3+ Br-
Zn2+ Easier I-
Fe2+ to discharge OH-
Sn2+
Pb2+
H+
Cu2+
Ag+
Activity 6 :-
1 Select the ion to be discharged from the following pairs of ions.Write the discharge equation and state at which electrode it occurs.
pair of ionsIon to be
dischargedDischarge equation
At Anodeor
Cathode
(i) OH- & SO42-
(ii) OH- & NO3-
(iii) H+ & Cu2+
(iv) H+ & K+
(v) H+ & Sn2+
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2 Complete the following table for the electrolysis of 0.1 mol dm-3 sodium nitrate solution, using carbon electrodes.
Apparatus set-up :
Write the electrolyte ionisation equation
Ions exist in electrolyte Na+, NO3-, H+, OH-
At the Anode (+) At the Cathode
Write the formulae of ions that are attracted to the anode and cathode
NO3-, OH-
Na+, H+
Ions that are selectively discharge OH-
H+
Reason
Write half equation (discharge equation) of the reaction occured at the anode and cathode
Name of the products at the anode and cathode
Observations
Confirmatory test[ method and observations of the test ]
Method :
Placed a glowing wooden splinter in the test tube.
Method :
Placed a burning wooden splinter near the mouth of the test tube.
Observation :
Glowing wooden splinter is lighted up.
Observation :
A ‘pop’ sound is produced.
Chapter 6 Electrochemistry12
A
Carbon electrodes
sodium nitrate solution0.1 mol dm-3
4541 CHEMISTRY Chapter 6
3 Electrolysis of 0.1 mol dm-3 sulphuric acid, using carbon electrodes.
Apparatus set-up :
Complete the following table :
Write the electrolyte ionisation equation
Ions exist in electrolyte
At the Anode At the Cathode
Write the formulae of ions that are attracted to the anode and cathode
Ions that are selectively discharge
Write half equation (discharge equation) of the reaction occured at the anode and cathode
Name of the products at the anode and cathode
Observations
Confirmatory test[ method and observations of the test ]
Chapter 6 Electrochemistry13
A
4541 CHEMISTRY Chapter 6
4 Complete the following table for the electrolysis of 0.1 mol dm-3 aqueous dilute sodium chloride solution, using carbon electrodes.
Apparatus set-up :
Write the electrolyte ionisation equation
Ions exist in electrolyte
At the Anode At the Cathode
Write the formulae of ions that are attracted to the anode and cathode
Ions that are selectively discharge
Write half equation (discharge equation) of the reaction occured at the anode and cathode
Name of the products at the anode and cathode
Observations
Confirmatory test[ method and observations of the test ]
Chapter 6 Electrochemistry14
A
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(II) Concentration of electrolyte
When electrolysis is carried out using inert electrodes and concentrated solutions, ions that
are more concentrated will be selected to discharge, but this is only true for halide ions;
which are Cl-, Br- and I-.
Activity 7 :-
State the selected ions to be discharged at the anode and cathode for the following concentrated solutions, using carbon electrodes.
1 Electrolysis of :Set I : 0.001 mol dm-3 hydrochoric acid, HCl(aq) Set II : 2.0 mol dm-3 hydrochoric acid, HCl(aq)using carbon electrodes.
Based on the diagram above, complete the following table :
Chapter 6 Electrochemistry15
A A
0.001 mol dm-3 hydrochoric acid
2.0 mol dm-3 hydrochoric acid
Carbon electrode
Set I Set II
4541 CHEMISTRY Chapter 6
The electrolyte ionisation equation
Ions exist in electrolyte
Set I : 0.001 mol dm-3 dilute HCl Set II : 2.0 mol dm-3 concentrated HCl
The ions that are move to the cathodeIons that are selectively dischargeDischarge equation at the CathodeName of the product at the CathodeThe ions that move to the Anode
Selective ion
Discharge equation at the AnodeName of the product at the Anode
Observation at Cathode
Observation at Anode
Confirmatory test for product form at anode*.[ method and observation of the test ]
2 Electrolysis of 2.0 mol dm-3 concentrated sodium iodide solution, using carbon electrodes.
Apparatus set-up :
Complete the following table :
Chapter 6 Electrochemistry16
A
4541 CHEMISTRY Chapter 6
Write the electrolyte ionisation equation
NaI Na+ + I-
H2O H+ + OH-
Ions exist in electrolyte
At the anode At the cathode
Write the formulae of ions that are attracted to the anode and cathode
Ions that selected to discharge
Write the reaction equation at the anode and cathode
Name of the products at the anode and cathode
Observations
Confirmatory test
(III) Types of electrode
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There are two types of electrode :
Inert electrode
An electrode that acts as a conductor of current only and does not undergo any chemical changes. Normally are made of carbon or platinum.
These electrodes do not participate in an electrolysis but merely provide a surface for the release of electrons.
Non-inert/ active electrode
An electrode that not only acts as a conductor of current but also undergoes chemical changes. During the electrolysis, the metal anode will corrode while the reaction at the cathode is similar to the reaction at carbon electrode. Consist of metal anodes such as copper, silver and nickel.
Activity 9 :-
Chapter 6 Electrochemistry18
A
Carbon rod
Silver nitrate solution
Silver plate
A
A
Platinum
Dilute copper(II) nitrate solution
Copper
A
A
Carbon rod
Copper(II) sulphate solution
Copper plate
A
Inert electrodes Active electrodes
Set I
Set II
Set III
4541 CHEMISTRY Chapter 6
Complete the following table for the electrolysis of 1.0 mol dm-3 of copper(II) sulphate solution, using carbon and copper electrodes.
Apparatus set-up :
The electrolyte ionisation equation
Ions exist in electrolyte
Set I : Carbon electrodes Set II : Copper electrodes
The ions that move to the cathodeDischarge equation at the cathodeName of the product at the cathodeThe ions that move to the anodeDischarge equation at the anodeName of the product at the anode
Observations at the
Cathode
Anode
Observation at the electrolyte after a while
Blue colour becomes paler/fade//Blue colour becomes colourless.
No change (Blue colour remains)
The concentration of ions in the electrolyte after a while and the explanation
Activity 10 :-
Chapter 6 Electrochemistry19
Set I : Carbon electrodes Set II : Copper electrodes
1.0 mol dm-3
CuSO4 solution
Carbon electrodes
A
Copper plate
A
the rate of formation of copper(II) ions, Cu2+ at the anode is the same as the rate of discharge of copper(II) ions, Cu2+ at the cathode.
4541 CHEMISTRY Chapter 6
Complete the following table for the electrolysis of :
Set I : Electrolysis of 0.02 mol dm-3 silver nitrate solution using carbon electrodes
Set II : Electrolysis of 0.02 mol dm-3 silver nitrate solution using silver electrodes
Apparatus set-up :
The electrolyte ionisation equation
Ions exist in electrolyte
Set I : Carbon electrodes Set II : Silver electrodes
The ions that move to the cathodeDischarge equation at the cathodeName of the product at the cathodeThe ions that move to the anodeDischarge equation at the anodeName of the product at the anode
Observations at the
Cathode
Anode
Activity 11 :-
Chapter 6 Electrochemistry20
Set I : Carbon electrodes Set II : Silver electrodes
0.02 mol dm-3
AgNO3 solution
Carbon electrodes
A
Silver plate
A
4541 CHEMISTRY Chapter 6
1. Write half equations to represent the discharge of ions at the anode and the cathode for each electrolyte and then, predict the products formed.
ElectrolytesHalf equation Product
Cathode Anode Cathode Anode
1. Electrolysis of dilute
copper(II) sulphate solution
using carbon electrodes
2. Electrolysis of dilute
copper(II) sulphate solution
using copper electrodes
3. Electrolysis of dilute sodium
chloride solution using
carbon electrodes
4. Electrolysis of concentrated
sodium chloride solution
using carbon electrodes
2. Figure below shows the set-up of apparatus to investigate the electrolysis of a dilute aqueous sodium chloride solution using carbon electrode.
The anode : .....................................................................................................
The cathode : ....................................................................................................
(b) Write the formulae of all the ions present in the dilute sodium chloride solution.............................................................................................................................
.............
(c) Write the ionic equation for the reaction at :
(ii) the volume of gas collected in the test tube at electrode Q is 20.00 cm3. How many molecules of the gas in the test tube?[ 1 mol of gas occupies a volume of 24.0 dm3 at room condition ][ Avogadro constant, NA = 6.02 × 1023 ]
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C Evaluating Electrolysis in Industry
State the uses of electrolysis in industries
(a) Extraction
Aluminium metal is extracted from its ore, bauxite, which contains aluminium oxide, Al2O3.Aluminium metal is extracted by electrolysing a mixture of aluminium oxide and cryolite using carbon electrodes.Cryolite is added to lower the melting point of Al2O3 from 2045 0C to about 900 0C.
(i) Write the equation representing the process at the cathode and the anode.
Cathode : ………………………………………………………………………...
Anode : ………………………………………………………………………….
(ii)Which substances act as :
Cathode : ………………………………………………………………………...
Anode : …………………………………………………………………………..
(iii) Name the products formed at cathode : …………………… and anode : …………………..
(iv) State the function of cryolite.
Cryolite is used to …………………… the melting point of aluminium oxide.
Chapter 6 Electrochemistry23
Uses of electrolysis
Blocks of carbon as ……………..
Carbon lining as …………..Molten aluminium oxide + cryolite, Na3AlF6
Moltenaluminium
4541 CHEMISTRY Chapter 6
Activity 12 :-
Extraction of Aluminium
Fill in the blanks with the words provided in the boxes.
Al3+ Al3+ Al3+ + 3e → Al
O2- O2- 2O2- → O2 + 2e
cryolite lower aluminium oxide
electrolysis electrolyte carbon blocks
collected denser 4Al3+ + 6O2- → 4Al + 3O2
1. Aluminium is extracted from …………………………………………. by electrolysis.
2. The ……………………… is molten aluminium oxide dissolved in molten ……………….
3. This molten mixture melts at a …………………… temperature than pure aluminium oxide.
4. This enables the …………………………………… to be conducted at a lower temperature.
5. The electrolyte contains the …………………. (cation) and …………………….(anion).
6. The anode is made up of a series of ……………………………...
7. Liquid aluminium is …………………….. than the electrolyte and will be …………………... at the bottom of the electrolytic cell..
8. Electrolysis reactions at :
Anode :
…………………….. ions are discharged.
Half equation for the discharge of ion : ……………………………………………..
Cathode :
…………………….. ions are discharged.
Half equation for the discharge of ion : ………………………………………………
This produces molten aluminium which ………………… at the bottom of the tank.
9. The overall equation for the reaction :
…………………………………………………………………………………………………..
Chapter 6 Electrochemistry24
4541 CHEMISTRY Chapter 6
(b) Purification
Purification Of Copper
Look at the diagram below. Use the information given to label the diagram.
Fill in the blanks with the words provided in the box.