Chemistry, C6A Electrolysis (S)

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.

Tetrachloromethane, CCl4 molten aluminium oxide, Al2O3 sulphuric acid solution, H2SO4

glucose solution, C6H12O6 molten naphthalene, C10H8 sodium hydroxide solution, NaOH

Electrolytes Non-electrolytes

Chapter 6 Electrochemistry1


2 Write ( √ ) for the correct statement and ( X ) for the wrong statement.

(a) Solid sodium chloride, NaCl salt is a good electrical conductor.

(b) Mercury, Hg is an electrolyte.

(c) Sulphuric acid, H2SO4 is an electrolyte.

(d) Compounds can conduct electricity in the presence of freely moving ions.

(e) Covalent compounds can conduct electricity in any states.

(f)Common salt, NaCl is an electrolyte in molten or aqueous solution.

3 Explain why a sugar solution does not conduct electricity while a common salt solution conducts electricity.

.......................................................................................................................................................

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

4 Complete the chart with the correct answer. (Activity to determine the electrolyte / non-electrolyte)

*CompoundObservation / Bulb lights up

Solid Molten Aqueous solution

Sugar, C6H12O6

Lead(II) bromide, PbBr2

Sodium iodide, NaI

Sulphur, S8

Aluminium oxide, Al2O3

Naphthalene, C10H8

Iron, Fe


*Compound(aqueous solution)

Bulb

Carbon electrodes

Cell

Bulb

*Compound(molten)

Bunsen burner

Cell Switch

Carbon electrodes


5 Explain why the substances below can conduct electricity or cannot conduct electricity.

Substances Types of compound Explanation

5.1 Solid sodium chloride, NaCl

It ……………..…… ..conduct electricity

because the ……………..…… and

……………..…… are held together in a lattice

and do not ……………..………

5.2 Molten sodium chlorideorSodium chloride solution, NaCl

When sodium chloride is ……………..……

or ……………..…… in water, it

……………..…… conduct electricity.

This is because the ……………..…… and

……………..…… are free to move in the

molten state and aqueous solution.

5.3 Ammonia, NH3 in methylbenzene/ tetrachloromethane / prophanone / ether(organic solvents)

It ……………..…… conduct electricity

because it exists as ……………..……

in organic solvents.

5.4 Ammonia solution // ammonium hydroxide, NH4OH(Ammonia gas dissolved in water)

It ……………..…… conduct electricity

because it exists as ……………..………. and

……………..………. in water.

Activity 2:-

(i) What is meant by electrolyte?

……………………………………………………………………………………………..

……………………………………………………………………………………………..

(ii) Copper(II) sulphate crystal does not conduct electricity while copper(II) sulphate solution does. Explain why.

……………………………………………………………………………………………..

……………………………………………………………………………………………..



B ELECTROLYSIS

1. Electrolysis is a process whereby compounds in …………………… or ………………………..

state are ………… ………….. into their ………………………… …………………………..

by passing ……………………….. through them.

2. Electrolytic cell consists of ……………………, ………………………. and two ……………..

3. Conductor which is dipped into electrolyte, is called as ………………………..

4. There are two electrodes :

Anode (+) : ………………………………………………………………………………………

Cathode (-):………………………………………………………………………………………

Activity 3:-

Lable the diagram of electrolytic cell below :

5. During electrolysis, the negative ions or anions move to the …………. whereas the positive

ions or cation move to the ……………


A

R S

A

T U

A

P Q


B1 Electrolysis of Molten Compounds

Example : Electrolysis of molten lead(II) bromide, PbBr2

1. In molten lead(II) bromide, ……………… ions and ……………….. ions move freely.

2. When electricity is passed through molten lead(II) bromide, a reaction take place. This is

called ……………………….

3. Lead(II) bromide, PbBr2 is an …………… compound. It consist of ………………….., Pb2+

and ………………….., Br-.

In solid lead(II) bromide, these ions …… ……… move freely but are held in fixed

positions in a lattice.

In molten lead(II) bromide, these ions free to move.

4. During electrolysis of molten lead(II) bromide, the … …..… ions are attracted and moved to

the …………... whereas the ………… ions are attracted and moved to the ……………

At the cathode :

Pb2+ ions undergo discharged.

Each of Pb2+ ions accepts two electrons to form a neutral lead atom.

Half equation :

Pb2+ + 2e → Pb

Lead metal is formed.

At the anode :

Br- ions undergo discharged.

Each of Br- ions released one electron to form a neutral bromine atom.

Br- → Br + e

Two bromine atoms combine to form a bromine molecule.

Half equation :

2Br- → Br2 + 2e

Bromine gas is released.

Overall equation :

Pb2+ + 2Br- → Pb + Br2



Activity 4 :-

1. Electrolysis of molten sodium chloride, NaCl using carbon electrodes.

Diagram : (Lable the diagram below)

Molten sodium chloride consist of …………… ions and …………………..ions.

Electrode Cathode (-) Anode (+)

Ions that are attracted to //Ions that are move to

Na+ Cl-

Half equation at Na+ + e → Na 2Cl- → Cl2 + 2e

Overall equation

Substances formed //Products form at

Sodium Chlorine gas

Observation at Shiny silvery metal deposited Greenish-yellow gas released

2. Electrolysis of molten lead(II) oxide using carbon electrodes

Diagram : (Lable the diagram below)


- + A


Molten lead(II) oxide consist of …………… ions and …………………..ions.



Half equation at

Overall equation

Substances formed at //Products form at

Observation at

B2 Electrolysis of Aqueous Solutions

Example : Electrolysis of copper(II) sulphate solution using carbon electrodes.

1. When electricity is passed through copper(II) sulphate solution, a reaction take place.

This is called ……………………….

2. copper(II) sulphate solution, CuSO4 is an …………… compound. It consist of

………………….. ions, Cu2+ , ………………….. ions, SO42- , ……………. ions, H+ and

………………….. ions, OH-.

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.



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.



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.



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?

……………………………………………………………………………………………………………


Dilute solution Inert electrode


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)



(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+



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.


A

Carbon electrodes

sodium nitrate solution0.1 mol dm-3


3 Electrolysis of 0.1 mol dm-3 sulphuric acid, using carbon electrodes.

Apparatus set-up :

Complete the following table :


Ions exist in electrolyte

At the Anode At the Cathode





Observations



A


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 :



At the Anode At the Cathode





Observations



A


(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.

ElectrolyteIons exist in the

electrolyteIons that are selectively discharge at

Cathode Anode

(a)Concentrated hydrochloric acid solution, HCl

(b)Concentrated potassium iodide solution, KI

(c)Concentrated sodium chloride solution, NaCl

(d)Concentrated silver nitrate solution, AgNO3

(e)Concentrated copper(II) sulphate solution, CuSO4

Activity 8 :-

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 :


A A

0.001 mol dm-3 hydrochoric acid

2.0 mol dm-3 hydrochoric acid

Carbon electrode

Set I Set II


The electrolyte ionisation equation


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 :


A



NaI Na+ + I-

H2O H+ + OH-


At the anode At the cathode


Ions that selected to discharge

Write the reaction equation at the anode and cathode


Observations

Confirmatory test

(III) Types of electrode



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 :-


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


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 :



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 :-


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.


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 :



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 :-


Set I : Carbon electrodes Set II : Silver electrodes

0.02 mol dm-3

AgNO3 solution

Carbon electrodes

A

Silver plate

A


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.

(a) (i) What is meant by cation?

............................................................................................................................

(ii) Identify the anode and the cathode.


electrode Q

electrode P

Dilute sodium chloride solution

A


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 :

(i) electrode P :

............................................................................................................................

(ii) electrode Q :

............................................................................................................................

(d) If a concentrated sodium chloride solution is used in the above electrolysis,

(i) name the gas collected in the test tube at electrode P. Explain why?

............................................................................................................................

............................................................................................................................

(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 ]



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.


Uses of electrolysis

Blocks of carbon as ……………..

Carbon lining as …………..Molten aluminium oxide + cryolite, Na3AlF6

Moltenaluminium


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 :

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



(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.

Cu2+ atom impurities

smaller bigger Cu2+ + 2e → Cu

anode cathode Cu → Cu2+ + 2e

pure copper impure copper 2Al3+ + 6O2- → 2Al + 3O2

1. The cathode is made of ………………………….

2. The anode is made of …………………………….

3. Electrolysis reactions :

Anode :

Copper ………………… are ionised.

Half equation for the ionisation : ………………………………………

The anode slowly gets …………………………

Cathode :

……………………… are discharged.

Half equation for the discharge of ion : ……………………………….

The cathode slowly gets ………………………

4. The …………………….. from the anode drop to the bottom of the cell.


A

B

C

D


Activity 13 :-

1. The electrolytic cell shown below is a single representation of an industrial process.

(a) Identify the anode and the cathode.

Cathode : ………………………………………………………………………...

Anode : ………………………………………………………………………….

(b) State the observation at the :

cathode : ………………………………………………………………………...

anode : ………………………………………………………………………….

(c) Write the half equation for the reaction at :

cathode : ………………………………………………………………………...

anode : ………………………………………………………………………….

(d) Named the product at the :

cathode : ………………………………………………………………………...

anode : ………………………………………………………………………….


Impure copper plate Pure copper plate

Copper(II) sulphate solution

A

Electrical source


(c) Electroplating

Fill in the blanks with the words provided in the box.

discharged cathode coating ions

anode dissolves copper Cu2+

1. Electroplating is ………………………. an object with a metal by electrolysis.

2. The …………………… is made of the plating metal (metal to be plated onto the object).

3. The …………………… is the object to be plated.

4. The electrolyte is a solution that contain the ………………………. of the plating metal.

5. During electroplating, metal from the anode ……………………….. in the electrolyte as a metal ions.

6. These ions go to the cathode where they are ………………….. onto the object as a layer of metal.

7. An example of electroplating copper onto an object is shown in the diagram below.Use the information given to label the diagram drawn.

8. Electrolysis reactions :

Anode :

……………………….. atoms are ionized.

Half equation for the ionization of the atom : ……………………………………

Cathode :

………………………………… ions are discharged.

Half equation for the discharged of ion : ………………………………………………



Activity 14 :-

1 The diagram shows the set-up of the apparatus used to electroplate an iron nail with silver.

(a) State two mistakes in the set-up of apparatus in the diagram above?

…………………………………………………………………………………………..

…………………………………………………………………………………………..

(b) After the mistakes are fixed, the electroplating process is continued.What is observed at the anode and cathode after 30 minutes?

Anode :…………………………………………………………………………………

Cathode : ………………………………………………………………………………

(c) Show your analysis :

2 Mamat wants to electroplate his coin with copper.


Silver foil

Silver nitrate solution

Iron nail


(a) Draw a diagram to show the apparatus set-up.In your diagram, label

(i) a suitable electrolyte(ii) the anode and cathode

(b) Show your analysis.

(c) Two steps that must be taken to get a better electroplated coin.



The Effects of the uses of electrolysis in industries

1. State the effects of the uses of electrolysis in industries :

(a)

(b)


Benefits

Reactive metals can be …………………… by electrolysis.

A very thin layer of metal can be …………………. on an object using electrolysis.

Electrolysis can be used to produce a very ………………. metal.

Disadvantages

Electrolysis may cause …………………………….

Electrolysis is an ……………………………. process because it require a large amount of energy.

Documents

Chemistry, C6A Electrolysis (S)