ELECTROLYSIS

A set of apparatus which uses electrical energy to produce chemical

reactions is called Electrolytic Cell.

The process of decomposition of a compound by electricity is called

Electrolysis.

The anode is the positive electrode and the cathode is the negative

electrode of an Electrolytic cell.

A substance which conducts electricity in the molten state or in aqueous

solution is called electrolyte.

Cations are positive ions and Anions are negative ions.

The process of gaining or losing electrons at the electrodes is called

discharge of ions.

An electrode which does not take part in chemical reactions during

electrolysis is called inert electrode.

An electrode which participates in chemical reactions during Electrolysis

is called Reactive Electrode.

The process of coating an object with a thin layer of metal using

Electrolysis is called Electroplating.

A device that uses chemical reactions to produce Electricity is called

Simple Cell (Electric Cell).

ELECTROLYSIS:

1. The process of break down or decompose of a compound by using

electricity is called Electrolysis.

2. Usually the compound should be molten or dissolved in water.

BASIC CONCEPTS

1.1

3. Electrolysis is important for extracting useful pure elements

from compounds.

4. Electrolysis takes place in an electrolytic cell.

5. The electrolytic cell consists of the following parts:

a) A battery to supply electricity

b) Two electrodes

c) An electrolyte

Connecting wire

BATTERY

+ -

switch

anode cathode

+ -

AN ELECTROLYTIC CELL

6. When electricity is passed through an electrolyte, chemical reactions take

place at the electrodes and the electrolyte is decomposed.

Electrolyte

A molten ionic

compound or an

aqueous solution

that conducts

electricity.

Dissociates to

form positive ions

(cations) and

negative ions

(anions).

Examples: dilute

sulfuric acid,

molten sodium

chloride and

cooper (II) sulfate

solution.

Electrodes

Conduct electric current

The anode is the positive

electrode connected to the

positive terminal of the

battery.

The cathode is the negative

electrode connected to the

negative terminal of the

battery.

Examples: carbon

(graphite) rods and

metal plates.

7. The reactions taking place at the electrode are called electrolytic reactions.

In the solid state, ionic compounds do not conduct electricity and thus

cannot act as Electrolyte.

8.The process of electrolysis involves three things, as shown in the diagram

below.

+ -

e- Battery e-

Anode Cathode

MOVEMENT OF ELECTRONS IN THE EXTERNAL CIRCUIT

(connecting wires)

The electrons flow from anode to cathode in

the external circuit.

The flow of electrons constitutes the flow of

electricity through the connecting wire.

MOVEMENT OF IONS IN THE ELECTROLYTE

Cations (positive ions) move towards

cathode (negative electrode).

Anions (negative ions) move towards anode

(positive electrode).

The flow of positive and negative ions

towards the electrodes constitutes the flow

of electric current through the solution.

DISCHARGE OF IONS AT THE ELECTRODES

At the electrode, ions are discharged by gaining or losing electrons to become neutral atoms or molecules.

Cations are discharged at the cathode that is gaining electrons to become neutral atoms or molecules.

Anions are discharged at the anode that is losing electrons to from neutral molecules.

+

+ +

-

+

-

-

-

-

+

9. Redox reactions occur at the electrodes of an electrolytic cell.

At the anode, oxidation occurs.

At the cathode, reduction occurs.

TEST YOURSELF:

Predict the likely products of the electrolysis of molten binary compound and

aqueous electrolyte

Use the idea of selective discharge based on :

a) Cation: Link to the reactivity series

b) Anions: Halides, hydroxides and sulfates are essentially the electrolysis of water

c) Concentration effects

Write ionic equations for the reactions occurring at the electrodes during the

Electrolysis.

ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS:

1.2

1. Electrolysis of molten ionic compounds produces metals at the cathode and non-

metals at the anode.

2. Electrolysis of molten sodium chloride

a) When solid sodium chloride is heated strongly, it melts at 801OC.

The molten sodium chloride contains,Na+ ions, and anions, CI- ions.

b) The diagram below shows the apparatus set-up for the electrolysis of

molten sodium chloride .

+ -

e- e-

CARBON ROD (+) CARBON ROD (-)

CRUCIBE MOLTEN SODIUM

CLAY TRIANGLE

TRIPOD

ELECTROLYSIS OF MOLTEN SOLUTION CHLORIDE

The Na+ ions are attracted to the

cathode and are discharged at the

cathode.

The CI- ions are attracted to the anode

and are discharged at the anode .

This method is

commonly used

to obtain the

metal sodium.

HEAT

c) The table below shows the redox reactions at the electrodes during the electrolysis of molten sodium chloride.

AT THE CATHODE

AT THE ANODE

a) Na+ ions gain electrons to from sodium

metal.

Na+(l) + e- Na(I)

b) This is a reduction process.

Na+ ions are reduced to form sodium metal

a) Cl- ions lose electrons to form chlorine

molecules.

2Cl-(l) Cl2 (g) + 2e-

b) This is an oxidation process.

Cl- ions are oxidised to form chlorine gas.

OVERALL REACTION:

The overall reaction is the decomposition of sodium chloride by

by electricity to form sodium metal and chlorine gas .

electrolysis

2NaCl (l) 2Na(l) + Cl2 (g)

During electrolysis of molten sodium chloride,

Tiny drops of sodium metal float to the surface and produce flashes of yellow light

when burnt in air,

A pale greenish – yellow gas of chlorine is seen around the anode.

A half equation is an

ionic equation that

shows either the gain

or loss of electrons

in an oxidation or

reduction process .

1.1 EXAMPLE:

Predict the products formed when the following molten compounds are electrolysed using

carbon electrodes;

a) Lead (II) Bromide

b) Magnesium Oxide

SOLUTION:

a) Ions presents in electrolyte: Pb2+ and Br- ions.

At the cathode, lead metal is formed.

Pb2+ (I) + 2e- Pb(l)… reduction

At the anode, the non-metal, bromine gas, is formed.

2Br-(I) Br2(g) + 2e-… oxidation

OVERALL REACTION:

Lead (II) bromide is decomposed by electricity to form lead and bromine.

electrolysis

PbBr2(l) Pb(l) + Br2(g)

TIPS FOR STUDENTS:

When a molten ionic compound is electrolysed using

inert electrodes then

The metal is always produced at the negative

electrode (cathode), where reduction of metal ion

(cation) occurs.

The non-metal is always produced at the positive

electrode (anode), where oxidation of a non-metal ion

(anion) occurs.

b) Ions presents in electrolyte: Mg2+ and O2- ions.

At the cathode, magnesium metal is formed.

Mg2+(l) + 2e- Mg(l) … reduction

At the anode, the non-metal, oxygen gas, is formed.

2 O2-(l) O2(l) + 2e-…oxidation

OVERALL REACTION:

Magnesium oxide is decomposed by electricity to form magnesium and oxygen.

electrolysis

2 Mg O(l) 2 Mg(l) + O2(g)

ELECTROLYSIS OF AQUEOUS SOLUTION OF COMPOUNDS:

1. An aqueous solution of a compound contains

a) Cations and Anions of the compound,

b) Hydrogen ions, H+, and hydroxide ions, OH-, from water molecules.

2. In electrolysis, when more than one type of cation or anion is present in the

solution, only one cation and one anion are preferentially discharged.

This is called the selective discharge of ions.

3.The selective discharge of ions depends on three factors:

1.3

a) The position of ions in the reactivity series

b) The concentration of the ions

c) The nature of the electrode used

4. The reactivity series and the selectivity discharge of ions

a) The lower the position of the ion in the reactivity series,

The more readily the ion gains or loses electrons,

The higher the tendency of the ion to be selectivity discharged

CATIONS ANIONS

K+ CI-

Na+ Br-

Ca2+ I-

Mg2+ OH-

Zn2+ SO42- and NO3- will not be discharged

Pb2+ during electrolysis.

H+

Cu2+

Ag+

b) If a solution containing Cu2+ions and H+ ions is electrolysed,

Ease of

discharge

increases

H+ ions will

remain in the

solution.

Cu2+ ions will be preferentially discharged to form copper metal

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

c) If a solution containing SO42- ions and OH- ions is electrolysed

OH- ions will be preferentially discharged to form oxygen gas.

4OH-(aq) 2H2O(l) + O2(g)

5. Electrolysis of water:

a) Pure water is a poor conductor of electricity. If a small of ionic compound or dilute

sulphuric acid is added to water, the solution becomes a good conductor of electricity .

b) When water acidified with dilute sulfuric acid is electrolysed ,two volumes of hydrogen

is produced at cathode and one volume of oxygen is produced at the anode .

electrolysis

2H2O(l) 2H2(g) + O2(g)

COMMON ERROR ACTUAL FACT

All covalent compounds cannot be

electrolysed. Only ionic compounds

can be electrolysed.

Some Covalent compounds, such as

acids (HCI or H2SO4) are electrolytes

when dissolved in the water. Hence,

these covalent compounds can be

electrolysed in a aqueous solution.

In contrast, all ionic compounds can be

electrolysed in the molten state or in

aqueous solution.

SO42- Ions

will remain in

the solution .

HYDROGEN GAS OXYGEN GAS

DILUTE SULFURIC ACID

ELECTROLISIS OF WATER

6. Electrolysis of dilute sodium chloride solution

a) An aqueous solution of sodium chloride contains four different types of

ions:

Ions from sodium chloride: Na+(aq) and Cl-(aq)

Ions from water: H+(aq) and OH-(aq)

b) The diagram below shows the apparatus set-up for the electrolysis of dilute sodium

- +

chloride solution .

OXYGEN GAS HYDROGEN GAS

PLATINUM ELECTRODE (+) PLATINUM ELECTRODE (-)

DILUTE SODIUM

CHLORIDE SOLUTION

e- e-

+ -

ELECTROLYSIS OF DILUTE SODIUM CHLORIDE SOLUTION

AT THE CATHODE

AT THE ANODE

a) H+ and Na+ ions are attracted to the

platinum cathode . H+ ions gains

electrons from the cathode to form

hydrogen gas .

2H+ (aq) + 2e- H2( g )

a) OH- and Cl- ions are attracted to the

platinum anode . OH- ions give up to the

anode to form water and oxygen gas .

4OH-(aq) 2H2O ( l ) + O2( g ) + 4e-

Na+ ions remain in solution. Cl- ions remains in solution.

OVERALL REACTION:

electrolysis

2H2O ( l ) 2H2( g ) + O2 ( g )

The electrolysis of dilute sodium chloride is equivalent to the electrolysis

of water. Two volumes of hydrogen are liberated for one volume of oxygen.

Since water is being removed during electrolysis, the concentration of sodium

chloride solution increases gradually.

7. Effect of concentration on selective discharge of anions:

a) The selective discharge at the cathode is determined by the position

of the cation in the reactivity series.

b) However, the selective discharged at the anode is influenced by two

factors:

The position of the anion in the reactivity series

The concentration of the anion in the electrolyte

c) When a solution containing OH- ion and a halide ion ( Cl- , Br- or I- ion )

is electrolysed , the halide ion is selectively discharged at the anode,

if it has a high concentration .

The product of electrolysis at the anode is always oxygen, unless the

electrolyte contains a high concentration of halide ions.

8. Electrolysis of concentrated sodium chloride solution:

The electrolysis of concentrated sodium chloride solution can be carried

out in the set – up shown in the diagram below .

CHLORINE GAS HYDROGEN GAS

CARBON ELECTRODE (+) CARBON ELECTRODE (-)

CONCENTRATED SODIUM

CHLORIDE SOLUTION

e- e-

+ -

ELECTROLISIS OF CONCENTRATED SODIUM CHLORIDE SOLUTION

AT THE CATHODE AT THE ANODE

Na+ and H+ ions attracted to the

cathode.

H+ ions accept electrons more readily

than Na+ ions.

H+ ions are selectively discharged

because H+ is lower than Na+ in the

reactivity series.

2H+ (aq) + 2e- H2 (g)

Na+ ions remains in solution.

Cl- and OH- ions are attracted to the

anode.

Although the OH- ions are more

easily discharged, the Cl- ions are

selectively discharged because the

concentration of Cl- ions is higher

2Cl-(aq) Cl2 (g) + 2e-

OH- ions remains in solution.

OVERALL REACTION:

electrolysis

2NaCl(aq) + 2H2O(l) 2NaOH(aq) + H2(g) + Cl2(g)

During the electrolysis of concentrated sodium chloride solution, equal volumes of

hydrogen and chlorine are liberated at the electrodes.

The solution becomes alkaline as H+ ions and Cl- ions are discharged, leaving behind

Na+ ions and OH- ions in the solution.

9. Effect of nature of electrodes on selective discharged of ions:

a) Inert electrodes are electrodes that do not take part in chemical reaction

during electrolysis.

b) When electrolysis is carried out using inert electrodes, such as carbon or

platinum electrodes selective discharge depends on two factors:

Position of the ion in the reactivity series

Concentration of the ion

In the electrolysis of an aqueous solution using inert electrodes,

A metal or hydrogen gas is formed at the cathode (negative electrode),

A non – metal (other than hydrogen) is formed at the anode (positive

electrode).

c) Electrodes which are not inert undergo chemical reactions during electrolysis. These

electrodes are called active electrodes ( reactive electrodes ) .

d) Copper electrode is an active electrode when it is used for the electrolysis of

electrolytes containing Cu2+ ions .

10. Electrolysis of copper (II) sulfate solution using copper

electrodes:

The diagram below shows the electrolysis of aqueous copper (II) sulfate using copper

electrodes.

+ -

e- e-

COPPER ELECTRODE(+) COPPER ELECTRODE(+)

COPPER(II)

SULFATE SOLUTION

ELECTROLYSIS OF COPPER (II) SULFATE SOLUTION

AT THE CATHODE

AT THE ANODE

Cu2+ and H+ ions are attracted to the

cathode.

Cu2+ ions are selectively discharged,

giving copper metal (pink solid) .

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

H+ ions remains in solution

SO42 – and OH- ions are attracted to the

anode.

Copper anode dissolves, giving Cu2- ions .

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

SO42 – and OH- ions remains in solution .

OVERALL REACTION:

The transfer of copper from the copper anode to the copper cathode.

During electrolysis, the copper anode slowly becomes smaller and the copper cathode

slowly becomes bigger as pure copper is deposited on it.

The concentration of aqueous copper(II) sulfate remains unchanged. Thus, the blue

colour of the solution remains unchanged during the electrolysis.

1.2 EXAMPLE:

a) Predict the products formed (i) at the cathode, (ii)at the anode, when copper(II) sulfate

solution is electrolysed using carbon electrodes.

b) Describe the colour changes in the electrolyte.

SOLUTION:

a) Ions present:

From copper(II) sulfate: Cu2+ ions and SO42- ions

From water: H+ ions and OH- ions

At cathode, Cu2+ and OH- ions are presents.

Cu2+ ions are selectively discharged to form copper metal.

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

At anode, OH- and SO42-ions are presents.

OH- ions selectively discharged to form oxygen gas.

4OH-(aq) 2H2O(l) +O2(g) + 4e-

b) The intensity of the blue colour of the electrolyte decreases because the concentration

of copper (II) sulfate solution decreases as more copper is deposited on the cathode .

COMMON ERROR

ACTUAL FACT

Only solid electrodes can be used for

electrolysis.

The common electrodes are solids,

such as carbon, platinum and other

solid metals (for example, Cu and Ag).

But mercury is a liquid electrode and

is used in the manufacture of sodium

hydroxide and chlorine in a special

electrolytic cell called mercury cell.

The passage of electric current through matter is called electrical conduction.This phenomenon has led to great advances in science and technology. Many substances conduct electricity and many substances even do not. conducting substances may behave differently when electric current is passed through them – some are decomposed and some are not.We will discuss all these in some detail in this chapter. Question1. HOW DO SUBSTANCES CONDUCT ELECTRICITY? Answer: A substance that allows electric current to pass through it must contain mobile charged particles.These charged particles, which can be either electrons or ions, are called charge carriers.The charge carriers carry charge across the substance and we say that the substance conducts electricity.On the basis of the nature of the charge carriers, conducting substances are classified into electronic (metallic) conductors and electrolytic conductors.

Question2. What do you mean by Electronic (Metallic) conductors? Answer: Substances which conduct electricity due to the presence of mobile electrons are called electronic or metallic conductors. Metals,alloys and the non metal graphite are electronic conductors.graphite is the only one non metal which conduct electricity.they have comparatively loose i.e.mobile electrons which carry charge across them. They conduct in the solid as well as in the molten state and no chemical change occurs when on electric current pass through them. Question3. What are Electrolytes? Answer: There are many substances(e.g.NaCl and KOH) which do not have mobile electrons and yet conduct electricity in the molten state or in an aqueous solution.Under these conditions,they form mobile ions which carry charge across them. Such substances which allow the passage of electric current through them in the molten state or in solution due to the presence of ions are calld electrolytes.And the conduction of electricity by ions is known as electrolytic conduction. Strong electrolytes: Electrolytes which dissociate completely (or almost completely)into ions in solution are called strong electrolytes. Due to complete dissociation, strong electrolytic exist only as ions (not as molecules) in solution. Many strong electrolytes dissociate completely in the molten state also, whereas others do so only in solution.The following types of compounds are strong electrolytes. Ionic Compounds,e.g. salts and soluble hydroxides of metals, i.e. strong bases are strong electrolytes. Some examples are given below:

a.)Salts like NaCl,KCl,MgCl2,CaCl2,PbBr2,NH4Cl and CuSO4

NaCl Na+ + Cl-. CuSO4 Cu2+ + SO4

2-. b.)Soluble hydroxides of metals, i.e.strong bases, such as NaOH,kOH and Ba(OH)2 NaOH Na+ + OH- Ba(OH)2 Ba2+ + 2OH-.

c.)Strong acids: Strong acids like HCl,HNO2,H2SO4,HCIO4,HBr and HI are strong electrolytes as they dissociate completely in aqueous solutions. HA + H2O H3

O+ + A- HCI + H2O H3O

+ + CI- H2SO4 + H2O H3O + + HSO4

- HNO3 + H2O H3O

+ + NO3- HSO4

- + H2O H3O+ + SO4

2- Weak electrolytes: Electrolytes which dissociate into ions in solution only to a small extent are called weak electrolytes. Due to partial dissociation ,weak electrolytes exist both as ions and molecules in solution.The following types of compounds are weak electrolytes.

(i) Weak inorganic acids: e.g.- H2CO3,H2SO3 and H3PO4 H2CO3 + H2O H2O

+ + HCO-2

HCO-3 + H2O H2O

+ + CO2-3

(ii) Organic Acids: e.g.- CH3COOH (acetic acid) and HCOOH (formic acid) CH3COOH + H2O H3O

+ + CH3COO- HCOOH + H2O H3O

+ + HCOO-

(iii)Weak bases: e.g.-NH3 and its derivatives NH3 + H2O NH+

4 + OH-

Summary of acids, bases and salts as electrolytes: Class of compound Strong electrolytes

( ions only)

Weak electrolytes ( ions

as well as molecules)

Acids Strong acids ,e.g.-HCl,

HNO3,H2SO4,HCIO4

HBr, HI.

Weak acids, e.g.-H2

CO3 ,H2SO3, organic

acids.

Bases Strong bases, e.g.-

NAOH, KOH, Ba(OH)2

Weak bases, e.g.- NH3

and its derivatives.

Salts Most salts e.g.-NaCI,

Na2SO4, K2SO4,

Na2CO3, NH4CI

Halides(and some other

salts of Zn,Cd and Hg.

Non electrolytes: The Substances which do not dissociate into ions in the molten state or in solution are called non-electrolytes. In other words, non-electrolytes remain as molecules in the liquid state – melt or solution – and do not conduct electricity. Common examples of non-electrolytes are covalent compounds like pure water, pure acids (not in solution), ethanol, benzene, carbon tetrachloride, sugar and paraffin wax. Pure water does dissociate into ions (H+ and OH-) but to such a small extent ([H+] = [OH -] = 10- mol L-3) that it does not conduct electricity. Charge Conduction of Electricity through Electrolytes: The following experiments show that a chemical change takes place when an electric current passes through the electrolyte. 1) A molten Salt: Experiment: Melt some solid lead (II) bromide in a porcelain crucible and pass electric current through the molten salt.

Bulb

Battery Ammeter

+

Red - Brown Vapours

Molten PbBr2 Porcelain Crucible

A Grey Deposit

Heat

Molten Lead Bromide Decomposes When Electric Current is Passed through it.

Observation: A silver-grey material collects around the cathode and red-brown vapours evolve at the anode. Inference: Lead(I)bromide decomposes into lead the silver grey material collecting at the cathode) and bromine (the red-brown vapours evolving at the anode).

2. An aqueous solution of a salt:

Experiment: Pass electric current through an aqueous solution of NaCl mixed with litmus solution and placed in a U-tube.

Observation: The solution in the cathode wing turns blue and that in the anode wing is bleached.

Bulb

Battery Ammeter

Cathode + Anode

Aqueous Solution of NaCI + litmus

Fig. NaCI Decomposes When Electric Current Is Passed Through Its Aqueous Solution.

Inference: A base(NaOH) might have been produced at the cathode (since the litmus turns blue) and a bleaching agent (Cl2) at the anode. Take an aqueous solution of table salt (NaCl) in a small wide-mouth glass and add some turmeric powder to it. Its solution will become yellow. Let the turmeric particles settle down. Pass electric current through the solution you will soon see bubbles of gases evolving at the terminals. Also,

you will find that the turmeric particles below the negative electrode turn red. Turmeric is a household item which can be used as an acid-base indicator in acidic and neutral solutions,it is yellow and in alkaline solutions. it is red. 3. Acidified (acidulated) water: water decomposes into hydrogen and oxygen when an electric current is passed through acidified water. OXIDATION AND REDUCTION IN TERMS OF ELECTRON TRANSFER: The modern view of oxidation and reduction is based on the transfer of electron(s) from one species to another.According to the electronic view, loss of electrons is oxidation and gain of electrons is reduction. In an oxidation – reduction reaction (also called a redox reaction), the oxidizing agent gets reduced and the reducing agent gets oxides. Therefore, an oxidizing agent is one which gains electron(s) and a reducing agent is one which loses electrons. The following examples will make the points clear.

1) The formation of NaCI from Na and Cl2 2) To form NaCI from Na and Cl2,Na atom loses an electron to form a Na-

ion and a CI atom gains that electron to form a Cl- ion.Thus the formation of Na from Na is an oxidation process and that of Cl- from Cl is a reduction process the formation of NaCl is a redox reaction, which is a combination of the two individual reactions – the oxidation and the reduction half-reactions, as they are called.

3) As a Cl2 molecule requires two electrons to form two Cl- ions, two Na atoms are ionized to furnish the electrons. The reaction is represented as follows:

oxidation(2e) Half-reaction

2Na + Cl2 2Na+ + 2Cl-

reduction (2e)

2Na 2Na+ + 2e (oxidation)

Cl2 + 2e 2Cl-

(reduction)

2Na +Cl 2Na+ + 2Cl- (redox reaction)

2NaCl

The formation of an ionic compound from its elements is always a redox reaction as it involves the transfer of electron(s) from one atom to another. 2. Formationof MgO from Mg and O2: In this reaction, an Mg atom loses two electrons to form an Mg2- ion and an O atom gains them to form an O- ion. Thus the formation of Mg 2- from Mg is an oxidation reaction and that of O 2- from O is a reduction reaction. Two Mg atoms are ionized to give four electrons for converting one O2 molecule into two O2- ions.

oxidation (-4e)

2Mg + O2 2Mg2+ 2O2-

reduction (+4e)

2Mg 2Mg 2+ + 4e (oxidation)

2 + 4e 2O2- (reduction)

2 Mg + O2 2mg2+ + 2O2- (redox reaction)

2MgO

Mg acts as the reducing agent as it loses electrons and O2 acts as the oxidizing agent as it gains electrons.Finally,the Mg2+ and O2- ions combine to form MgO. 3. Conversion of FeCl2 to FeCl3 by Cl2: 2FeCl2 + Cl2 2FeCl3

FeCl2 is made up of one Fe2+ ion and two Cl- ions. FeCl3 is made up of one Fe3+ ion and three Cl- ions.Hence,the reaction may be represented as 2(Fe2+ + 2Cl-) + Cl2 2(Fe3+ +3Cl-) or 2Fe2+ + 4Cl- + Cl2 2Fe3+ + 6Cl- or 2Fe2+ + Cl2 Fe3+ + 2Cl- (cancelling 4Cl from both sides) The overall change is: oxidation (-2e)

2Fe + Cl2 2Fe3+ + 2Cl-

reduction (+2e)

Half-reaction

2Fe2+ 2Fe3+ + 2e (oxidation)

Cl2 + 2e 2Cl- (reduction)

2 Fe2+ +Cl2 2Fe3+ + 2Cl- (redox reaction)

Here, two Fe2+ ions are oxidized to two Fe3+ ions,and two Cl atoms (in Cl2)are reduced to two Cl- ions.The Cl2 molecule is the oxidizing agent and the Fe2+ ion (or FeCl2) is the reducing agent. 4. Displacement of a Metal by another metal form its salt in solution: A more active (ie.More electropositive)metal displaces a less active (i.e. less electropositive)one from a solution of a salt of the latter. These are redox reactions,Some examples are mentioned:

a) Fe displaces Cu from a solution of a Cu salt.This happens because Fe

is more electropositive than Cu. Fe(s) + CuSO4 (aq) FeSO4 (aq) + Cu(s) (Cu2+ + SO2-

4) (Fe2+ + SO2-4)

We find that the SO4

2- ion does not undergo a change. The reaction can be depicted as follows:

oxidation (-2e)

Fe + Cu2+ 2Fe2+ + Cu-

reduction (+2e)

Half-reaction

Fe Fe2+ + 2e (oxidation)

Cl2+ + 2e Cu (reduction)

Fe + Cl2+ Fe2+ + Cu (redox reaction)

Here, Fe is oxidized to Fe2+ and Cu2+ is reduced to Cu.

b) Zn displaces Ag from a solution of an Ag salt Zn, being more electropositive than Ag, displaces Ag from a solution of its salt.

Zn(s) + 2AgNO3 (aq) Zn(NO3)2 (aq) + 2Ag(s) (2Ag+ + 2NO-

3) (Zn2+ + 2NO-3)

This is a redox reaction which can be depicted as follows:

oxidation (-2e)

Zn + 2Ag+ Zn2 + 2Ag-

reduction (+2e)

Half-reaction:

Zn Zn2+ + 2e (oxidation)

2Ag+ + 2e 2Ag (reduction)

Zn + 2Ag+ Zn2+ + 2Ag (redox reaction)

ELECTROLYSIS: The decomposition of a compound in the molten state or in Solution when electric current is passed through it is known as electrolysis. Electrolysis is schematically shown in Figure:

1. The Electrolytic Cell or Voltameter: The vessel in which electrolysis is carried out is called the electrolytic cell or voltameter.

2. Electrodes: The terminals (usually metal or graphite rods) through

which electric current enters and leaves the electrolyte are called electrodes.

Bulb

Battery Ammeter

Cathode + Anode

Electrolytic Electrolyte

Cell OR

Voltameter

Fig: ELECTROLYSIS

M+ A-

The electrode connected to the negative terminal of the battery is called the cathode and the one connected to the positive terminal, the anode. While choosing the electrode material, one should ensure that it does not react either with the electrolyte or with the product liberated at the electrode.

3. The electrolyte: An electrolyte may be a melt or an aqueous solution.Often,in the case of a solution, the solute being decomposed is called the electrolyte.For example,if an aqueous solution of CuSO4 is placed in the voltameter,the CuSO4 is said to be the electrolyte.

4. Dissociation or lonisation of the electrolyte: In the molten state or in solution, an electrolyte dissociates into cations (positive ions) and anions (negative ions).

Mn+ An- Mn+ + An- HA + H2O H3O+ + A-

(Electrolyte) Cation anion acid hydronium ion Here M is a metal and A is a non metal or a negative redical like hydroxyl or sulphate. In ionic solids like NaCl or PbBr2, the ions are fixed in their lattice positions and are not free to move.But in the liquid state or in solution),the ions are mobile.

5. Discharge of ions at the electrodes: Being positively charged,the cations migrate(i.e.Move)in an electric field towards the negative electrode i.e.,the cathode.And the anions, being negatively charged, migrate towards the positive electrode, i.e., the anode.The cations take up electron(s)from the cathode and are discharged (i.e. lose their charge) there.The anions,in contrast give up electron(s) to the anode and are discharged there.

Cathode Mn+ + ne M (cathodic reduction) 2H3O

+ + 2e H2

Anode An- A + ne (anodic oxidation)

The number of electrons taken up at the cathode must be equal to that given up at the anode. As electrons are taken up by the cation at the cathode,the reaction occurring at this electrode is called cathodic reduction. And since electrons are given up by the anion at the anode, this reaction is called anodic oxidation. Metal atoms are deposited at the cathode, but H atoms, if deposited combine to form H2 molecules O2, Cl2 and Br2 respectively.We will soon learn how negative radicals,like OH- and SO4

2-, behave. EXAMPLE: 1) Sodium Chloride (Molten)

2NaCl Cathode: 2Na+ + 2e 2Na electrolysis 2Na+ + 2Cl- anode: 2Cl- 2Cl + 2e (molten)

Cl2 2) Magnesium chloride (Molten)

Mgcl2 Cathode: Mg2+ + 2e Mg electrolysis Mg2+ + 2cl- anode: 2Cl- 2Cl + 2e

3) Lead bromide (Molten) Cl2 PbBr2 Cathode: Pb2+ + 2e Pb electrolysis Pb2+ + 2Br- anode : 2Br- 2Br + 2e Br2

6. Selective Discharge of Ions:

Suppose we choose a molten mixture of NaF and MgCl2 for electrolysis. The Na2+ and Mg2+ ions will migrate to the cathode but only the Mg2+ ions will be discharged there. At the same time, the F- and Cl- ions will migrate to the anode but only the Cl- ions will be discharged there. The discharge of an ion in preference to another at an electrode is called selective discharge. The selective discharge of ions takes place because the tendencies of elements to form ions differ from each other.an element having a smaller tendency to form ions will be deposited at an electrode in preference to the one having a greater tendency to form ions.Let us see how this principle helps to understand the selective discharge of cations and anions from a mixture of electrolytes.

Cations:The reactivity series represents,in general:

the order of the tendency of metals and hydrogen

to form cations.so,the cation of a tendency of

metals and hydrogen to form cations. So, the

cation of a less active metal is discharged at the

cathode in preference to that of a more active

metal.

Thus,Mg being lower than Na in the activity series,Mg2+

ions will be discharged at the cathode in preference to

Na+ ions from a mixture of the two. Similarly, Fe2+ ions

will be discharged in preference to Mg2+ ions, and Cu

ions in preference to Fe2+ or H+ ions at the cathode.

Exception: Though Ca is below Na in the activity series,

Na+ ions are discharged in preference to Ca2+ ions at the

cathode.For example, the electrolysis of a molten

mixture of CaCl2 and NaCl will yield Na at the cathode.

Reactivity series

K

Na

Ca

Mg Increasing

Al ease of

Zn discharge

Fe

H

Cu

Ag

With the positions of Na and Ca inverted, the activity Ease of

series gives the correct order of the tendencies of metals

(and H) to form and remain as ions. The character of an

element to form and remain as ions is called its

electrochemical character and the order of the

tendencies of elements to do so is known as the

electrochemical series.This series for cations is often

called the electropositive series and that for anions, the

electronegative series.

Anions: Anions are also arranged in the order of the

ease of being discharged: the order is often called the

electronegative series (given alongside). We can easily

understand now why Cl ions are discharged in

preference to F- ions on the anode when a molten

mixture of NaF and MgCl2 is electrolysed.

discharge of

anions

F-

SO42-

NO3-

OH- Increasing

Cl- ease of

Br- discharge

l-

Discharge of ions from Aqueous Solutions: The products of the electrolysis of an aqueous solution of a salt may be different from those obtained if the electrolyte is molten. This is because of the simultaneous dissociation of water, though to a very small extent, into H3O

+(H+ for simplicity) and OH-. So,in the electrolytic cell,the H+ ions migrate towards the cathode and the OH- ions towards the anode.thus,there is more than one ion competing for discharge at each electrode.

(i) the metal ion and the H+ ion at the cathode ,and (ii) the anion of the salt and the OH- ion at the anode.

One ion will be discharged in preference to the other at each electrode as follows: Cathode:

1. If the metal M is more active than H, it will remain in ionic form in the solution and H+ will be discharged.

2H+ + 2e 2H H2 2H3O

+ + 2e H2 + 2H2O Thus, H3O

+ (or H+) ions will be discharged in preference to K+, Ca2+,Na+, Mg2+, Al3+ and Zn2- ions.

2. On the other hand, if Mn+ is less active than H,M ions will be discharged in preference to H ions i.e. H3O

+ ions. Mn+ + ne M So,the ions Cu2+ and Ag+ will be discharged in preference to H3O

+ ions at the cathode. Anode: It is obvious that

i) Cl-, Br- and I- are discharged in preference to OH- and the products are Cl2, Br2 and I2 respectively.

2Cl- 2Cl + 2e Cl2

ii) OH- ions are discharged in preference to SO2 and NO2 ions, and the prduct is O2

4OH- 4OH + 4e O2 + 2H2O EXAMPLE1: What are the products at the cathode and the anode if an aqueous solution of NaCl is electrolysed using graphite electrodes? Solution: In an aqueus solution NaCl and H2O dissociate as follows NaCl Na+ + Cl- 2H2O H3O

+ + OH- At the cathode Na and H3O ions compete for discharge. Being more active than hydrogen, sodium stays back in the form of Na ions, and H3O ions are discharged. 2H3O

+ + 2e H2 + 2H2O At the anode Cl and OH ions copete for discharge.Since the former are discharged more easily than the latter,Cl is formed at this electrode.

2Cl- 2Cl + 2e Cl2 Thus,H2 is produced at the cathode and Cl2 at the anode. EXAMPLE2: Draw a flowsheet to represent the electrolysis of an aqueous solution of sodium sulphate (Na2 SO2) using graphite electrodes. Solution: Na2SO4 dissociation +

8H2O dissociation + 4H3O

+ + 4e 4H + 4H2O 4OH- 4OH + 4e 2H2 O2 + 2H2O (H3O

- ions are discharged (OH- ions are discharged in preference to Na ions) in preference to SO2-

4 ions) Molten lead (II) Bromide (PbBr2): Principle:

1. Lead (II) bromide (PbBr2) is an ionic compound and it must dissociate almost completely into ions and act as a strong electrolyte.

2. The electrodes should be so chosen that they do not react either with

the electrolyte or with the respective products – lead and bromine. Graphite would suit our purpose.

3. The electrolytic cell should be made of a material that can withstand

the temperature (-400C0) required and does not react with the electrolyte oir the products. A silica crucible would meet these requirements.

2Na+

4H3O+

SO42-

4OH-

Scheme:

PbBr2 Pb 2Br (Molten) +2e -2e Pb Br Cathode Anode (graphite) (graphite)

Experiment:

1. Take some solid PbBr2 in a silica crucible. 2. Introduce two graphite electrodes into the solid. 3. Heat the crucible slowly till the solid melts. Continue heating on a very

low flame just to maintain the molten state of the salt. 4. Switch on the current using the circuit shown in Figure 6.5.

Battery Switch Ammeter

+

Graphite Electrodes Red - Brown Vapours

Molten PbBr2 Silica Crucible

A Silver - Grey

Metal

Heat

Fig: Electrolysis of Molten Lead Bromide

OBSERVATION:

A silver grey metal (Pb) deposits at the cathode, and red-brown vapours

of Br2 are evolved at the anode.

Net Chemical change:

PbBr2 electrolysis Pb + Br2

(Molten) (at cathode) (at anode)

Acidified (or acidulated) water: Principle:

1. Water, being a covalent compound, is a non-electrolyte though H3O+

and OH- ions are formed in the extremely low concentration of 10-7 mol L-1 due to auto dissociation.

2H2O H3O+ + OH-

So, pure water does not conduct electricity.

2. Acidified water (also called acidulated water) conducts electricity due to the formation of ions.

H2SO4 + 2H2O 2H3O+ + SO2-

4

3. The HO+ ions move in an electric field towards the cathode and get discharged there.

2H3O+ + 2e 2H + H2O

H2 The anions (SO2-

4 and OH-) move towards the anode but the OH ions are discharged there in preference to the SO ions. Four OH groups thus produced combine to form one molecule of oxygen and two of water.

4OH- 4OH + 4e

O2 + 2H2O As OH- ions are discharged, more water undergoes auto dissociation to replenish the ions and the electrolysis continues.

4. The electrodes should be such that they do not react with the electrolyte as well as the respective products since platinum is fairly unreactive,electrodes of this metal are suitable.

5. The electrolytic cell may be made of glass.

Scheme +

H2SO4 + 2H2O 8H2O +

Experiment:

1. The hoffmann voltameter (Figure 6.6) is used for the electrolysis of acidified water.

The voltameter consist of two vertical graduated tubes A and B, fitted with stopcocks at the upper ends. These tubes are connected to each other by a horizontal tube, which in turn is connected to a long vertical funnel C. The electrodes, made of platinum foils, are introduced into A and B and connected respectively to the negative and positive terminals of a battery.

2 H3O+

4H3O+

+4e

2H2 + 4H2O

SO42-

4OH-

-4e

O2 + 2H2O

The electrical circuit is shown in Figure:

Acidified Water

C

Hydrogen

Oxygen

A B

Pt Cathode Pt Anode

Switch Battery

- +

Ammeter

Fig: Hofmann Voltameter for Electrolysis of Acidified Water

2. Introduce acidified water into the voltameter through the funnel C.

3. Switch on the current.

Observation: Two volumes of a gas collect in tube A and one volume in tube B.

1. Gas collected at the cathode: The gas released from tube A, when kindled, burns with a ‘pop’. So, it is H2.

2. Gas collected at the anode: A glowing taper starts burning in the gas released from tube B. So, the gas is O2.

Experiment:

1. Dissolve some crystals of blue vitriol in water (20 gl-1).Add about 5-10 ml of concentrated H2SO4 to make the clear solution.Place the solution in the voltameter.

2. Complete the circuit as shown in the figure, using copper electrodes. 3. Switch on an electric current using a 6-V or 12-V battery or a battery

eliminator.

Observation:

A shining brown-red metal is deposited at the cathode.

1. The anode thins out. 2. There is no change in the intensity of the color of the electrolyte.

Battery Switch Ammeter

Cu Cathode + Cu Anode

CuSo4 Solution Voltameter

Fig: The Electrolysis of An Aqueous Solution of CuSO4 using Copper Electrodes.

APPLICATIONS OF ELECTROLYSIS:

Electrolysis has many industrial applications. Electroplating: Depositing a film of one metal over another by the process of electrolysis is known as electroplating. Electroplating with nickel or chromium protests a metal like iron from rusting and at the same time gives it a shining look. Several parts of motorcycles, cars, bathroom fittings, etc.which look so bright, are made of steel or brass, plated with nickel or chromium.Similarly, cutlery made of iron or steel may be made to look like silver by silver plating. And copper or silver jewellery can be made to look like gold jewellery by gold plating.

M+ A-

It should be realized that electroplated materials are not alloys as they are not solid solutions – there is only a film of one metal over the other. Electroplating with nickel: Something made of steel may be electroplated with nickel in order to protect it from rusting. An aqueous solution of the double salt nickel ammonium sulphate, (NH4) SO4 NiSO4, 6H2O, is used as the electrolyte for this purpose. This double salt, available in the form of bluish green crystals, is highly soluble in water. Battery Switch Ammeter

+

Nickel Anode

Article To Be

Plated Electrolyte

Fig: Electroplating A Steel Article With Nickel

A nickel bar is made the anode and the thing to be plated, the cathode. On electrolysis, the Ni2+ ions get discharged and deposit slowly over the cathode, i.e, the thing to be plated. At the same time, nickel dissolves out of the anode and replenishes the Ni2- ions in the electrolyte.

Ni2+ + 2e Ni (at the cathode) Ni Ni2+ + 2e (at the anode) The SO and OH ions move towards the anode, but neither is discharged. The metal should deposit uniformly and firmly over what you are electroplating.The happens if the deposition is slow. Among other things, this can be ensured by not passing a high current through the electrolyte. Nickel sulphate may be also used as an electrolyte, but the double salt works better. Electroplating with silver: Silver plating is mostly used for cutlery and some fancy articles. Nickel silver or German Silver,an alloy containing Cu , Zn and Ni (but no silver), has been found to be the most suitable metallic base over which silver can be firmly deposited. AgNO3 can be used as an electrolyte, but a coordination compound of the formula K[Ag(CN)2] is preferred to AgNO3.The compound K[Ag(CN)2] is commonly known as potassium argentocyanide.it is prepared as follows. An aqueous solution of AgNO3 is treated with one of KCN, and a white precipitate of AgCN is formed. A KCN solution is again added till AgCN dissolves to form the argentocyanide.

KCN + AgNO3 AgCN + KNO3

Silver cyanide

(white) KCN + AgCN K[Ag(CN)2] Potassium argentocyanide

The electrolytic bath is arranged as shown in Figure 6.9.A silver bar is made the anode and the article to be polated is made the cathode.The electrolyte dissociates as follows. K[Ag(CN) 2] K+ + Ag(CN)2

-

Ag(CN) 2 Ag+ + 2CN-

Battery Switch Ammeter

+

Silver Anode

Article To Be

Plated Solution of K (Ag(CN)2)

(cathode)

Fig: Electroplating With Silver

On electrolysis, Ag deposits at the cathode and the same amount of the metal dissolves out of the anode. Ag+ + e Ag (at the cathode) Ag Ag+ + (at the anode) Advantages of K (Ag(CN)2]over AgNO3: The ion Ag (CN) -

2 dissociates only slightly to Ag+. This affords a very slow and so a firm and uniform deposition of Ag at the cathode. On the other hand, the dissociation of AgNO3 leads to a high concentration of Ag+ ions. As a result, the deposition is fast and, therefore, not very uniform. Purification of Copper: Electroefining: Copper of purity 98-99% obtained by conventional methods is further

purified to 99.98% by the electrolytic method. Thick rectangular r block of impure copper are taken as anodes and thin plates of pure copper as cathodes. The cathodes and anodes are alternately hung in an electrolytic both containing a CuSO4 solution acidified with H2SO4.

Pure Copper Impure Copper

( Cathode ) ( Anode )

CuSO4 Solution

( Acidified With H2 SO4 )

Anode Sludge

Fig . Electro refining of Copper

On passing current, CuSO4 undergoes electrolysis. Pure Cu deposits at the cathodes and the metal dissolves out of the anodes to replenish the the concentration of the Cu2+ ions in the electrolyte. CuSO4 Cu2+ + SO2-

4

Cu2+ + 2e Cu (at the cathode) Cu Cu2+ + 2e (at the anode) In effect, copper dissolves out of the anode blocks of impure copper and deposits in the purest form on the cathodes.Some impurities like Fe,Zn and Ni, which are more electropositive (i.e. more active) than Cu, pass into solution as sulphates. And others like Ag, Au and possibly Pt, which are less

+

-

electropositive than Cu, settle as a mud called anode mud or anode sludge.The anode sludge is processed for the separation of precious metals. The purification of a metal by the electrolytic process also called electrorefining is used for several metals like Cr,Ni,Cu,Ag,Au and Pt. POINTS TO REMEMBER

1. The passage of electric current through matter is called electrical conduction.

3. Metallic or electronic conductors conduct electric current without

undergoing a chemical change. All metals, alloys and the nonmetal (graphite) do so.

4. The decomposition of a compound when electric current is passed

through it in the molten state or in solution is known as electrolysis. 5. Substances which allow the passage of electric current through them

in the molten state or in solution due to the presence of ions are called electrolytes.

6. Substances that do not dissociate into ions in the molten state or in

aqueous solutions are called non-electrolytes. 7. An electrolyte dissociates in the molten state or in aqueous solutions

into cations (positive ions) and anions (negative ions), which are free to move. In an electric field, the cations migrate towards the cathode (the negative electrode) and get discharged there by taking up electrons from it. Similarly, the anions migrate towards the anode (the positive electrode) and get discharged there by giving up electrons to it.

8. Ions compounds, e.g. salts and bases, act as electrolytes in the molten state as well as in aqueous solutions. Some polat covalent compounds like acids also act as electrolytes, but only in acqueous solutions.

9. Electrolytes which are almost completely dissociated into ions in aqueous solutions are known as strong electrolytes, and those dissociated into ions in aqueous solutions only to a small extent as weak electrolytes.

10. Substances containing ions only are strong electrolytes,Ionic compounds and strong acids belong to this category.

11. Substances containing ions as well as molecules are weak electrolytes. Weak acids and weak bases fall in this category.

12. Substances containing molecules only are nonelectrolytes,e.g. pure water, pure acid, benzene, ethanol.

13. The discharge of an ion in preference to another at an electrode is called selective discharge.

14. An element having a smaller tendency to form ions is deposited at an electrode in preference to one having a greater tendency to form ions.

15. The cation of a less active metal is discharged at the cathode in preference to that of a more active metal.

16. The ease of discharge of anions follows the order F42- < NO3

- < OH- < CI- < Br- < I-.

17. The electrolysis of molten PbBr2 using graphite electrodes given Pb at the cathode and Br2 at the anode.

18. The electrolysis of acidified (acidulated) water using platinum electrodes gives hydrogen at the cathode and oxygen at the anode in the volume ratio 2:1.

19. When an aqueous solution of copper(II)sulphate is electrolysed using copper electrodes, the copper metal dissolves out of the anode and pure copper deposits at the cathode.

20. Depositing a film of one metal over the other by the process of electrolysis is known as electroplating. You can electroplate something with nickel by using an aqueous solution of nickel ammonium sulphate [a double salt of the formula (NH4)2 SO4 NiSO4.6H2 O] as an electrolyte,

The thing to be plated as the cathode,and A nickel rod as the anode.

21. You can electroplate something with silver using an aqueous solution o f K[Ag(CN)2] as the electrolyte,

The object to be plated as the cathode,and A silver rod as the anode.

22. Cu can be electrorefined using an aqueous solution

of CuSO4,acidified with H2SO4,as the electrolyte, Thin plates of pure Cu as cathodes,and Thick plates of impure Cu as anodes.

Among the impurities in Cu, the more active metals, namely Fe, Zn and Ni, dissolve in the electrolyte and the less active metals, namely Ag, Au and Pt, collect below the anodes as mud or sludge.

“Objective Questions”:

(I) Choose the correct answer from the options given below each of the following questions:

1) Which of the following substances is an electrolyte?

a) Mercury b) Copper c) Sodium sulphate d) Aluminium

2) Which of the following substances is a weak electrolyte? a) Dilute hydrochloric acid b) Dilute sulphuric acid c) A solution of potassium bromide. d) Carbonic acid

3) Which of the following statements is true for the formation of sodium chloride by the direct combination of sodium with chlorine?

a) Sodium is reduced (b) Chlorine is oxidized. c) Chlorine is the oxidising agent (d) Sodium is the oxidizing agent.

4) Which of the following species will be deposited at the cathode on the

electrolysis of an aqueous solution of potassium bromide? a) K b) H2 c) Br2 d) O2

5) If you want to electrolyse concentrated HCI,which of the following will you

choose for making the anode? a) Graphite b) Aluminium c) Iron d) Copper

6) For electroplating a brass jug with copper, the anode must be made of a) Zinc b) copper c) nickel d) platinum

7) Which of the following equations truly represents the anode reaction in nickel plating?

a) Ni2+ + 2e Ni b) Ni Ni2+ + 2e c) 4OH- O2 + 2H2O + 4e d) SO4

2- SO2 + O2 + 2e

8) Figure represents the electrorefining of silver. A(the anode) and C(the cathode) respectively, are

a) Pure silver, impure silver ) impure silver, pure silver c) pure silver, pure silver d) impure silver, impure silver.

AgNO3 Solution

Anode Sludge

A C A C A C A

-

+

9) During the electrolysis of molten lead bromide ,which of the following takes place? a) Bromine is released at the cathode. b) Lead is deposited at the anode. c) Bromine ions gain electrons. d) Lead is deposited at the cathode. Fill in the blanks using the appropriate option(s) given within brackets:

1. Sodium hydroxide is --------------- (a metallic conductor, an electrolyte, A non electrolyte) 2. Salts do not conduct electricity in the -------------- state.(molten, solid) 3. Potassium bromide is a -------- electrolyte. (weak , strong) 4. A sample of chloroform consists of molecules only and so it is ----- (an electrolyte, a nonelectrolyte) 5. Pure water consists almost entirely of--------- (ions , molecules) 6. Pure water -------- conduct electricity. ( does , does not ) 7. Electrolysis is the passage of --------- through a liquid or a solution accompanied by a ---------- change.(electric current,electrons, physical ,chemical,) 8. During the electrolysis of acidified water using platinum electrodes,

Hydrogen is liberated at the ----------- and oxygen at the --------. (cathode , anode)

9. Select appropriate words to fill in the blanks from the following list: (anode, cathode, electrode, anions, cations, electrolyte, nickel, Voltameter) To electroplate an article with nickel requires an -------- which must be a solution containing --------ions.the article to be plated is placed at the ------- of the cell in which the plating is carried out. The ------- of the Cell is made from pure nickel. The ions that are attracted to the negative Electrode and discharged are called --------.

10.) Complete the following table, which refers to two practical applications of electrolysis: Anode Electrolyte Cathode

1.)Silver plating

of a spoon

---------- Solutionof

potassium

argento cyanide

-----------

2.)Purificationof

copper

---------- ------------ ------------

11.) Complete the following table:

Cathode: Anode: Electrolyte Material reaction product material Reaction

1. Copper(II)

chloride

solution

Platinum --------- ---------- Graphite ----------

2. Concentrated

hydrochloric

acid

Platinum ---------- ---------- Graphite ----------

3. Nickel

ammonium

sulphate

Iron ---------- ---------- Nickel ----------

solution

12.) As we descend the electrochemical series containing cations, the tendency of the cations to get ----------- at the cathode increases. (oxidised,reduced) 13.) The ----------- the concentration of an ion in a solution, the greater is the probability of its being discharged at the appropriate electrode. (higher,lower) 14.) Correct the following statements by adding or substituting appropriate words:

1. All covalent compounds are strong electrolytes. 2. Lead bromide conduct electricity. 3. Formic acid in solution contains only ions. 4. For nickel plating, the cathode must be made of nickel. 5. During the electrorefining of a metal, the cathode is made of impure

metal slabs.

Short Answer Type question: 1. Define electroplating.

Ans. The electrolytic process of deposition of a superior metal on the surface of

a base metal or article is called electroplating.

For e.g.: electroplating of an article of iron with Nickel.

2. Give reasons for the following:

a.)The article to be electroplated is always placed at the cathode.

Ans: During electrolytic reaction the metal is always deposited at the cathode by gain of

electrons. Since the electroplating is to be done on the surface of article, therefore it is

connected to the cathode.

b.) The metal to be plated on the article is always made the anode and has to be

replaced periodically.

Ans: The metal anode is the superior metal which is to be electroplated on the article.

Anode continuously dissolves as ions in the solution which migrate towards cathode and

is hence replaced periodically.

c.) Electrolyte must contain, ions of the metal used for plating on the article.

Ans: The electrolyte dissociates into ions of the metal which migrate towards the

cathode and are deposited as neutral metallic atoms on the cathode (article.)

d.) During electroplating a low current and for a longer time should be used.

Ans: Higher current caused uneven deposition of the metal. Longer time and low

current initiates a thicker uniform deposition.

e.) During electroplating a direct current and not A.C. current should be used.

Ans: A.C. current causes discharge and ionisation to alternate at the cathode thus

giving no effective coating.

3. What is electrorefining ?

Ans: It is a process by which metals containing impurities are purified electrolytically

to give a pure metal.

4. State how activity series of metal plays a role in extraction of metals from their oxides.

Ans: Metals which are at the top of activity series (like K, Na, Ca, Mg and Al) are extracted by

electrolysis of their fused salts. Their oxides are highly stable and hence do not decompose

thermally and conventional reducing agents like carbon monoxide and H2 cannot reduce

them.

Metal below Al in the activity series such as zinc, iron lead etc can be extracted from their

oxides by reduction using conventional reducing agents. Metals like Hg and Ag can be

extracted from their oxides by thermal decomposition.

5. Name the product at cathode and anode during electrolysis of:

Ans:

a.) Molten lead Bromide with inert electrode.

Cathode : lead metal , Anode : Bromine vapours

b.) Acidified copper sulphate solution with inert electrodes.

Cathode : Sodium metal , Anode : Chlorine gas

c.) Acidified copper sulphate solution with inert elecrodes.

Cathode : Copper metal , Anode : Oxygen gas

d.) Acidified water with inert electrodes.

Cathode: Hydrogen gas, Anode: Oxygen gas

e.) Dilute Hydrochloric acid with inert electrodes.

Cathode: Hydrogen gas , Anode: Oxygen gas

f.) Concentrated hydrochloric acid with inert electrodes.

Cathode: Hydrogen gas , Anode: Chlorine gas

6. What will you observe (a) at cathode (b) at anode (c) in electrolytic, during the

electrolysis of copper sulphate solution with copper electrodes?

Ans:

a.) At cathode – size of cathode increases

b.) At anode – size of anode decreases

c.) Electrolyte - The colour of electrolyte remains same i.e. blue

7. What is the material for cathode and anode during electro refining of impure copper?

Ans: Cathode : Pure copper

Anode : Impure copper

8. a.) Name some metals extracted by electrolysis.

Ans: Sodium, Potassium, Calcium, Magnesium etc.

b.) Name the compounds from which these metals are extracted.

Ans: These metals are extracted from their chloride i.e.

Sodium from Sodium Chloride

Potassium from Potassium chloride

Calcium from Calcium chloride

Magnesium from Magnesium chloride

c.) What is the state of compounds from which these metals are extracted?

Ans: They are extracted in their fused or molten state.

9.) Name all the particles present in

a. Sodium chloride solution.

Ans: Sodium ions, chloride ions, hydrogen ions, hydroxyl ions, water molecule.

b. Molten sodium chloride.

Ans: Sodium ion, chloride ion.

c. Sulphurous acid.

Ans: Hydrogen ion, sulphate ion, hydroxyl ions, sulphurous acid molecule, water molecule.

d. Carbon tetrachloride.

Ans: Molecule of carbon tetrachloride.

10.) Write equations for the reaction taking place at cathode and at anode during

the electrolysis of:

a. Acidified nickel sulphate with nickel electrode

Ans: At Cathode : Ni2+ + 2e- ---------> Ni

At Anode : Ni ---------> Ni2+ ions + 2e-

b. Acidified copper sulphate solution with copper electrodes.

Ans: At Cathode : Cu2+ + 2e- ---------> Cu

At Anode : Cu ---------> Cu2+ions + 2e- -

c. Acidified copper sulphate solution with platinum electrodes.

Ans: At Cathode : Cu2+ + 2e- ---------> Cu

At Anode : OH- ---------> OH + 2e-

40H ---------> 2H2O + O2

d. Acidified water with inert Electrodes

Ans: At Cathode : H+ +e- ---------> (H)

2(H)- ---------> H2 gas

40H ---------> 2H2O + O2

e. Molten lead Bromide with inert electrodes

Ans: At Cathode : Pb2+ + 2e- ---------> Pb

At Anode : 2(Br) ---------> Br2 gas

11. Name the solution which contain.

a. Only ions or molten ions.

Ans: Any strong electrolyte e.g.- dil. Sulphuric acid

b. Only molecules.

Ans: Any non - electrolyte e.g. Distilled water.

c. Ions as well as molecules.

Ans: Any weak electrolyte e.g. carbonic acid.

12. Following questions relate to the electroplating of an article with silver.

a. Of what substances must the anode be made of?

Ans: Block of silver

b. What will be the cathode?

Ans: Cleaned article.

c. What ions must be present in the electrolyte?

Ans. Silver ions.

d. What is the equations for the reaction at Cathode?

Ans. Reaction at cathode is

Ag++ e- ---------> Ag0 (deposited)

13. If molten magnesium chloride is electrolysed suggest a suitable anode for it.

Ans. Anode ---------> Graphite

14. Choosing only words from the following list, write down the appropriate words to fill in

the blanks below: anions, anode, cathode, cations, electrode, electrolyte, nickel, voltameter.

Ans. To electroplate an article with nickel requires an:

(i) Electrolyte: Which must be a solution containing nickel ions.

(ii)The article to be plated is placed as the cathode of the cell in which the

plating is carried out.

(iii) The Anode of the cell is made from pure nickel. The ions which are attracted to

the negative electrode and discharged are called anions.

15. Select the correct word from the words in bracket to complete the sentence.

(i) The electrode at which anions donate excess electrons and are oxidised to

neutral atoms is the (anode/cathode) Ans: anode

(ii) On electrolysis, Ag1+ and H1+ ions migrate to the (cathode / anode )

and (Ag1+ / H+1) are discharged. Ans: cathode , Ag1+

(iii) Electrolysis is a/an (oxidation, reduction/ redox) reaction in which

reduction reaction takes place at (anode/cathode)

Ans: redox , cathode

(iv) According to Arrhenius theory, the amount of electricity conducted by the

electrolyte depends on (nature/concentration) of the ions in the solution.

Ans: concentration

(v) Salts ionize in aq. solution, on passage of electric current to give

(negative /positive ) ions other the H+ ions.

Ans: positive ions

16.Write down the words or phrases from the brackets that will correct the sentences.

(i) Pure water consists of almost molecules (ions/molecules). We can expect that pure

water will not (will / will not) normally conduct electricity.

(ii) With platinum electrodes hydrogen is liberated at cathode (Cathode/anode)

and oxygen at the anode (anode/cathode) during the electrolysis of acidified water.

(iii) Electrolysis is the passage of electricity (electrons /electricity) through a

liquid or a solution accompanied by a chemical (physical / chemical ) change.

17. If the compound formed between x (a metal with a valency 2) and y (a nonmetal

with a valency 3) is melt and an electric current passed through the molten

compound. What will happen?

Ans: element x will be obtained at the cathode and y at the anode of

the electrolytic cell.

18. Select the correct answer from the list in bracket:

(i) The cation discharged at the cathode most readily

(Fe2+, Cu2+, Pb2+, H1+) Cu2+

(ii) The anion discharged at the anode with most difficulty

(So2-4, Br1-, NO1-

3, OH1- ) SO42-

(iii) The metallic electrode which does not take part in an electrolytic reaction.

(Cu, Ag, Fe, Ni) Fe

(iv) A covalent compound which in aqueous state conducts electricity

(CH4, CS2, NH3, C2H4 ) NH3

19. Match the statement 1 to 5 with their answer selected from A to J.

A : Cathode, B. Sucrose soln. C: CI1-, D formic acid,

E : Electro metallurge, F : Ammonia, G : Mg2+, H: electro refining,

I : Sulphur dioxide, J : Anode

1. A compound containing molecule only.

Ans: Sucrose Solution

2. A compound which ionizes in solution state but not in gaseous state.

Ans: Ammonia

3. The ion which accepts electrons from the cathode and gets reduced to neutral atoms.

Ans: Mg2+

4. The Electrode to which the cyanide ions of aq. Na(Ag(CN)2) migrate

Ans: Anode

5. An application of electrolysis in which anode does not generally diminishes in size.

Ans: Electro metallurgy

20.Complete the table given below:

(i) Electroplating an iron rod with silver.

(ii) Electroplating a copper sheet with nickel.

(iii) Electro refining of silver.

(iv) Extraction of potassium from kCI.

(v) Extraction of aluminium from AI2O3.

Ans:

Nature of

Anode

Nature of

Cathode

Ions present

in

electrolyte

Ions discharged at

cathode Anode

i. Thick block

of silver

Metal

Clean iron

rod

Na+, Ag+ CN-

Ag + le ----> Ag+e Ag ---> Ag++e-

formed

ii. Thick block

of nickel

metal

Clean copper

sheet

2+, SO42-, H+

On-

2+ + 2e- ----> Ni

(pure nickel)

Ni ---> Ni2 + 2e-

ions formed

Impure

block of

silver

Pure thin

sheet of

silver

Ag + So42-, H+

OH-

+ + le -----> Ag0

Pure silver

deposited

Ag ----> Ag + e-

cation

formed

Graphite

(inert)

Iron

(inert)

, CI- ions

+1 + le- --->K0

Pure

potassium

----> CI + e-

CI- + CI ---> CI2(g)

Chlorine gas

Thick carbon

rods

Gas carbon

lining

3+, O2- 2AI3++ 6e---->2AI 2- ---> 3[0] +6e-

3[0] + 3[0] ---->

3[02] (g)

21. Name the following: (1) Compounds which conduct electricity when dissolved in water in the molten State.

Ans. Electrolytes

(2) The decomposition of a chemical compound in the aqueous or fused state by

the passage of direct electric current.

Ans. Electrolysis

(3) Electrode connected to the positive terminal of the battery.

Ans. Anode

(4) Electrode connected to the negative terminal of the battery.

Ans. Cathode

(5) Compound which do not conduct electricity in aqueous or molten state.

Ans. Non – Electrolytes

(6) Electrolytes which almost completely dissociate in fused or aqueous solution.

Ans. Strong electrolyte

(7) Electrolytes which are partially dissociated in fused or aqueous state.

Ans. Weak electrolytes

(8) It is the vessel in which electrolysis is carried out.

Ans. Electrolytic cell.

(9) Ions which migrate to anode.

Ans. Anions

(10) Ions which migrate to cathode.

Ans. Cations

(11) The number of postitive charges equals the number of negative charges in

the electrolytic solution.

Ans. Electrolytic equilibrium

(12) The process due to which an ionic compound in the fused or in aqueous

state dissociates into ions by passage of electric current.

Ans. Electrolytic Dissociation

(13) An Ionic compounds, when added to water, it dissociates.

Ans. NaCI

(14) A Polar solvent.

Ans. Water

(15) A metals which ionize most readily.

Ans. Potassium / calcium / sodium.

(16) Metals which ionize least readily.

Ans. Silver /mercury

(17) It is the preferential discharge of ions present in an electrolyte at respective

electrodes.

Ans. Selective discharge of ions.

(18) An inert electrode.

Ans. Iron / graphite / platinum.

(19) An active electrode.

Ans. Copper /nickel /silver

(20) Product formed at anode during electrolysis of molten lead bromide.

Ans. Bromine vapours.

(21) Acid used for the acidification of pure water during electrolysis.

Ans. Dilute sulphuric acid.

(22) Product formed at cathode during electrolysis of acidified water.

Ans. Hydrogen gas.

(23) Product at anode during electrolysis of acidified water.

Ans. Oxygen gas.

(24) Product at anode during electrolysis of aqueous copper sulphate inert electrodes.

Ans. Oxygen gas.

(25) Product at anode during electrolysis of aqueous copper sulphate using active

copper electrodes.

And. Copper ions.

(26) The electrolytic process of deposition of superior metal on the surface of base

metal.

Ans. Electroplating.

(27) The article to be plated is placed at the electrode.

Ans. Cathode.

(28) The metal to be plated on the article is always placed at the electrode.

Ans. Anode

(29) Anode used during electroplating of an article with nickel.

Ans. Plate or block of nickel.

(30) The electrode which diminishes in mass during electroplating.

Ans. Anode

(31) The process by which metals containing impurities are purified electrolytically

to give a pure metal.

Ans. Electro refining

(32) Metals generally refined by electrolysis.

Ans. Zinc, lead, copper, mercury, silver.

(33) The Process of extraction of the metals by electrolysis.

Ans. Electro metallurgy.

(34) Metals which are generally extracted by electrolysis.

Ans. Potassium, sodium, calcium, magnesium, aluminium.

(35) Metals that can be extracted from their oxides by by thermal decomposition.

Ans. Mercury and silver.

(36) Substances which do not allow an electric current to flow through them.

Ans. Insulators or non – Conductors.

EQUATION SUMMARY

a.) Electrolysis of fused lead bromide:

Electrode reaction:

Dissociation of PbBr2

PbBr2 ⇌ Pb2+ + 2Br -

(molten)

Reaction at cathode : Pb2+ + 2e- –––––> Pb0 (Product hydrogen gas)

Graphite

Reaction at anode : Br -1 ––––––> Br + e-

Graphite Br + Br ––––––> Br2 (product bromine vapours)

b. Electrolysis of acidified water:

Electrode reaction:

Dissociation of water (acidified):

H2SO4 ⇌ 2H + SO42-

H2 O ⇌ H + OH- {ions present}

Reaction at Cathode : H1+ + le- –––> H

Platinum 2H + 2H –––> 2H2 (product hydrogen gas)

Reaction at Anode : OH-1 –––> OH + e-

platinum 40H –––> 2H2O + O2 (product hydrogen gas)

c. Electrolysis of aqueous copper sulphate:

(active copper electrodes)

Electrode reaction:

Dissociation of aqueous of CuSO4

CuSO4 ⇌ Cu2+ + So42-

H2O ⇌ H+ + OH-

Reaction at cathode : Cu2+ + 2e- –––> Cu (product copper metal)

Reaction at anode : Cu –––> Cu2+ + 2e- ions

(Product -.... Cu2+ions are produced)

Electrolysis of CuSO4(aqueous) with inert Pt. electrodes:

Electrode reaction :

Dissociation of CuSO4 (aq)

CuSO4 ⇌ Cu2 + SO42-

H2O ⇌ H+ OH-

Reaction at cathode : Ni2+ + 2e- –––> Ni

(article to be electroplated ) (Ni deposited)

Reaction at anode : Ni –––> Ni2+ ions (product Nil) +2e-

(pure Nickel block)

Electroplating of an article with silver Electrode reaction : Dissociation of Sodium Silver cyanide Na (Ag(CN)2) ⇌ Na+ + Ag+ + 2CN- H2O ⇌ H+ + OH- Reaction at cathode : Ag1+ + le- –––> Ag0 (article to be plated) Reaction at Anode : Ag – le –––> Ag+ ions (Product Nil Ag + 1 ions are formed) Electro refining of copper Electrode Reaction : Dissociation of aqueous Copper sulphate CuSO4 ⇌ Cu2+ + SO42- H2O ⇌ H+ + OH- Reaction at cathode : Cu2+ + 2e –––> Cu0 atoms deposited on pure Sheet Reaction at Anode : Impure block of Cu –––> Cu2+ + 2e- ions are formed (product –Nil)

Electro metallurgy : Extraction of sodium metal Dissociation of sodium chloride fused NaCl ⇌ Na+ + Cl- Reaction at Cathode : inert electrode Na+1 + le- –––> Na (metal) product Reaction at Anode : Cl-1 –––> Cl + e- Cl + Cl –––> Cl2(g) Product chlorine gas Extraction of potassium metal Dissociation of fused potassium bromide : KBr ⇌ K++ Br- fused Reaction at Cathode : (inert) K+1 + le- –––> K (Product potassium metal) Reaction at Anode : (inert) Br1- –––> Br + e- Br + Br –––> Br2(g) (product metal calcium) Extraction of calcium metal. Dissociation of fused calcium chloride } CaCl2 ⇌ Ca2+ 2Cl

Reaction at Cathode (inert) Ca2+ + 2e- –––> Ca (product metal calcium) Reaction at Anode: Cl + Cl –––> Cl + e Cl + Cl –––> Cl2 (gas) product clorine gas Extraction of Aluminium Dissociation of pure alumina Al2O3 ⇌ 2Al3+ + 302-

Reaction at Cathode : Al3 + 3e- –––> Al (inert electrode) Product Al. metal Reaction at Anode : 302- –––> 30 + 2e- (inert) 3[3] + 3[0] –––> 302 (g) (product oxygen gas)

REVISION EXERCISES:

1) What are the followings called? i) The ratio of the mass of an atom of an element to one-twelfth the

mass of an atom of 12C

ii) The ratio of the mass of a molecule of a substance to one-twelfth

the mass of an atom of12 C iii) The amount of a substance that contains the same number of

particle – atoms, molecules, or ions, - as the number of atoms in exactly 12 g of carbon –12.

iv) The volume occupied by one mole of a gas at stp. v) The decomposition of a compound by passing electric current

through it in the molten state or in solution. vi) A substance in which the presence of ions in solution or in the

molten state allows the passage of electric current through it. vii) Depositing a film of one metal over the other by the process of

electrolysis. 2.Name the law or hypothesis involved in each of the following cases:

i) When gaseous hydrogen reacts with gaseous chlorine to form

gaseous hydrogen chloride, their volumes bear a whole-number ratio under the same conditions of temperature and pressure.

ii) 1 L of hydrogen contains the same number of molecules as 1 L of oxygen at the same temperature and pressure.

3. Fill in the blanks from

appropriate word or value

from the brackets:

i) The ionization energy

and electron affinity of

representative elements

have a general

increasing trend along a

period, with breaks at

groups ------------- and --

-----------

(2/3/14/15)

ii) The maximum number

of covalent bonds

between any two atoms

(two/three/four)

is -------------

iii) Covalent compounds in

the pure state -------------

conduct electricity

(do/do not)

iv) If the cation or the anion

is coloured, the salt is --

-----------

(colourless/coloured)

v) An NaOH solution

furnishes sufficient ------

------- ions to precipitate

insoluble metal

hydroxides.

(Na+/OH-/H3O+)

vi) Cu(OH)2 and Pb(OH) 2

are ______ in an

excess of an NaOH

Solution.

(soluble / insoluble)

vii) In electrolysis, cations

are discharged at the ---

-- electrode and anions

at the ----- electrode.

(positive / negative)

viii) Substance containing

ions as well as

molecules are -----

electrolytes.

(weak/strong)

ix) In electrolysis, there is a

cathodic ----- and an

anodic -----.

(oxidation/reduction)

4. Justify the following statements: i) A molecule is never less than one, but a mole may be so. ii) The relative atomic mass of a gaseous element existing as diatomic

molecules is the same as its vapour density. 5. How many molecules of a gas are present in 22.4 L of it at stp? 6. The empirical formula of a compound containing H and O is HO, and

its relative molecular mass is 34. What is the molecular formula of the compound (H = 1, 0 = 16)?

7. Ethylene and cyclobutane have the same empirical formula, Ch, How will you arrive at their molecular formulae if their vapour densities are 14 and 28 respectively (H=10, C=12)?

8. Write balanced chemical equations for the following:

i) When an NaOH solution is added to a CaCl2 solution, a white precipitate is obtained.

ii) When an NH4OH solution is added slowly to a CuSO4 solution, a light-blue precipitate is first.

iii) An ammonium salt when boiled with an NaOH solution liberates NH3-.

iv) NH3 forms dense white fumes with HCl vapours. v) H2SO4 dissociates in two steps in an aqueous solution. vi) K[Ag(CN) 2]dissociates in an aqueous solution to give Ag ions.

9. Give reasons for the following:

i) Ionic solids are usually soluble in water. ii) H2SO4 forms two series of salts. iii) Cu(OH) 2 may form basic salts. iv) The electrolysis of acidulated water produces O2 at the anode. v) The concentration of the electrolyte does not decrease during

the electroefining of metals. vi) No products are formed at th anode during the electroplating of

an object.

10.Calculate the percentage of nitrogen in urea, CO(NH2)2 (H=1, C=12, N=14, 0=16). Give answer upto the second place of decimal. (Ans. 46.67%)

11.Lead nitrate decomoposes on heating as follows:

2Pb(NO3)2(s) Heat 2PbO(s) + 4NO2(g) + O2(g)

If 132.4 g of Pb(NO3)2 is heated, what is the i) Total volume of gases evolved when measured at stp. ii) Percentage by volume of O2 in the gases evolved, and iii) Mass of the residue (N = 14, O=16,Pb=207)?

[���. 22.4 � , 20% , 89.2�]

12. Choose the correct answer from the options given below each of the following questions:

i) Which of the following hydroxides is soluble in NaOH solution? a) Ca(OH)2 b) Zn(OH)2 c) Mg(OH)2 d) Fe(OH)3

ii) Which of the following gases will contain the minimum number of molecules? Answer that the volumes of the gases have been measured at stp. a) 2.8 L of H2 b) 5.6 L of CO2 c) 11.2 L of NH3 d) 22.4 L of NO2

iii) Which of the following is a strong electrolyte?

a) CH3COOH b) NH4OH c) H2CO3 d) NaOH 13. Indicate which of the following statements are true and which are false:

i) Ionic compounds have high melting and boiling points. ii) Mineral acids are strong acids. iii) Al(OH)3 is an amphoteric hydroxide. iv) There is no change in the colour of the turmeric mixed with NaCl

solution when the latter is electrolysed. v) Electroplated materials are alloys.

14. A thin layer of metal can be deposited on an object by the process of

electroplating.

a) List two uses of electroplating.

b) Which of the following objects cannot be electroplated: Clay Bowl, Iron Tray,

Porcelain Plate, Steel Fork, Wooden Spoon ? Why ?

c) i) Draw a labeled diagram of a set – up that can be used to coat a layer of

nickel on the surface of a steel coin .

ii) Write balanced equations, including state symbols, for the reactions at the

cathode and the anode .

Solution: a) To protect steel objects from corrosion, to give objects a shiny , attractive

appearance.

b) The clay bowl, porcelain plate and wooden spoon cannot be electroplated .

they are non conductors of electricity.

c) i)

PIECE OF NICKEL

STEEL COIN

AQUEOUS NICKEL (II)

NITRATE

ii) At the cathode: Ni2+ (aq) + 2e- Ni(s)

At the anode: Ni(s) Ni2+ (aq) + 2e-

15. The diagram below shows an experimental set – up used to investigate

the production of electrical energy in an electric cell . Zinc was used for

one electrode, while the other electrode was either made of Copper, Iron,

Magnesium, Silver, Tin, or Zinc .

ZINC ELECRODE METAL ELECRODE

ELECTROLYTE

a) Suppose the second electrode is made of copper.

i) Suggest a suitable electrolyte for the cell.

ii) Write ionic equations for the reaction taking place at the electrodes.

iii) Write a balanced equation, with state symbols, for the overall chemical

reaction in the cell.

b) The table below shows the voltages obtained when the different metals were

used as the second electrode . Deduce the metal which corresponds to each

voltage.

VOLTAGE / V

0.4

-1.6

1.1

0.0

1.6

0.8

V

METAL

c) Name another metal, which when used for the second electrode, will register a

reading larger than 1.6 V on the voltmeter.

Solution:

15. a) i) aqueous sodium chloride solution

ii) Zn Zn2+ + 2e-

Cu2+ + 2e- Cu

iii) Zn (s) + Cu2+ (aq) Zn2+ (aq) + Cu (s)

b)

VOLTAGE / V

0.4

-1.6

1.1

0.0

1.6

0.8

METAL

IRON

MAGNESIUM

COPPER

ZINC

SILVER

TIN

d) Gold or platinum

16. The diagram below shows a simple electric cell. A rod made of metal E

was used as one electrode, while the other electrode was a rod made

of either metal A, B, C, or D .

METAL E METAL A, B, C, or D .

ELECTROLYTE

a) Explain why deionised water is used as the electrolyte .

b) The table below shows the reading on the voltmeter when the different metals

were used in turn .

ELECTRODE 1

ELECTRODE 2

ELECTRODE 3

E

A

0.98

E

B

-0.34

E

C

-1.08

E

D

1.45

V

i) When metal C is placed in an aqueous solution containing E2+ ions, metal E is

obtained. Hence, rank metals A to E decreasing order of reactivity.

MOST

REACTIVE

LEAST

REACTIVE

ii) Which two metals amongst the five, when used as electrodes 1 and 2 , will

produce the largest voltage reading ?

Solution:

a) Deionised water contains only negligible amounts of H+ and OH- ions. There

are no other ions present .Deionised water is thus considered a non –

conductor of electricity, and is unsuitable as an electrolyte.

b) i)

MOST

REACTIVE

LEAST

REACTIVE

C

B

E

A

D

iii) Metals C and D

17. Complete the following table:

ELECTROLYTE

IONS PRESENT

IONIC EQUATION, WITH STATE

SYMBOLS, OF THE REACTION AT INERT CATHODE

IONIC EQUATION, WITH STATE

SYMBOLS, OF THE REACTION AT INERT ANODE

DILUTE SULFURIC ACID

MOLTEN SODIUM BROMIDE

DILUTE HYDROCHLORIC ACID

AQUEOUS LEAD (II) NITRATE SOLUTION

Solution:

ELECTROLYTE

IONS PRESENT

IONIC EQUATION, WITH STATE SYMBOLS, OF THE REACTION

AT INERT CATHODE

IONIC EQUATION, WITH STATE SYMBOLS, OF THE REACTION AT

INERT ANODE

DILUTE SULFURIC ACID

H+, SO4 2-, OH-

H+(aq) + 2e- H2 (g)

4OH- (aq) O2 (g) + 2H2O (l) + 4e-

MOLTEN SODIUM BROMIDE

Na+, Br -

Na+(l) + e- Na (l )

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

DILUTE HYDROCHLORIC ACID

H+, Cl-, OH-

2H+(aq) + 2e- H2(g)

2Cl- (aq) Cl2 (g) + 2e-

AQUEOUS LEAD (II) NITRATE SOLUTION

Pb2+, NO4 -, H+ , OH-

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

4OH- (aq) O2 (g) + 2H2O (l) + 4e-

18.) A student wanted to purify a piece of impure copper. the apparatus was set

up as shown in the diagram below.

PURE COPPER IMPURE COPPER

COPPER(II)

SULFATE CRYSTALS

a) Correct two mistakes that have been made in this set – up.

b) Assume the mistake have been corrected.

i) Write ionic equations, with state symbols, for the reactions taking place at the

electrodes.

ii) Suppose the initial masses of the pure copper and impure copper rods are 15.0

g and 25.0 g, respectively . After a certain time, t, the mass of the impure

copper rod was 21.5 g . sketch, on the axes below, graphs showing how the

masses of the two rods vary with time .

iii) How does the amount of Cu2+ ions in the beaker vary with time ?

c) Suppose graphite electrodes are used in place of the two copper rods.

i) Describe what would be observed during electrolysis.

ii) Write balanced ionic equations, with state symbols, for the reactions taking

place at the electrodes .

Solution:

18. a)1. The pure copper rod should be connected to the negative terminal of the

battery while the impure copper rod should be connected to the positive

terminal.

2. Water should be added into the beaker to create an aqueous solution.

b) i) At the cathode : Cu2+(aq) + 2e- Cu(s)

At the anode: Cu(s) Cu2+(aq) + 2e-

ii) Mass / g

25 .0

Impure Copper

15 . 0

Pure Copper

iii) The amount of Cu2+ remains unchanged.

d) i)Gas bubbles are formed at the surface of the anode. While a pink-brown solid

is deposited at the cathode.

ii) At the cathode: Cu2+ (aq) + 2e- Cu(s)

At the anode: 4OH- (aq) O2 (g) + 2H2O(I) + 4e-

19.) The diagram below is a schematic representation of an electrolysis tank used

industrially to extract aluminium from impure aluminium oxide, Al2O3 .

Time/s

Carbon

Cathode Molten Aluminium Oxide

And Cryolite at 9500 C

ALUMINIUM

Aluminium Out Steel Container

a) Why is it necessary to melt the aluminium oxide?

b) The melting point of pure aluminium oxide is 20540 C . Suggest why cryolite has to

be added to aluminium oxide.

c) Is the carbon cathode maintained at a positive or negative potential with respect

to the carbon anodes ?

d) Construct ionic equations, with state symbols, for the reactions that take place at

the electrodes .

e) i) Name two allotropes of carbon . Identify the allotrope used to make the carbon

electrodes. Justify your answer.

ii) explain, with the help of an equation, why it is necessary to replace the carbon

anodes regularly.

f) The most common aluminium oxide ore used industrially is bauxite . other than

aluminium oxide, bauxide also contains varying amounts of water and other metal

oxides as impurities . For the purpose of this question, we shall consider bauxite to be

impure aluminium oxide . Suppose 1478 kg of aluminium was extracted from 4220 kg

of bauxite and that the yield of aluminium from aluminium oxide was 90.5 %

.Calculate percentage – purity of this batch of bauxite .

g) The melting point of pure aluminium is 6600C .explain why it is important to

recycle aluminium .

Solution:

a) In the solid state, the ions in aluminium oxide are tightly held in their lattice

position. The ions are only free to move when aluminium oxide is melted.

b) The addition of cryolite lowers the melting point of aluminium oxide from

20540C to 9500C.This reduces considerably the amount of electrical energy

that would be required for heating, thus lowering the cost of extraction .

c) Negative

d) At the cathode: Al3+ (l) + 3e- Al (l)

At the anode: 2O2-(l) O2 (g) + 4e-

e) i) Graphite and diamond . Graphite is used as it is a good conductor of

electricity.

ii) C(s) + O2 (g) CO2 (g)

At the high operating temperatures, oxygen gas produced at the anode

reacts with the carbon electrode to form carbon dioxide .

f) Molar mass of Al2O3 = 2( 27) + 3 (16) = 102 g / mol

No. of mol of Al formed = 1 478 000 ÷ 27 = 54 741 mol

54 741 mol corresponding to a 90.5 % yield,

Thus max. no. of mol of Al= ( 100 ÷ 90.5 ) X 54 741 = 60 487 mol

��.�� ��� �������

�� �� ��� �� ��

No of mol of Al2O3 present in bauxite = 30 244 mol

Mass of Al2O3 present in bauxite = 30 244 X 102 = 3 084 837 g = 3085 kg

% purity of bauxite = ( 3085 ÷ 4220 ) X 100% = 73.1% (3 s.f.)

g) The melting point of aluminium is much lower then the melting point of

aluminium oxide (even with cryolite added). A lot less energy is thus required

to recycle aluminium as compared to the extraction of an equivalent amount

of aluminium from its ore.

TYPICAL PROBLEMS:

20. An electric current was passed through dilute sulfuric acid.

a) Give the formulae of all the ions present in dilute sulfuric acid.

b) Identify the cathode and the anode in the diagram below Also, fill in the

blanks with the gases that are collected in the respective burettes .

+ -

Dilute Sulfuric Acid

Platinum Electrodes

c) Write ionic equation to represent the electrode reactions.

d) i) After the current was passed through the sodium for 180 s, 30.6 cm3 of

gas was collected above the cathode the volume of gas that was collected

above the anode .

ii) The volumes of the gases given above were measured at room

temperature and pressure . The amount of electric charge ( units of

coulombs, C ) per mole of electrons is known as the Faraday constant, F,

where F = 96 485 C / mol . Calculate the total amount of charge

transfered in the production of the gas at the cathode .

iii) The quantity of charge, Q, can be calculated using the equation below .

Q ( In Coulombs, C ) = Current ( In Amperes, A ) X Time ( In Seconds, s )

21. An element E is soft and conducts electricity. It is highly reactive and reacts

Readily with water. Its chloride form , ECl , occurs naturally and dissolves readily

in water.

a) Give the formulae of the ions formed when ECI dissolves in water. hence, state the

group number of the Periodic Table to which E belongs.

b) Briefly explain how E can be extracted from ECl .

c) The oxide of E reacts violently with water, forming an aqueous hydroxide

solution.

i) Write a balanced equation, with state symbols, for this reaction.

ii) When 2.40 g of the oxide of is reacted with water, 2.86 g of the soluble

hydroxide was formed . Calculate the relative atomic mass of E.

iii) Hence, with the aid of a Periodic Table, identify E.

22. An electrochemist, Dr Patrik, carried out the electrolysis of aqueous sodium

nitrate solution. Initially, the burettes above the electrodes were completely filled

with solution . after a certain time, 22.5 cm3 and 36.0 cm3 of gases were collected

above the two electrodes, as shown in the diagram below.

Aqueous Sodium Nitrate Solution

22.5 cm3

30.0 cm3

Graphite Electrode Graphite Electrode

+ To Battery -

a) Identify the gas that is collected above the cathode. write an ionic equation, with

state symbols, to show the production of this gas .

b) The gas collected above the anode was found to be a mixture of oxygen, carbon

dioxide and carbon monoxide .

i) Account for the presence of the three anode gases. For each gas, write a

balanced equation, with state symbols, that describes its formation .

ii) Dr Patrik Found that the mixture contained 9.0 cm3 of carbon dioxide .

Calculate the volumes of carbon monoxide and oxygen collected in the mixture .

c) i) If Dr Patrik had used an aqueous potassium sulfate solution instead of an

aqueous sodium nitrate solution, will there be any difference in the products

formed ? Explain your reasoning.

ii) Supposing that platinum electrodes had been used in place of the graphite

electrodes, will Dr Patrik observe any difference in the products formed ?

Explain your reasoning.

TIPS FOR STUDENTS: Some structured and data – based questions may involve physical equations that you may not be familiar with . Read the question carefully and do not panic. You should be able to complete the calculations based on your knowledge and the information given in the question .