ELECTROLYSIS
BASIC CONCEPTS
 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).
1.1
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.
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
Electrolyte
Connecting wire
Electrodes
BATTERY
 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.
+
-
 Conduct electric current
switch
anode cathode
+
 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.
AN ELECTROLYTIC CELL
 Examples: carbon
(graphite) rods and
metal plates.
6. When electricity is passed through an electrolyte, chemical reactions take
place at the electrodes and the electrolyte is decomposed.
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.
MOVEMENT OF IONS IN THE ELECTROLYTE
MOVEMENT OF ELECTRONS IN THE EXTERNAL CIRCUIT
 Cations (positive ions) move towards
(connecting wires)
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.
 The electrons flow from anode to cathode in
the external circuit.
 The flow of electrons constitutes the flow of
electricity through the connecting wire.
+
e-
-
Battery
e-
Anode
Cathode
+
-
+
-
+
-
+
+
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.
1.2
ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS:
1. Electrolysis of molten ionic compounds produces metals at the cathode and nonmetals 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 .
 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.
-
e-
e-
CARBON ROD (+)
CARBON ROD (-)
CRUCIBE
MOLTEN SODIUM
CLAY TRIANGLE
TRIPOD
HEAT
ELECTROLYSIS OF MOLTEN SOLUTION CHLORIDE
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 a)
Cl- ions lose electrons to form chlorine
metal.
molecules.
Na+(l) + e-
Na(I)
2Cl-(l)
Cl2 (g) + 2e-
b) This is a reduction process.
b) This is an oxidation process.
Na+ ions are reduced to form sodium metal
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 .
A half equation is an
ionic equation that
shows either the gain
or loss of electrons
in an oxidation or
reduction process .
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.
1.1 EXAMPLE:
Predict the products formed when the following molten compounds are electrolysed using
carbon electrodes;
a) Lead (II) Bromide
TIPS FOR STUDENTS:
When a molten ionic compound is electrolysed using
inert electrodes then
b) Magnesium Oxide
 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.
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)
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)
1.3
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:
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+
Zn2+
Pb2+
Ease of
discharge
increases
OHSO42- and NO3- will not be discharged
during electrolysis.
H+
Cu2+
Ag+
b) If a solution containing Cu2+ions and H+ ions is electrolysed,
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)
will remain in
the solution .
2H2O(l) + O2(g)
COMMON ERROR
All covalent compounds cannot be
electrolysed. Only ionic compounds
can be electrolysed.
5.
SO42- Ions
ACTUAL FACT
 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.
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)
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
a)
OH- and Cl- ions are attracted to the
platinum cathode . H+ ions gains
platinum anode . OH- ions give up to the
electrons from the cathode to form
anode to form water and oxygen gas .
hydrogen gas .
2H+ (aq) + 2e-
H2( g )
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.
 Cl- and OH- ions are attracted to the
anode.
 H+ ions accept electrons more readily
than Na+ ions.
 Although the OH- ions are more
easily discharged, the Cl- ions are
selectively discharged because the
concentration of Cl- ions is higher
 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.
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
SO42 – and OH- ions are attracted to the
cathode.
anode.
Cu2+ ions are selectively discharged,
Copper anode dissolves, giving Cu2- ions .
giving copper metal (pink solid) .
Cu2+(aq) + 2e-
Cu(s)
H+ ions remains in solution
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
Only solid electrodes can be used for
electrolysis.
ACTUAL FACT
 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
Na+ + Cl-.
NaCl
CuSO4
Cu2+ + SO42-.
b.)Soluble hydroxides of metals, i.e.strong bases, such as NaOH,kOH
and Ba(OH)2
NaOH
Na+ + OHBa(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.
HCI + H2O
HNO3 + H2O
HA + H2O
H3 O+ + AH3O+ + CIH2SO4 + H2O
H3O + + HSO4H3O+ + NO3HSO4- + H2O
H3O+ + SO42-
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+ + CH3COOHCOOH + H2O
H3O+ + HCOO(iii)Weak bases: e.g.-NH3 and its derivatives
NH3 + H2O
NH+4 + OHSummary 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,
Halides(and some other
salts of Zn,Cd and Hg.
Na2CO3, NH4CI
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 redbrown 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
Cathode
Ammeter
+ 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 Naion 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+ + 2e
2Na
2Na + Cl2
2Na+ + 2Cl-
2Cl-
Cl2 + 2e
(reduction)
2Na +Cl
(oxidation)
2Na+ + 2Cl-
(redox reaction)
reduction (2e)
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
2 + 4e
2Mg2+ 2O2-
2Mg + O2
2 Mg + O2
2Mg 2+ + 4e
(oxidation)
2O2-
(reduction)
2mg2+ + 2O22MgO
reduction
(+4e)
(redox reaction)
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+ + 6Clor
2Fe2+ + Cl2
Fe3+ + 2Cl- (cancelling 4Cl from both sides)
The overall change is:
oxidation
Half-reaction
(-2e)
2Fe2+
2Fe + Cl2
2Fe3+ + 2Cl-
reduction
2Fe3+ + 2e
(oxidation)
Cl2 + 2e
2Cl-
(reduction)
2 Fe2+ +Cl2
2Fe3+ + 2Cl-
(redox reaction)
(+2e)
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)
(Cu2+ + SO2-4)
FeSO4 (aq) + Cu(s)
(Fe2+ + SO2-4)
We find that the SO42- ion does not undergo a change.
The reaction can be depicted as follows:
oxidation
(-2e)
Half-reaction
Fe + Cu2+
2Fe2+ + Cu-
Fe
Cl2+ + 2e
reduction
(+2e)
Fe + Cl2+
Fe2+ + 2e
(oxidation)
Cu
(reduction)
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)
(2Ag+ + 2NO-3)
Zn(NO3)2 (aq) + 2Ag(s)
(Zn2+ + 2NO-3)
This is a redox reaction which can be depicted as follows:
oxidation
(-2e)
Half-reaction:
Zn
Zn + 2Ag+
reduction
Zn2 + 2Ag-
(+2e)
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
+ Anode
Cathode
M+
A-
Electrolytic
Electrolyte
Cell OR
Voltameter
Fig: ELECTROLYSIS
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).
n+ nM A
Mn+ + AnHA + H2O
H3O+ + ACation anion
acid
hydronium ion
(Electrolyte)
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+ +
2H3O+ +
ne
2e
M
H2
(cathodic reduction)
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 SO42-, 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
anode:
2Cl-
Mg2+ + 2cl3) Lead bromide (Molten)
PbBr2
Pb2+ + 2Br-
Mg
electrolysis
2Cl + 2e
Cl2
Cathode:
Pb2+ + 2e
anode :
2Br-
Pb
electrolysis
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.
Reactivity series
K
Na
Ca
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.
Mg
Increasing
Al
ease of
Zn
discharge
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.
H
With the positions of Na and Ca inverted, the activity
Ease of
Fe
Cu
Ag
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
FSO42NO3OH- 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
2+
+
So,the ions Cu 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
4OH4OH + 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+ +
ClH3O+ +
2H2O
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
2Na+
+
8H2O
dissociation
4H3O
+
4H3O+ + 4e
+
SO424OH-
4H + 4H2O 4OH-
2H2
(H3O ions are discharged
in preference to Na ions)
-
4OH + 4e
O2 + 2H2O
(OH ions are discharged
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.
Scheme:
PbBr2
(Molten)
Pb
2Br
+2e
-2e
Pb
Cathode
(graphite)
Br
Anode
(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
Molten PbBr2
A Silver - Grey
Metal
Red - Brown Vapours
Silica Crucible
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
(Molten)
electrolysis
Pb
+
(at cathode)
Br2
(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+ + OHSo, 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
2 H3O+
H2SO4 + 2H2O
8H2O
4H3O+
+4e
2H2 + 4H2O
+
SO424OH-
+
-4e
O2 + 2H2O
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.
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 Anode
Cu Cathode
M+
A-
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.
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)
2+
Ni
Ni +
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]
Ag(CN) 2
K+ +
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+ +
Ag
e
Ag (at the cathode)
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+ + 2e
Cu
Cu2+ + SO2-4
Cu
(at the cathode)
2+
Cu +
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) 4OHO2 + 2H2O + 4e
d) SO42SO2 + 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.
+
A
C
A
C
A
C
A
AgNO3 Solution
Anode Sludge
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
1.
Copper(II)
chloride
solution
Platinum ---------
2.
Concentrated Platinum ---------- ---------- Graphite ---------hydrochloric
acid
3.
Nickel
ammonium
sulphate
Iron
reaction product material
Reaction
---------- Graphite ----------
---------- ----------
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.
2.
3.
4.
5.
All covalent compounds are strong electrolytes.
Lead bromide conduct electricity.
Formic acid in solution contains only ions.
For nickel plating, the cathode must be made of nickel.
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 + 2eb. 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 + 2e40H
---------> 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
i.
Nature of
Cathode
Thick block Clean iron
of silver
rod
Metal
ii. Thick block Clean copper
of nickel
sheet
metal
Impure
Pure thin
block of
sheet of
silver
silver
Graphite
(inert)
Iron
(inert)
Ions present Ions discharged at
in
cathode
Anode
electrolyte
+
Na , Ag+ CN- Ag + le ----> Ag+e Ag ---> Ag++eformed
2+
, SO42-, H+
On-
2+
Ag + So42-, H+
+
+ 2e- ----> Ni
(pure nickel)
Ni ---> Ni2 + 2eions formed
+ le -----> Ag0
Ag ----> Ag + e-
OH-
, CI- ions
+1
Pure silver
cation
deposited
formed
+ le- --->K0
----> CI + e-
Pure
potassium
CI- + CI ---> CI2(g)
Chlorine gas
Thick carbon Gas carbon
rods
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 + eGraphite
Br + Br ––––––> Br2 (product bromine vapours)
b. Electrolysis of acidified water:
Electrode reaction:
Dissociation of water (acidified):
H2SO4 ⇌ 2H + SO42H2 O ⇌ H + OH- {ions present}
Reaction at Cathode : H1+ + le- –––> H
Platinum
2H + 2H –––> 2H2 (product hydrogen gas)
Reaction at Anode : OH-1
–––> OH + eplatinum
40H
–––> 2H2O + O2 (product hydrogen gas)
c. Electrolysis of aqueous copper sulphate:
(active copper electrodes)
Electrode reaction:
Dissociation of aqueous of CuSO4
CuSO4 ⇌ Cu2+ + So42H2O ⇌ H+ + OHReaction 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 + SO42H2O
⇌ H+ OHReaction at cathode : Ni2+ + 2e- –––> Ni
(article to be electroplated )
(Ni deposited)
Reaction at anode
: Ni
–––> Ni2+ ions (product Nil) +2e(pure Nickel block)
Electroplating
Electrode
Dissociation
Na
(Ag(CN)2)
H2O
Reaction
at
(article
Reaction
of
article
with
silver
reaction
:
of
Sodium
Silver
cyanide
⇌
Na+
+
Ag+
+
2CN⇌
H+
+
OHcathode
:
Ag1+
+
le–––>
Ag0
to
be
plated)
at
Anode
:
+
Ag – le –––> Ag ions (Product Nil Ag + 1 ions are formed)
Electro
Electrode
Dissociation
CuSO4
H2O
Reaction at cathode
Reaction
Impure block of
at
an
refining
of
:
of
copper
Reaction
:
aqueous
Copper
sulphate
⇌
Cu2+
+
SO42⇌
H+
+
OHCu2+
+
2e
–––> Cu0 atoms deposited on pure
Sheet
Anode
:
Cu
–––> Cu2+ + 2e- ions are formed
(product
–Nil)
Electro
Extraction
Dissociation
NaCl
Reaction
inert electrode
Reaction
at
-1
Cl
Cl
Extraction
Dissociation
KBr
fused
Reaction
(inert)
Reaction
at
(inert)
metallurgy
of
:
sodium
metal
sodium
chloride
fused
⇌
Na+
+
Clat
Cathode
:
Na+1
+
le–––> Na (metal) product
Anode
:
–––>
Cl
+
eCl
–––>
Cl2(g)
Product
chlorine
gas
of
+
of
of
potassium
potassium
⇌
fused
at
Cathode
+ le-
K+1
Anode
Br
Extraction
Dissociation
of
CaCl2
Reaction
(inert)
Reaction at Anode:
Cl
Extraction
Dissociation
Al2O3
Reaction
at
Cathode
(inert electrode)
Reaction at Anode
(inert)
+
Br1–––>
Br
of
⇌
+
Cl
Cl
+
metal
:
Br-
:
–––> K (Product potassium metal)
:
–––>
Br + eBr2(g) (product metal calcium)
calcium
calcium
fused
Ca2+
bromide
K++
at
+ 2eCl
–––>
metal.
chloride
}
Ca2+
2Cl
Cathode
–––>
Ca (product metal calcium)
–––>
Cl
+
e
Cl2 (gas) product clorine gas
of
Aluminium
of
pure
alumina
3+
⇌
2Al
+
3023
:
Al
+
3e
–––>
Al
Product Al. metal
: 302–––>
30 +
2e3[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.
1 L of hydrogen contains the same number of molecules as 1 L
of oxygen at the same temperature and pressure.
ii)
3. Fill in the blanks from
appropriate word or value
from the brackets:
i)
The ionization energy (2/3/14/15)
and electron affinity of
representative elements
have
a
general
increasing trend along a
period, with breaks at
groups ------------- and ------------
ii)
The maximum number (two/three/four)
of
covalent
bonds
between any two atoms
is ------------iii)
Covalent compounds in (do/do not)
the pure state ------------conduct electricity
iv)
If the cation or the anion (colourless/coloured)
is coloured, the salt is ------------
v)
An
NaOH
solution (Na+/OH-/H3O+)
furnishes sufficient ------------ ions to precipitate
insoluble
metal
hydroxides.
vi)
Cu(OH)2 and Pb(OH) 2 (soluble / insoluble)
are ______ in an
excess of an NaOH
Solution.
vii)
In electrolysis, cations (positive / negative)
are discharged at the ---- electrode and anions
at the ----- electrode.
viii)
Substance containing (weak/strong)
ions
as
well
as
molecules
are
----electrolytes.
ix)
In electrolysis, there is a (oxidation/reduction)
cathodic ----- and an
anodic -----.
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)
ii)
iii)
iv)
v)
vi)
When an NaOH solution is added to a CaCl2 solution, a white
precipitate is obtained.
When an NH4OH solution is added slowly to a CuSO4 solution, a
light-blue precipitate is first.
An ammonium salt when boiled with an NaOH solution liberates
NH3-.
NH3 forms dense white fumes with HCl vapours.
H2SO4 dissociates in two steps in an aqueous solution.
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:
Heat
2Pb(NO3)2(s)
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) + 2eAt the anode: Ni(s)
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 .
V
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
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
METAL
0.4
-1.6
1.1
0.0
1.6
0.8
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 .
V
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
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
DILUTE
SULFURIC ACID
MOLTEN
SODIUM
BROMIDE
DILUTE
HYDROCHLORIC
ACID
AQUEOUS LEAD
(II) NITRATE
SOLUTION
IONS PRESENT
H+, SO4 2-,
OH-
Na+, Br -
IONIC EQUATION, WITH STATE
SYMBOLS, OF THE REACTION
AT INERT CATHODE
IONIC EQUATION, WITH STATE
SYMBOLS, OF THE REACTION AT
INERT ANODE
H+(aq) + 2e-
4OH- (aq)
O2 (g) + 2H2O (l) + 4e-
Na+(l) + e-
H2 (g)
Na (l )
2Br - (l)
Br2 (g) + 2e-
H+, Cl-, OH-
2H+(aq) + 2e-
H2(g) 2Cl- (aq)
Cl2 (g) + 2e-
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) + 2eAt the anode: Cu(s)
Cu(s)
Cu2+(aq) + 2e-
ii) Mass / g
25 .0
Impure Copper
15 . 0
Pure Copper
Time/s
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) + 2eAt the anode: 4OH- (aq)
Cu(s)
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 .
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:
At the anode:
Al3+ (l) + 3e2O2-(l)
Al (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 .