1. Electrolysis is the process of using an electric current to drive nonspontaneous chemical reactions. It involves electrochemical cells with an electrolyte solution and two electrodes connected to an external power source. 2. During electrolysis, ions migrate within the electrolyte - positive ions move toward the negatively charged cathode and negative ions move toward the positively charged anode. Reduction occurs at the cathode as ions gain electrons, while oxidation occurs at the anode as ions lose electrons. 3. Applications of electrolysis include electroplating metals onto surfaces, extracting reactive metals like aluminum from their ores, and producing chlorine and sodium hydroxide via the chlor-alkali process. Factors like electrode material and electrolyte concentration

1. CHEMISTRY ELECTROLYSIS Issue 1
11
Chemistry
In this issue
 Electrochemical cells
 Parts of an electrochemical cell; electrolytes, salt bridge,
electrodes.
 Redox Reactions
 Electrolytic cells
 Applications of electrolysis
From the lectures of Ms. Sadaf Gulzar
Electrochemistry
Electrochemistry
By: Fatima Laraib
By Fatima Laraib
Electrochemistry is the study of the relations cathode of a galvanic cell is its
between electrical and chemical phenomena positive terminal. In both galvanic
in terms of chemical changes produced by and electrolytic cells, oxidation
electrical current and the production of takes place at the anode and
electricity by chemical reactions electrons flow from the anode to the
cathode.
An electrochemical reaction is any chemical
reaction which is either caused or  The anode and cathode of an Parts of a cell
accompanied by the passage of an electric electrolytic cell are connected to a This section will deal with the components of an
current and involving the transfer of battery or other direct current electrochemical cell in particular. For e.g, the salt
electrons between two substances. source, whereas a simple cell serves bridge.
as a source of electrical energy.
Electrochemical Cells Page 2
 In electrolytic cells, electrical
Electrochemistry involves electrochemical energy from an external source
cells which is a device capable of either causes non-spontaneous redox
deriving electrical energy from chemical reactions to occur, whereas in
reactions or facilitating chemical reactions simple cells, spontaneous redox
through the introduction of electrical energy. reactions produce electricity.
It is also a device where electron transfer is
forced to take an external pathway instead  In electrolytic cells, the electrodes
of going directly between the reactants. can be of the same or different
metals but in simple cells, the
The differences between a simple cell and an electrodes have to be of different
electrolytic cell are: elements only.
 The anode of an electrolytic cell is  In electrolytic cell Ions are Redox Reactions
positive (cathode is negative), since discharged on both the electrodes. This section will talk about the preliminary ideas behind
the anode attracts anions from the While in a galvanic cell ions are only one of the most important electrochemical reactions.
solution. However, the anode of a discharged at the cathode i.e. redox reactions.
galvanic cell is negatively charged,
since the spontaneous oxidation at  The electrons flow from the cathode Page 3
the anode is the source of the cell's to anode in an electrolytic cell. This
electrons or negative charge. The is not true in case of galvanic cells.

2. CHEMISTRY ELECTROLYSIS | Issue 1 2
Parts of an electrochemical
cell
Electrolytes
The electrolyte is a molten ionic
compound or an aqueous solution that
conducts electricity. It provides the
medium for transfer of ions inside the
cell between the anode and cathode. It
only conducts electricity because it
contains charged particles (ions) that are
mobile. It should be a non-conductor of
electrons to avoid self-discharge of the
cell.
e.g:- acids, alkalis, aqueous solutions of
salts, molten salts.
Strong electrolytes KEY POINT!! ELECTRICAL & ELECTROLYTICAL
The substances, which ionize almost CONDUCTION
Weak electrolytes Metals and ionic compounds are conductors of
completely into ions in aqueous phase In aqueous phase environments, the
environments, are known as strong electricity. Ionic compounds in their molten or
substances which ionize to a small extent
aqueous states are ELECTROLYTICAL
ELECTRICAL & called electrolytic
electrolytes. Strong electrolytes fall into into ions are known as weak electrolytes.
The weak electrolytes include weak conductors because they conduct electricity by
CONDUCTION
three categories: strong acids, strong
acids and weak bases. In this case, the movement of ions across the electrolyte.
bases, and salts. (Salts are sometimes Thus the electrolytes are decomposed to form
also called ionic compounds) the molecules are in equilibrium with
new substances. In case of electrical conduction,
The equation which involves their ions. The equation which involves
electricity is conducted by the flow of electrons
the ionization of weak electrolytes is
the ionization of strong electrolytes is from one end of the conductor to the other end.
represented with double headed
represented with only single headed Thus metals and graphite remain chemically
arrows. For example,
arrow directed to the right. For unchanged when an electric current flows
+ -
example, CH3COOH + H2O ⇌ H3O + CH3COO through them.
+
HCl + H2O ———-> H3O + Cl
releasing or sending oppositely The anode or negative electrode is the
charged ions into the solution. reducing electrode. It gives up electrons
to the external circuit and is oxidised
Electrodes during the electrochemical (discharge)
reaction. It is generally a metal or an alloy
Electrodes are metallic or graphite but hydrogen is also used. The anodic
terminals which are responsible for process is the oxidation of the metal
bringing in or carrying out electric reducing agent to form metal ions.
current within or outside the
electrolytic cell. There are two types The cathode or positive electrode is the
of electrodes, active electrodes and oxidising electrode. It accepts electrons
inert electrodes. from the external circuit and is reduced
during the electrochemical (discharge)
Salt bridge Active electrodes reaction. It is usually a metallic oxide or a
Electrodes like copper and silver sulfide but oxygen is also used. The
which participate in the cathodic process is the reduction of the
It is the separator which electrically isolates the electrochemical processes are known oxidising agent (oxide) to leave the metal.
positive and negative electrodes. It is a device as active electrodes.
used to connect the oxidation and reduction half-
cells of a galvanic cell. Salt bridges usually come Inert Electrodes
Inert electrodes such as graphite or
in two types: glass tube and filter paper. The salt
platinum are non-reactive and are
bridge: not affected by the ions surrounding
a) Acts as a link between the two aqueous them in an electrochemical reaction.
solutions.
b) Overcomes liquid junction potential.
c) Maintains the electrical neutrality of the
aqueous solution of the electrodes by

3. CHEMISTRY ELECTROLYSIS | Issue 1 3
REDOX REAGENTS
The picture shows the rusting
of iron which is a redox
reaction
Potassium Iodide test
Potassium Iodide changing its colour from colourless
to brown in the presence of an oxidising agent
Redox Reactions
Redox reactions Oxidation State
Redox reactions describe all chemical The charge of an element in a compound
reactions in which atoms have their with respect to the number of electrons
oxidation number (oxidation state) it has lost or gained.
changed or a reaction in which electrons
are transferred, thereby oxidizing some Principles governing oxidation
atoms, and reducing others. states
Potassium Manganate Test Oxidation 1. All the elements/molecules in
Potassium manganate changes its colour from purple to A substance is said to be oxidized when their atomic or diatomic forms
colourless in the presence of a reducing agent. carry a charge of 0.
it enters into a combination with oxygen,
2. The oxidation state of a simple
loses hydrogen, loses electrons or ion is the same as the charge on
FAST FACTS
increases its oxidation state after a the ion.
reaction. 3. The oxidation states of the
atoms present in the formula of
Decomposition of compounds to form a compound must add up to 0.
elements are redox reactions.
Reduction
A substance is said to be reduced when it 4. The total of the oxidation states
A single replacement reaction is of a polyatomic ion is equal to
always a redox reaction because it loses oxygen, gains hydrogen, gains
its charge
involves an element that becomes electrons or decreases its oxidation state 5. Oxygen in all its oxides will
incorporated into a compound and an after a reaction. carry a charge of -2 where as in
element in the compound being peroxides will carry a charge of
released as a free element. -1.
A double displacement reaction is Oxidising agent (oxidant) 6. Hydrogen when bonded with a
usually not a redox reaction. Is a substance which oxidises another non-metal will carry a charge of
Neutralization reactions, acid- +1 whereas with metals it will
substance and itself is reduced by
carbonate reactions and precipitation exhibit the -1 charge.
reactions are usually non-redox. accepting electrons from the other
7. Group I, II and III elements will
reactant. E.g:-
carry a charge of +1,+2 and +3
 Potassium dichromate (VI) consecutively.
 Potassium Manganate (VII)
A half-cell reaction is a reduction or an  Concentrated Sulfuric acid
oxidation reaction component of a redox
reaction. Reducing agent (reductant) FAST FACT
Is a substance which reduces another
A half reaction is obtained by considering In voltaic cells the oxidation and reduction
the CHANGE in oxidation states of the substance and itself is oxidised. Good
reactions are separated via half cells.
INDIVISUAL substances involved in a redox donors of electrons E.g:-
reaction.  Metals
 Hydrogen
The atoms and charges must be balanced on  Sulfer dioxide
both the sides of the equation.
 Potassium Iodide
Equations can be combined to give an  Ammonia
overall picture of the reaction or left as
individual equations to illustrate the
oxidation or the reduction steps separately.

4. CHEMISTRY ELECTROLYSIS | Issue 1 4
Electrolytic Cell
The electrolyte which is the substance
Voltaic cells use a spontaneous Electrolysis depends over:
being electrolyzed must contain free
chemical reaction to drive an
electric current through an external moving ions. 1. The electrochemical series
circuit. These cells are important 2. Molarity/concentration of
Connection of the electrodes, to a source solution.
because they are the basis for the
of direct electric current renders one of 3. Types of electrodes.
batteries that fuel modern society.
them negatively charged and the other 4. Type of electrolyte; molten or
But they aren't the only kind of
positively charged. Positive ions in the aqueous.
electrochemical cell. It is also
electrolyte migrate to the negative
possible to construct a cell that
electrode (cathode) and they combine
does work on a chemical system by
with one or more electrons in order to
driving an electric current through
gain their stable atomic state back. Thus
the system. These cells are
reduction occurs at the cathode.
called electrolytic cells.
Simultaneously, negative ions migrate to
The redox reaction in an electrolytic the positive electrode (anode) and
cell is nonspontaneous. Electrical transfer one or more electrons to it, losing
energy is required to induce the their charge and becoming neutral
electrolysis reaction. particles. Thus, oxidation occurs at the
anode.
Electrolysis refers to the
decomposition of a substance by an The process of gaining or losing electrons
electric current. It involves the at the electrodes is called discharge.
reactions within the electrolyte and When ions are discharged at the
the reactions at the surface of the electrodes they form atoms or ions.
electrodes.
Factors affecting discharge of
ions in the Electrolyte
The factors that influence the
discharge of ions are
1. Relative positions of the ions
in the reactivity series.
2. Concentration of the ions in
the electrolytes.
3. Nature of the electrodes
RULES FOR PREDICTING THE RULES FOR PREDICTING THE SELECTIVE
SELECTIVE DISCHARGE FOR DISCHARGE FOR CATIONS
ANIONS
Positive ions from the metals lowest in the
OH- ions from water are reactivity series are discharged at the
preferentially discharged when the cathode in preference to any other ions
solutions are dilute to form O2 present in the solution.
Negative ions such as chloride, Ions of less reactive metals are
bromide and iodide can be preferentially discharged. (e.g copper and
preferentially discharged when silver)
their concentrations are high
enough when compared to OH – Otherwise, H+ ions from water will be
discharged/reduced to form hydrogen gas.
When sulfate and nitrate ions are
present in water, it is the 2H+(aq) + 2eH2(g)
hydroxide ions that are
preferentially discharged. Ions of very reactive metals cannot be
discharged in the presence of water.

5. CHEMISTRY ELECTROLYSIS | Issue 1 5
METAL EXTRACTION
Applications of electrolysis Obtaining metals from their ores
generally involve refining crude metal to
obtain the pure metal. This is done via
The numerous applications of electrolysis electrolysis.
include: ELECTROPLATING In general the more reactive the metal
The process of depositing a layer of is, the harder it is to extract the metal
1. Electrolytic purification metal on another substance using from its ore. Reactive metals such as
2. Electroplating electrolysis is called electroplating. sodium, potassium, calcium, magnesium
3. Metal Extraction and aluminum cannot be extracted by
Both the anode and the cathode are reduction with Carbon. The compounds
immersed in a solution which contains a of these metals are very difficult to split
ELECTROLYTIC PURIFICATION dissolved metal salt (e.g., an ion of the up. Hence electricity is used to extract
metal being plated) and other ions which these metals.
Electrolysis is used to purify metals as in
act to permit the flow of electricity
the electrolytic purification of Copper.
through the circuit. Direct current is CHLOR-ALKALI EXTRACTION
supplied to the anode, oxidizing its
Electrolysis of aqueous copper (II)
metal atoms and dissolving them in the
sulfate using copper electrodes. electrolyte solution. The dissolved metal
ions are reduced at the cathode, plating
At the Cathode
the metal onto the item. The current
through the circuit is such that
Both Cu2+ and H+ ions are attracted to the rate at which the anode is dissolved
the Cathode but Copper ions are is equal to the rate at which the cathode
preferentially discharged and deposited is plated.
on the Cathode as a brown layer of solid Hence:
copper.
The object to be plated is made
Cu2+(aq)+2eCu(s) the cathode.
The anode is the source of the
At the Anode plating material.
The electrolyte is an aqueous The chlorine-alkali (chloralkali) industry is an
Both hydroxide and Sulfate ions are solution of a salt of the plating important part of the chemical industry, and
attracted but NEITHER is DISCHARGED. material. produces chlorine and sodium
hydroxide through the electrolysis of table salt
The net result is the transfer of
In fact, the copper anode dissolves to (NaCl). The main raw material is brine which is
the plating material from the
form Cu2+ ions in aqueous solution. a saturated solution of sodium chloride (NaCl)
anode to the cathode. that is obtained from natural salt deposits (read
Cu(s)  Cu2+(aq)+2e more at page 8)
Electroplating of metals helps to prevent
Overall reaction their corrosion and rusting and also
improves their appearance. A common ALUMINIUM EXTRACTION
1. Cathode gains copper ions and example includes galvanizing; i.e.
becomes larger. coating iron/steel with a layer of
2. No gas is evolved at the anode. another metal like zinc in order to
In fact, the anode loses copper prevent rusting.
ions and becomes smaller. Another example can be silver or gold
3. Concentration and colour of the plating which is used to coat a relatively
copper(II) sulfate solution cheap metal to make it look more
remains unchanged. expensive.
4. Amount of copper ions
discharged to form copper Conditions favoring a good quality metal
deposits at the cathode (from plating
the solution) = Amount of
The metal object to be plated
copper atoms (from the anode)
must be clean and free of Aluminum is extracted from the ore bauxite
which ionizes and enters the (Al2O3) by electrolysis. The Al2O3 is insoluble,
grease.
solution as copper ions. so it is melted to allow the ions to move when
The concentration of metal ions
an electric current is passed through it. The
in the electrolyte must be low.
Hence to refine copper, impure copper is anodes are made from carbon and cathode is
The electric current must be
used as the anode. During electrolysis the carbon-lined steel case.
not too large; otherwise the At the Cathode: Al3+(l)+3e-Al(l)
the impure copper anode dissolves and a
coating layer will form too At the anode: 2O2-(l) O2(g)+4e-
layer of pure copper is deposited at the
rapidly and peel off easily. The oxygen reacts with the carbon anodes to
cathode. form carbon dioxide. The constantly need to be
replaced because of this.

6. CHEMISTRY ELECTROLYSIS | Issue 1 6
ELECTROLYSIS OF Electrolysis of molten sodium As the hydrogen ions are lower in the
chloride reactivity series than sodium, they accept
BINARY COMPUNDS electrons more easily. The hydrogen ions
Sodium chloride contains Sodium ions
are discharged.
Many ionic compounds are binary (Na+) and Chloride ions (Cl-).
2H+(aq)+2e-H2
compounds
At the cathode:
A binary compound is a compound At the anode:
containing only two elements. It  Each Na+ ion gains one electrons Both the chloride ions and hydrogen ions
contains a metal cation and a to form a sodium atom. It is migrate to the anode but the chloride ions
non-metal anion. reduced are preferentially discharged because of
their higher concentration.
The electrolysis of a molten  Na+(l) + e- Na(l)
2Cl-(aq)Cl2(g)+2e
binary compound will yield a Hence the colourless chlorine gas is given
At the anode:
metal and a non-metal as off.
products.  Each Cl- ion gives up one electron As the hydrogen and chloride ions are
to form a chlorine atom. It is discharged, sodium and hydroxide ions
Electrolysis of molten lead bromide
oxidised. Two chlorine atoms remain in the solution. The solution
Lead(II) Bromide, PbBr2 is an ionic binary then combine to form a chlorine becomes sodium hydroxide.
salt made of the ions Pb2+ and 2xBr- molecule.
Electrolysis of aqueous Sodium
 Cl-(l) Cl(l)+e-
Chloride
2Cl(l) Cl2(g)
An aqueous solution of sulfuric acid
ELECTROLYSIS OF contains four different ions.
Ions from NaCl: Na+(aq) and Cl-(aq)
AQUEOUS SOLUTIONS Ions from water: H+(aq) and OH-(aq)
An aqueous solution of a compound is a At the cathode:
mixture of two electrolytes. The hydrogen and sodium ions are
Ions discharged depend on their position in attracted to the cathode. H+ ions gain
the electrochemical series. electrons from the cathode to form
hydrogen gas.
Electrolysis of Concentrated 2H+(aq)+2e-H2(aq)
Sodium Chloride (Brine)
Sodium ions remain in solution.
The electrolytic cell used for electrolysis
At the cathode: of concentrated sodium chloride solution At the anode:
2+ is designed to collect gaseous products The hydroxide and chloride ions are
 Pb ions gain electrons from the attracted to the anode. Hydroxide ions
at both electrodes.
electrode to become lead atoms. give up electrons to the anode to form
water and oxygen gas.
 The Pb2+ ions are reduced
4OH-(aq)2H2O(l)+O2(g)+4e-
 The Pb2+ have been discharged
and molten greyish globules of Chloride ions remain in the solution.
lead metals are formed below the
electrolyte Hence the overall reaction is
2H20(l) 2H2(g)+O(g)
 Electrode reaction at cathode:
Pb2+(l)+2e Pb(l) Since water is being removed by
decomposition into hydrogen and oxygen,
At the anode: the concentration of sodium chloride
solution increases gradually.
 Br- ions lode electrons to become With electrolysis of aqueous solutions of
bromine molecules. dilute acids or alkalis, the volume of
The cathode can be platinum or
hydrogen given off at the cathode is twice
 The Br- ions are oxidised. carbon but the anode must be
that of the oxygen gas at the anode.
carbon to resist attack by chlorine.
 Bromide ions are discharged NOTE: The test tube containing hydrogen
forming an effervescence of Four ions are present in the
mostly (more than half) appears to be
pungent, red-brown bromine gas. solution:
empty as compared to the one containing
Cations: H+(aq) and Na+(aq)
oxygen because hydrogen is completely
 Electrode reaction at the anode: Anions: OH-(aq) and H+(aq
insoluble in water while oxygen is slightly
2Br-(l) Br2 (g) + 2e soluble.
At the cathode:
The sodium and hydrogen ions
move to the cathode.

7. CHEMISTRY ELECTROLYSIS | Issue 1 7