Conduction of electricity
 conductor is a solid which allows
electricity to pass through but is not
chemically changed during the
conduction
 presence of freely moving valence
electrons e.g. all metals and graphite
 conducting rods & wires are used in
electrolysis cells
 non-conductor is a solid which does
not allow electricity to pass through,
a.k.a insulator
 valence electrons are held in fixed
positions e.g.. sulfur, phosphorus,
diamond, solid state crystalline salts,
wood and glass etc.
Electrolysis
Cell
(Source of electricity)
 electrolysis is the process of breaking up an
Conducting wire


Beaker

Cathode Electrode
Anode Electrode

Electrolyte

ionic compound into simpler substances,
usually elements, using electricity
electrolysis occurs when the ionic compound
is in aqueous solution or molten because it
allows electric current to pass through
electrolysis is carried out in an electrolysis
cell
electricity is passed from a cell or battery
through a liquid which may be a aqueous
solution or molten solid
a molten ionic compounds or aqueous
solution of ionic compounds that allows
electricity to pass through is called an
electrolyte
the rods which carry the electric current to
and from the electrolyte are called electrodes
Electrodes
 electrodes are rods that carry the
electric current to and from an
electrolyte
 they are usually; platinum, copper,
graphite
 Cathode:
 electrode that is connected to the
negative terminal of the cell or battery.
 positively charged ions, cations,
moved towards the cathode
 Anode:
 electrode that is connected to the
positive terminal of the cell or battery.
 negatively charged ions, anions,
moved towards the anode
Electrolytes and non-Electrolytes
 electrolytes:
 molten ionic compounds or
aqueous solution of ionic
compounds that allows electricity
to pass through and are
decomposed in the process
 e.g.. acids, alkali, salts dissolved in
water, molten salts
 non-electrolytes:
 does not allow passage of electricity
 e.g.. distilled water, alcohol,
turpentine, oil, paraffin, organic
solvents
Electrolysis
 when electricity is passed through an




electrolyte, chemical decomposition
occurs
this involves the ‘splitting up’ of the
electrolyte
since all electrolytes are ionic, composed
of positively charged ions (cations) &
negatively charged ions (anions)
when an electric current pass through the
electrolyte, ions in the solution or liquid
migrate towards the oppositely charged
electrode
discharge of ions at the electrodes results
in the chemical decomposition of the
electrolyte to form its elements
 at the anode, negatively charged ions




lose their electron(s) to the anode to
form neutral atoms
the negatively charged ions are said to be
oxidized and discharged at the anode oxidation occurs at the anode
at the cathode, positively charged ions
gain electron(s) from the cathode to
form neutral atoms
the positively charged ions are said to be
reduced and discharged at the cathode reduction occurs at the cathode
metals or hydrogen are formed at the
negative electrode (cathode), and that
non-metals (other than hydrogen) are
formed at the positive electrode (anode).
Electrolysis of molten Compounds
 many ionic compounds are binary
compounds i.e. compounds
containing only 2 elements e.g. KI,
PCl2, NaCl
 binary ionic compound contains a
metal cation and a non-metal
anion
 electrolysis of a molten binary
compound will yield a metal and a
non-metal as products
 metal will be formed at the
cathode while non-metal will be
formed at the anode
Electrolysis of molten Compounds
Product formed at;
Compound Electrolysed
Cathode (-)
Anode(+)
Copper Bromide
Copper
Bromine
Zinc Chloride
Zinc
Chlorine
Sodium Chloride
Sodium
Chlorine
Aluminium Oxide
Aluminium
Oxygen
Electrolysis of Molten Lead(II) Bromide
Electrolysis of Molten Lead(II) Bromide
 At the cathode:
 Pb2+ ions gain electrons from the
electrodes to become lead atoms
 Pb2+ ions have been discharged and
molten greyish globules of lead metal
are formed below the electrolyte
 electrode reaction at the cathode;
 Pb2+(l) + 2e→ Pb(l)
 At the anode:
 Br- ions lose electrons to electrode to
become bromine molecules - Br- are
oxidized
 Bromine atoms combine to form
bromine molecules forming an
effervescence of pungent, red-brown
bromine gas
 electrode reaction at the anode;
 2 Br- (l) → Br2(g) + 2e-
Predicting products in the electrolysis of aqueous
solutions using inert electrodes e.g. graphite;
Electrolysis of Aqueous Solutions
 an aqueous solution of an ionic
compound is a mixture of 2 electrolytes –
one of which is water
 although water is molecular, in aqueous
solutions, a small % of it ionizes into
hydrogen ions & hydroxide ions;
 H2O(l) →
H+(aq) + OH-(aq)
 e.g. aqueous copper (II) sulphate;
contains water & copper (II) sulphate
 water ionises into;
 hydrogen ions & hydroxide ions
 copper (II) sulphate ionises into;
 copper (II) ions and sulphate ions
 ions discharged at the electrodes depends
on the position of the ions in the
electrochemical series
Rules for Predicting Selective Discharge of Cations
 cations from the metals lowest in the




reactivity series are discharged at the
cathode in preference to any other cation
present in the solution
cations of less reactive metals e.g. Cu2+,
Au+, Ag+ are preferentially discharged
otherwise, H+ ions from water will be
discharged to form H2 gas
2H+(aq) + 2e-  H2(g)
cations of very reactive metals (Na+, K+,
Ca2+) cannot be discharged in the
presence of water, so they remain in
solution
Rules for Predicting Selective Discharge of Anions
 OH- ions from water are preferentially
discharged when the solutions are
dilute, to form O2
 4OH-(aq) → O2(g) + 2H2O(l) + 4e anions such as Cl-, Br- and I- can be
preferentially discharged when their
concentrations are high enough when
compared to OH when SO42- and NO3- are present in
water, it is the OH- from water which
is preferentially discharged
Product formed at;
Electrolyte
Cations & anions
present
Cathode (-)
Anode(+)
Concentrated sodium
chloride
Na+, H+, Cl- & OH-
Hydrogen
Chlorine
Dilute sulfuric acid
H+, SO42- & OH-
Hydrogen
Oxygen
Dilute sodium chloride
Na+, H+, Cl- & OH-
Hydrogen
Oxygen
Concentrated
hydrochloric acid
H+, Cl- & OH-
Hydrogen
Chlorine
Dilute Silver nitrate
Ag+, H+, NO3- & OH-
Silver
Oxygen
Dilute Copper nitrate
Cu2+, H+, NO3- & OH-
Copper
Oxygen
Dilute hydrochloric acid
H+, Cl- & OH-
Hydrogen
Oxygen
Electrolysis of dilute sodium chloride solution
 at the anode;
 anions OH- & Cl- moves to the anode
 OH- ions are selectively discharge according to
the position of ions in the electrochemical
series
 gas bubbles are formed which re-lights a
glowing wooden splinter -this gas is oxygen
 4OH-(aq)  2H2O(l) + O2(g) + 4e at the cathode;
 cations H+ & Na+ moves to the cathode
 H+ ions are selectively discharge according to
the position of ions in the electrochemical
series
 gas bubbles are formed, when a lighted wooden
splinter is placed near the test tube, a 'pop'
sound is produced – this gas is hydrogen gas
 2 H+(aq) + 2e-  H2(g)
Electrolysis of dilute sulfuric acid
Electrolysis of dilute sulphuric acid
 dilute sulphuric acid can be electrolysed
using the apparatus shown
 at the cathode:
 H+ ions attracted to the cathode
 electrode reaction at the cathode(-);
2H+(aq) + 2e- → H2(g)
 hydrogen gas is produced at the cathode
(gas B)

 at the anode:
 both OH- and SO42- will be attracted to the
anode but OH- ions are preferentially
discharged
 electrode reaction at the anode(+);
 4 OH-(aq) → O2(g) + 2H2O(l) +4e oxygen gas is produced at the anode (gas A)
 overall Reaction;
 2H2O(l) → 2H2(g) + O2(g)
 this reaction is sometimes known
as electrolysis of acidified water
 sulphuric acid is added to increase
the number of mobile ions to help
conduct electricity
 in this process, the amount of acid
remains the same, but amount of
water decreases - hence, the
concentration of sulphuric acid
increases
 how do you test for gas A (oxygen)
& gas B (hydrogen)?
Electrolysis of aqueous copper(II)sulphate using carbon electrodes
 copper (II) sulfate solution contains the
following ions; blue Cu2+, SO42- and H+ &
OH- from water
 Cu2+ & H+ migrate to the cathode while
SO42- & OH- migrate to the anode
 at the cathode;
 Cu2+ ions are preferentially discharged

and deposited on the cathode as a brown
deposit of solid copper
Cu2+(aq) + 2e → Cu(s)
 at the anode;
 OH- ions are preferentially discharged and
oxygen bubbles are given off
 4 OH-(aq) → O2(g) + H2O(l) +4e-
 the blue colour of copper (II) sulfate
solution fades as Cu2+ ions are
discharged
Electrolysis of aqueous copper(II)sulphate using copper electrodes
 copper (II) sulfate solution contains the following ions;





blue Cu2+, SO42- and H+ & OH- from water
Cu2+ & H+ migrate to the cathode while SO42- & OHmigrate to the anode
at the cathode;
 Cu2+ ions are preferentially discharged and
deposited on the cathode as a brown deposit of
solid copper thus the cathode grows thicker
 Cu2+(aq) + 2e- → Cu(s)
at the anode;
 electrodes are not inert, copper loses electrons
forming copper ions which goes into solution
 Cu(s) → Cu2+(aq) + 2e the anode gets thinner
Cu2+ ions formed at the anode move to the cathode
where they are deposited
the blue colour of copper (II) sulfate solution does not
fade because Cu2+ ions removed from the solution are
replaced by those from the anode
Electroplating
 electroplating is the application of a




thin layer of metal to a metallic or
other conducting surface through
electrolysis
the object to be plated is made the
cathode (- ve electrode) of an
electrolysis cell
the anode is usually a bar of the
plating metal
the object is immersed in an aqueous
solution containing cation of the
plating metal
during electrolysis, the plating metal
(anode) dissolves forming cation
which are deposited on the object
(cathode) as metal atoms
Electroplating steel key with copper
 to which terminal of the battery is the copper foil & steel
key connected to?
 copper foil is connected to the + ve terminal to become
the anode & while the steel key is connected to the - ve
terminal to become the cathode
 suggest a suitable substance to be used as an electrolyte
 aqueous solution of copper (II)
sulfate/nitrate/chloride
 what observations would be made at the electrodes when
electric current is passed through the electrolyte for some
time?
 anode;- copper foil gradually become thinner
 Cu(s) → Cu2+(aq) + 2e cathode;- steel key gradually get coated with copper
 Cu2+(aq) + 2e→ Cu(s)
 giving a reason, predict what observation(s) will be made
in the electrolyte during electrolysis
 the colour of the copper sulfate electrolyte remains the
same as the Cu2+ ions removed from the solution at the
cathode are replaced by Cu2+ ions formed at the anode
Uses of Electroplating
 nickel, silver, tin & gold are the most
commonly used metals for
electroplating
 articles are electroplated to;
 improve their appearance;- plating
gives the object a shiny surface that
does not get dull, - gold plating is
used for jewelry
 to provide a protective coating;steel objects are coated with a layer of
tin which protects the metal
underneath from air and water thus
protecting it from rusting
 to give them special surface
properties
 to give them engineering or
mechanical properties
Manufacture of Cl2, H2 & NaOH from brine
 brine is a concentrated aqueous solution





of sodium chloride i.e. sodium chloride
(NaCl) dissolved in water (H2O)
brine is obtained from sea water or from
rock salt underground
electrolysis of brine is used to
manufacture chlorine, sodium hydroxide
& hydrogen on a large scale
chlorine is used for making disinfectants
& bleaches, manufacture of hydrochloric
acid and for water treatment
sodium hydroxide is used for making
soap & in extraction of aluminium
hydrogen is used for manufacture of
ammonia, making margarine & as a
pollution-free fuel
Electrolysing brine
 brine is electrolyzed in a diaphragm
cell shown in the diagram
 aqueous sodium chloride contains; Na+,
H+, Cl- & OH- ions
 Na+& H+ migrates to the cathode while
Cl- & OH- migrates to the anode
 at the anode;
 Cl- ions are preferentially discharged
forming chlorine gas
 2Cl-(aq)
→ Cl2(g) + 2e-
 at the cathode;
 H+ ions are preferentially discharged
forming hydrogen gas
 2H+(aq) + 2e- → H2(g)
 Na+ & OH- ions remain in the solution
forming an aqueous sodium hydroxide
(NaOH) solution
Manufacture of aluminium from aluminium oxide
 extraction of aluminium from aluminium



 aluminium is the most abundant metal
in Earth’s crust – it occurs as metal ore
called bauxite which contains about
50% Al2O3
 bauxite is purified to yield a white
powder, aluminium oxide, from which
aluminium can be extracted

oxide is done by electrolysis
first the aluminium oxide must be
molten to allow electricity can pass
through it
aluminium oxide has a very high melting
point (2,070°C), so it would be expensive
to melt it
instead, aluminium oxide is dissolved in
molten cryolite, sodium aluminium
fluoride (Na3AlF6), which has a lower
melting point (about 1000°C)
use of cryolite reduces some of the
energy costs involved in extracting
aluminium and also improves electrical
conductivity
 electrolysis is carried out using inert
graphite electrodes in steel electrolysis cell
 the electrolyte is molten aluminium oxide
 Al2O3(s) →
2Al3+(l) + 3O2-(l)
 at the cathode;
 aluminium ions (Al3+ )gain electrons forming
aluminium metal
 Al3+(l) + 3e- → Al(l)
 molten aluminium is siphoned out of the
steel electrolysis cell
 at the anode;
 oxide ions (O2-) lose electrons forming
oxygen atoms which combine to form oxygen
gas
 2O2-(l) → O2(g) + 4e oxygen reacts with hot carbon anodes
forming carbon dioxide thus the anodes
needs to be replaced from time to time
 overall equation for the electrolysis;
 2Al2O3(s) → 4Al(l) + 3O2(g)
Summary
Factors affecting discharge of Ions
Uses of electrolysis
 relative positions of the ions in the
 purification of metals
electrochemical series
 concentration of the ions in the
electrolyte
 nature of the electrode – inert or reactive
 electroplating
 extraction of metals
 manufacture chlorine, sodium hydroxide
& hydrogen from brine
Electricity and chemistry – Revision
 End of Topic C 5
 Read your textbook pages
70 to 81 and answer the
questions on page 84 & 85
 Conductors and insulators
(pp82 – 83) is not part of
your syllabus but it wont
harm your brain if you read
through!
 Best regards ~ Dnl