Chemistry

1. UNIT THREE
Electrochemistry
 is a field of chemistry that deals with the relationship between electrical
energy and chemical energy.
 It is also a field of chemistry concerned with processes that bring about
chemical reactions (changes)using electricity or generating electrical
energy from chemical reactions. Thus, electrical energy and chemical
energy are inter-convertible.
 Studies about inter conversion of one formto another form.
 They are also a role in the production of dry cells and lead storage batteries
used in the automotive industry.
 Electrochemical cells arethe devices that convertchemical energy to electrical
energy or electricalenergy to chemical energy. They classified as electrolytic
cells and galvanic cells.
 Electrolyticcells convertelectricalenergy to chemical energy.
 Galvanicorvoltaiccells convertchemicalenergy to electrical energy.
Examples :- flashlight batteries, wrist watches, cameras and car batteries.
 Electrolysisisused to manufacture metals like sodium, aluminium; non-
metals like chlorine, hydrogen and compounds like sodium hydroxide and
sodium hypochlorite.
 Electrical conductivityisthe capacity of a substance to transmit electricity.
 The materials that allow the passage of electricity through them are called
electrical conductors.
 Conductivity apparatus is used to test the conductivity of different solid
substances or that of aqueous solutions of different compounds. The basic
components of conductivity apparatus include electric wires,electrodes,a
d.c. sourceor dry cells,a switch andlight bulb.
 Depending on the nature of the particles responsible forthe flow of electric
charges through conductors, electrical conductivity canbe classified as
metallic conductivity or electrolyticconductivity.
 Metallic conductivity (electronic conductivity) :-
 refers to the transmission of electric current through metals. This
transmission is directly related to the structure of metals. In atoms of
metals, the valence electrons are bounded very loosely to their respective
nuclei and move very easily throughout the metal. This means metals
contain electrons that do not have fixed positions and are relatively free to
move. These electrons are called free electrons or mobile electrons or
delocalized electrons.
 It is caused by the flow of mobile electrons due to repulsion exerted on them
from the electrons entering the metal from the source of electricity.
 The charge carriers in metallic conductionare electrons.
 Non-metals are generally non-conductors of electricity,because they do not
have freely moving electrons.
 Electrolytic conductivity
 Are substances that transmit electricity in a molten state or in aqueous
solution.
 Strongelectrolytes ionizealmost completely in aqueous solutions.
 Weak electrolytes ionizeonly slightly.
 The charge-carriers in electrolytic conductivity are ions (anions and
cations).
 Non-electrolytes area substances that do not transmit electricity
either in solution or in a molten state. Examples:- sugar, ethanol, oil,
benzene
 Electrolysisisa process in whichelectrical energy is used to produce
chemical Changes. This process is carried out in an electrochemicalcell
knownas an Electrolytic or electrolysis cell.
 Electrodes are strips of metal or graphite that allow electrons to leave or
enter the electrolytes. They can be chemically active or inert.
 Activeelectrodes directly take part in reactions.
Examples :- zinc and magnesium.
 Inert electrodes do not directly take part in chemical reactions. They only
serve to transfer electrons. Examples:-platinum and graphite.
 Anode(+)is an electrode connected to the positive terminal of the source
 It is the electrode through whichelectrons leave the cell.
 It is also the electrode at whichoxidation takes place
 Cathode(-) is an electrode connected to the negative terminal of the
source
 It is the electrode through whichelectrons enter the cell.
 It is also the electrode at which reduction occurs
 Cations are a positive ions attracted to the cathode.
 Anions are a negative ions attracted by the positive electrode (anode)
 Oxidation is a process of losing electrons.
 Reduction is a process of gaing electron
 Half-cellreaction is the reaction that takes place at each electrode
 Oxidation half-reactions occurat anode and reduction half reaction occurs
a cathode.
 cell reaction is the net reaction that takes place in the electrolytic cell.

2. Galvaniccell (voltaiccells)
 is a device in whichchemical energy is changed to electrical energy.
 Electrons flow in the a spontaneous direction which means from anode to
the cathode
 They are classified into primary cells, secondary cells, and fuel cells
Primary Cells
 are not rechargeable. This is because the electrode reaction as well as the
entire cell reaction cannot be reversed on recharging.
 it is not possible to recoverthem by charging the cells.
 the reactionstaking placein them areirreversible.
 The electrolyte in a Galvanic cell can be in the form of a solution (wet cells)
or a paste( dry cells).
 Example:-Daniel’s cell and zinc-carbon(Leclanche)dry cells
Anodehalf-reaction:Zn (s)  Zn2+ (aq)+ 2e–
Cathodehalf-reaction:Cu2+ (aq)+ 2e–  Cu (s)
Over all reaction(Cell reaction):Zn(s) + Cu2+ (a q)  Zn2+ (a q) + Cu(s)
Galvanic(voltaic)cell Electrolyticcell
Convert chemical energy into
electrical
Convert electrical into chemical
energy
Spontaneous Non- spontaneous
Anode is positive Anode is negative
Cathode is negative Cathode is positive
Oxidation occurs at anode Oxidation occurs at anode
Reduction occurs at cathode Reduction occurs at cathode
Saltbridge
 is the connection between the two solutions
 is used to - maintain electrical neutrality between the twosolutions
- allow the movement of ion between the solution
- prevent mixing of the electrode solutions
Secondary Cells
 are rechargeable.
 It needs to be recharged when it stops producing electricity.
 the reactionstaking placein them arereversible.
Example :- A lead storage battery
Applicationofelectrochemistry
 Productionof metal and non-metal
 Purificationof metal
 Electroplating metals
 Productionof chemicals
 Productionof galvanic cell
Note :- - Electroplating isa process of coveringthe surface of a metal (metal
article) witha thin layer of another metal.
- Electro refining the process of purifying metals by electrolysis.