Electrochemistry

Electrochemistry

General Introduction
Redox reactions




Oxidation and reduction


Property Oxidation Reduction
1 Hydrogen Loss Gain
2 Oxygen Gain Loss
3 Electron Loss Gain
4 Oxidation No. Increase Decrease



Oxidising agent or oxidant Reducing agent or reductant

1 Under goes reduction Undergoes oxidation
2 Gain electrons Donates electrons
3 Oxidation no. of its atoms increases Oxidation no. of its atoms decreases

For example, consider the reaction
Zn(s)  CuSO4 (aq)
ZnSO4 (aq)  Cu(s)
Or Zn(s)  Cu2 (aq)
Zn2 (aq)  Cu(s) ……(i) Concept Ladder
Zn is oxidized on Zn2+ ions while Cu2+ has been
Displacement reactions
reduced to Cu.
can and can't be
Similarly in the reaction redox reactions. Single
Cu(s)  2 AgNO3 (aq)
Cu(NO3 )2 (aq)  2 Ag(s) displacement reactions
Or Cu(s)  2 Ag
Cu (aq)  2 Ag(s) ……(ii)
2 are redox reactions where
as double displacement
Electrochemistry




Cu has been oxidized to Cu2+ ions while Ag+ has
reactions are not redox
been reduced to Ag.
reactions.



1.

, Again in the reaction
Zn(s)  H2 SO4 (aq)
ZnSO4 (aq)  H2 (g )
Previous Year’s Question
Or Zn(s)  2 H (aq)
Zn2 (aq)  H2 (g) ……(iii)
Zn has been oxidized to Zn2+ whereas H+ In acidic medium H2O2 changes
ions have been reduced to H2 gas. Cr2O72- to CrO5 which has two
The substance which gets reduced (—O—O—) bonds. Oxidation state

oxidizes the other substance and is called of Cr in CrO5 is [NEET-2014]
oxidizing agent or oxidant while the substance (1) +5 (2) +3 (3) +6 (4) -10
which gets oxidized reduces the other substance
and is called reducing agent or reductant. Thus,
oxidizing agent is a substance which gains Concept Ladder
electrons while reducing agent is a substance
According to classical
which loses electrons.
concept, oxidation is an
Redox reaction is defined as the reaction addition of oxygen [or
which is considered to be made up of two half electronegative radical/
reactions, one involving oxidation, i.e. loss of element] or removal of
hydrogen [or electropositive
electrons and the other involving reduction, i.e.,
radical/element
gain of electrons. These are known as oxidation
half-reaction and reduction half-reaction. For
example, the reaction (i) may be split into two
half reactions as under:
Rack your Brain

All redox reactions are
exothermic. Why?


Electrochemistry
Electrochemistry is defined as the branch
of physical chemistry, which deals with the study Definition
of conversion of electrical energy from chemical Electrochemistry deals with
energy which is produced in a redox reaction relationship between electrical
Electrochemistry




and chemical energy
or how to get redox reaction by using electrical
energy, which is otherwise non-spontaneous.



2.

,3.
Electrochemistry

, • Electro chemical cell concept came from the study of redox reaction




• This reaction happens by its own. Therefore
its Gibb’s free energy is negative.
Definition
DG < 0 DG > O
The device in which
(spontaneous) (Non spontaneous
chemical energy is converted
• Work done by the system in electrochemical into electrical energy by
Electrochemistry




process Work done = –DG spontaneous redox reaction is
• Electrical work = Charge × potential called Galvanic cell Or Voltaic
Difference cell



4.