Electrochemistry: Class 12

Electrochemistry
CLASS 12
Manavi | Electrochemistry | September 13, 2017

Electrochemistry:
1. Production of electrical energy by spontaneous chemical reactions.
2. Use of electrical energy to bring out non spontaneous chemical reactions.
3. E.g.: battery and fuel cells converts chemical – electrical.
4. Less polluted way and energy efficient.
Electrochemical cells:
1. Simplest cell is the Daniell cell.
2. Rxn for this is: -
3.
Zn{s} + Cu2+{aq} Zn2+{aq} + Cu {s}
It has potential equal to 1.1 V. At Standard conditions.

If an external potential is applied, then we found that reaction goes till the opposing
voltage reaches 1.1 V.
= 1.1 V No current flows.
Any further increase in external potential again starts the rxn but in opposite sense. It now
functions as an electrolytic cell, a device that uses electrical energy to carry non
spontaneous chemical reactions.
In cell redox reaction takes place and we get electrical energy. Anode-Oxidation and
Cathode –Reduction. Electron flow in outer circuit. This creates potential difference.
Salt bridge allows indirect conduction between 2 salts. We take inert electrolyte here. Inert
because they can’t perform chemical reactions. They can only move in solutions.
E.g. NH4NO3, KNO3.
Gives electrical neutrality to anodic and cathodic division.
PAGE 1
LHS :- ANODE RHS:- CATHODE
PAGE 2
Galvanic Cell: -
Gibbs energy of spontaneous rex is converted into electrical work which may be used for running
motor, heater, fan, geyser.
Half-cell reaction:
Anode: -
Oxidation ---------------- loss of electrons.
Cathode: -
Reduction -------------gain of electrons.
Rxn occur indifferent portions of Daniell cell. The two portions of the cell are also called half-cells
or redox couples. Each half cell consists of a metallic electrodes dipped into an electrolyte and are
connected by a voltmeter, salt bridge internally and switch externally. Sometimes if both the
electrodes are dipped in same electrolyte then we don’t need the salt bridge.
At each electrode there is tendency of metal ions from the solution to deposit on metal electrode
trying to make it positively charged. At the same time meta atom of the electrode have the
tendency to go into the solution and thus leaving electrons ad making it negatively charged
Potential difference between the electrode and the electrolyte is called Electrode potential.
In a galvanic cell the half cell in which oxidation takes place, anode it has a negative potential w.r.t
to soln. And other one is cathode which have positive potential w.r.t to the soln.
As switch gets on the electron starts flow in direction opposite that of the current. The potential
difference measured is called the cell potential. It is called electromotive force[EMF] when no
current is drawn through the cell.
CONVENTION [Galvanic cell]:
ANODE: LEFT
CATHODE: RIGHT
In Daniell cell Zn2+ is increasing + charge and hence salt bridge anionic part plays the counterpart
and in cathodic division – charge dominates and hence their cationic part does counterpart’s rod
dissolves hence it weight decreases
Ecell = E Anode {Oxidation pot.} + E cathode {Reduction Pot}
=-E Anode {Reduction Pot} + E Cathode {Reduction Pot}
PAGE 3
NCERT FORMULA:
E cell = E right – E left
Cathode is on right and anode is on left.
The potential of individual half-cell can’t be measured. We can only measure potential between
two half cells potentials and get the emf.
According. To convention Simple hydrogen electrode [SHE] is taken as reference electrode

assuming its E cell=0.
Standard hydrogen electrode consists of a platinum electrode coated with platinum black. Dipped
in acidic soln and pure hydrogen gas is bubbled through it. The conc. of reduced and oxidised
forms of hydrogen maintains and unity. We take it as anode
Ecell = E right/E cathode
E.g. Measured Emf of Pt|H2|H=||Cu2+|Cu.
Standard conditions’ E cell= 0.34V
The + value indicates that Cu2+ ions get reduced more easily then H+ ions. Reverse cannot occur.
Thus Cu doesn’t dissolve in HCl. In Nitric acid it is oxidized by nitrate on not by hydrogen ion.
E.g. Measured Emf of Pt|H2|H=||Zn2+|Zn.
Standard conditions’ E cell= -o.76V
The + value indicates that Hydrogen ions can oxidize Zn or Zn can reduce hydrogen ions.
Condition for 1 ion to displace another: -
B reduction potential > A reduction potential.
PAGE 4
B reduction potential >- A oxidation potential.
B reduction potential + A oxidation potential>
PAGE 5

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Electrochemistry

Manavi | Electrochemistry | September 13, 2017

It has potential equal to 1.1 V. At Standard conditions.

Oxidation ---------------- loss of electrons.

Reduction -------------gain of electrons.

CONVENTION [Galvanic cell]:

Ecell = E Anode {Oxidation pot.} + E cathode {Reduction Pot}

=-E Anode {Reduction Pot} + E Cathode {Reduction Pot}

E cell = E right – E left

Cathode is on right and anode is on left.

According. To convention Simple hydrogen electrode [SHE] is taken as reference electrode

Ecell = E right/E cathode

E.g. Measured Emf of Pt|H2|H=||Cu2+|Cu.

Standard conditions’ E cell= 0.34V

E.g. Measured Emf of Pt|H2|H=||Zn2+|Zn.

Standard conditions’ E cell= -o.76V

Condition for 1 ion to displace another: -

B reduction potential > A reduction potential.

B reduction potential + A oxidation potential>

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