Electro chemistry

1. N.GOKILA
III B.Sc Chemistry
S.B.K College
Aruppukottai

2. Content
 Terminology
 Types of cells
 Types of electrodes
 Single electrode potential
 Measure of Single electrode potential
 Standard electrode potential
 Electro chemical series and significance
 Nernst equation derivation
 Electrochemical passivity
 Determination of solubility products of sparingly soluble
salts

3.  Calculation of ΔG, ΔH, ΔS and K for cell reaction
 Chemical cells with and without transference
 Application of EMF measurement
 Determination of PH using hydrogen electrode, glass
electrode and quinhydrone electrode
 Westom Cadmium cell

4. Terminology
Electrode
It is a device made by introducing metal into
its own ionic solution where either oxidation or
reduction taking place.
Oxidation
Removal of electron from a metal is called
oxidation.
m→mn++ne-

5. Reduction
Gain of electron by a metal ion is called
reduction.
mn++ne- → m
Oxidation electrode
It is a device made by introducing metal into its
own ionic solution where oxidation taking place.
e.g

6. When Zn metal rod is placed in the Zn2+ionic
solution (ZnSO4 solution)Zn metal goes into
Zn2+ions into the solution by leaving its electron
on the Zn metal rod . So that the electronic
charge on the Zn-metal rod is increased.
Zn→Zn2++2e-
Reduction electrode
It is a device made by introducing a metal rod
into its own ionic solution where reduction
tacking place.

7. e.g
When Cu-metal rod is placed in the Cu2+ ionic
solution (CuSO4 solution) Cu2+ions get deposited
as copper on the Cu-rod . So that the electronic
charge on the Cu-metal rod is decreased.
Cu2++2e- → Cu

8.  Electro chemical cell
It is a device made by joining oxidation
electrode and reduction electrode where
electrical energy is produced by redox reaction.

9.  At oxidation electrode (anode)
Zn→Zn2++2e-
At reduction electrode (cathode)
Cell reaction
Zn+ Cu2+ → Cu+ Zn2+
As a result of cell reaction electrons flow from
Zn-rod to Cu-rod and SO42- ions migrate from
reduction electrode to oxidation electrode through
salt bridge . That makes the flow of negative charges
in the above circuit is completed and the current
flow is noted in the ammeter . In the absence of salt
bridge , no current flow through the ammeter and no
reaction takes place in the cell
Cu2++2e- → Cu

10. Types of cells
 Reversible cell
 Irreversible cell
Reversible cell
It is a device in which a state of equilibrium is existed in
the cell reaction.
i.e,
a) If the external emf is equal to the cell emf , no cell
reaction takes place.
b) If the external emf is infinitesimally smaller than the cell
emf , the current will flow from the cell by the cell
reaction.
c) If the external emf is infinitesimally greater than the cell
emf , the current flow into the cell and the cell reaction
gets reversed.

12. Cell reaction
Zn+ Cu2+ Cu+ Zn2+
If EMFexternal infinitesimally smaller than the
EMFcell the forward reaction is occur.
Zn+ Cu2+ → Cu+ Zn2+
If EMFexternal infinitesimally greater than the
EMFcell the backward reaction is occur.
Cu+ Zn2+ → Zn+ Cu2+

13.  Irreversible cell
It is a cell in which a state of equilibrium is not
existed in the cell reaction.

14. If the EMFexternal infinitesimally smaller than the
EMFcell the , the cell reaction is
At anode : Zn→Zn2++2e-
At cathode :2H++2e- →H2
Cell reaction
Zn+ 2H+→Zn2++H2
If EMFexternal infinitesimally greater than the EMFcell
the cell reaction is
At anode :
At cathode : 2H++2e- →H2
Cell reaction
Cu+ 2H+→Cu2++H2
Cu → Cu2++2e-

15. Types of electrodes
 Metal – metal ion electrode
 Gas electrode
 Metal-metal salt electrode
 Oxidation – reduction electrode

16. Metal – metal ion electrode
It is made by introducing metal into its own ionic solution
in which the electrode reaction is reversible with metal ions.
(e.g) Zn –Zn2+ ion electrode
Electrodic reaction
Zn oxidation Zn2++2e-
reduction

17. Gas electrode
It is made by passing into the corresponding ionic
solution in the presence of platinum metal in which the
electrodic reaction is reversible with respect to the ions of the
gas.
(e.g) hydrogen electrode
it is made by passing through the hydrogen gas into the
HCl acid in the presence of Pt metal in which the electrode
reaction is reversible with respect to the H+ ions.

18. electrodic reaction
H2 2H++2e-
Metal-metal salt electrode
It is made by taking the metal and its sparingly
soluble salt in the saturated solution of salt having the
same anion in the presence of Pt wire in which the
electrodic reaction is reversible with respect to the
anion.
(e.g) calomel cell
It is made by taking the Hg and calomel(Hg2Cl2) to
the KCl solution in the presence of Pt wire
oxidation
reduction

19. If the electrode reaction is oxidation
2 Hg (l) 2 Hg++2 e-
2 Hg+ + 2 Cl- Hg2Cl2
2 Hg (l) 2 Cl- Hg2Cl2 +2 e-
i.e there is a fall in concentration of chloride ions

20. Oxidation – reduction electrode
It is made by taking platinum metal in the solution
containing ions of the same substance in two different
oxidation state like Fe2+ and Fe3+ ions or Ce 2+ and Ce3+

21. The electrode reaction is
Fe2+ Fe3++e-
Here the potential of the electrode arises from the tendency
of the ions in one of the oxidation state to change into the
other more stable oxidation state