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Class 10 Class 12

What is corrosion? Describe the electrochemical phenomenon of rusting of iron.

Corrosion is the process of slowly eating away of the metal due to attack of the atmospheric gases on the surface of the metal resulting into the formation of compounds such as oxides, sulphides, carbonates, etc.

The corrosion of iron is called rusting.
According to theory of rusting, impure iron surface behaves as a small electrochemical cell in the presence of water containing dissolved oxygen or CO2.
The pure iron acts as anode and impure surface as cathode.
At Anode : Iron atom undergo oxidation spontaneously forming Fe2+ ion.
Fe → Fe2+ (aq) + 2e-cell = – 0.44 V
Fe2+ ions move into solution and electrons into cathodic area where they are picked up by H+ ions of the solution.
At cathode:
             2H++12O2+2e-  H2O   E°red = 1.23 V
H+ ions are produced by secondary reaction either from H2O or from H2CO3 (CO2 + H2O)
H2O ---->H+ + OH

H2CO3------> H+ + HCO3

The overall reaction of the corrosion cell may be represented as:
Fe(s)+2H+(aq)+12O2 Fe2+(aq)+H2O                                                                    E0cell = 1.67 V                  

The Fe2+ ions move through water and come at the surface where these are further oxidized into Fe3+ ions by atmospheric oxygen to form hydrate ferric oxide known as rust, Fe2O3.xH2O.
2Fe2++12O2+2H2O  Fe2O3+4H+        Fe2O3+xH2O  Fe2O3.xH2O                                            Rust


How would you determine the standard electrode potential of the system Mg2+/Mg?


Use standard hydrogen electrode as anode and Mg2+ | Mg as a cathode we can measure the standard electrodepotential of systemMg2+ | Mg. Standard hydrogen electrode, represented by Pt(s), H2(g) (1 atm) | H+ (aq) and dip the electrode of Magnesium wire in a 1M MgSO4 solution .The standard hydrogen electrode is always zero.
Use formula
Eocell = Eo right  – Eoleft
The standard hydrogen electrode is always zero.
So that the value of
Eoleft =0
Eocell = Eo Mg|Mg2+
Eo Mg|Mg2+= Eocell

Calculate the potential of hydrogen electrode in contact with a solution whose pH is 10.


For hydrogen electrode, ,
given that      pH = 10
use formula [H+] = 10–pH
so that  [H+] = 10−10 M
Electrode reaction will
H+  + e –  →1/2 H2
Use the formula

EH+/H2  = EoH+/H2 - RTnFlnH2[H+]2              = 0-8.314×2982×96500ln IH+2              = 0.05915 log [H+]              =-0.05915 pH             =-0.05915 × 10 =- 0.59 V

Consult the table of standard electrode potentials and suggest three substance that can oxidize ferrous ions under suitable conditions.


oxidation of ferrous ion means :

Fe2+--> Fe3+ +e-EFe3+Fe2+0 = 0.77V
Any substance which standard electrode potential is more than that of Fe+3 /F+2 can oxidise ferrous ions. 
(refer to the table given in book)

The EMF of the substance whose reduction potentials greater than 0.77V will oxidised ferrous ion.
for example Br2, Cl2,and F2 .


Calculate the emf of the cell in which the following reaction takes place:
Ni(s) + 2Ag+ (0.002 M)  Ni2+ (0.160 M) + 2Ag(s)
Given that EoCell = 1.05 V


Ecell = Eocell + RT2FlnAg+(aq)2Ni2+ (aq)
or      Ecell = Eocell+0.0592 logAg+(aq)2Ni2+ (aq)
The equation is also written as

or    Ecell = Eocell-0.0592logNi2+(aq)Ag+ (aq)2         = 1.05 V - 0.0295 log 0.1600.002  

= 1.05 V – 0.0295 x log 80

= 1.05 V – 0.0295 x 1.9031

= 1.05 V – 0.056 = 0.99 V.


Can you store copper sulphate solutions in a Zinc pot?


No. Because zinc is more reactive than copper and thus holes will be developed in zinc pot.

Cu2+(aq) + Zn(s) → Zn2+ (aq) + Cu(s)