Electrochemistry - Part 3 | Class 12th

Quantitative Aspects of electrolysis – Faraday’s laws

ü Faraday’s first law:

The amount of substance(m) deposited at the electrode during electrolysis is directly proportional to the quantity of electricity(q) passed through electrolyte.

                                            m α Q

                                              OR

                     m = ZQ

Where z is a constant called electrochemical equivalent. Z = equivalent                                   weight/96500

But quantity of electricity is the product of current in ampere (I) and time in second                 (t).

                      i.e. Q = It

                     Therefore, m= Zit

ü Faraday’s second law:

When same amount of electricity is passed through different electrolytic solution, amount of Substance deposited is proportional to the chemical equivalent weights.

             For e.g. when same quantity of electricity is passed through solutions of two                           substances A and B, then

                   Mass of A deposited = Equivalent mass of A
                  Mass of B deposited     Equivalent mass of B

Products of electrolysis:  The products of electrolysis depend on the nature of the electrolyte and the type of electrodes used. If the electrode is inert (e.g. Pt, gold etc.), it does not participate in the electrode reaction. While if the electrode is reactive, it also participates in the electrode reaction.

For e.g. if molten NaCl is electrolysed, Na is deposited at the cathode and chlorine is liberated at the anode. NaCl → Na⁺ + Cl^-

At cathode: Na⁺ + e^- → Na

At anode: Cl^- → ½ Cl₂ + e^-

Batteries:

A battery is basically a galvanic cell in which the chemical energy of a redox reaction is converted to electrical energy. They are of mainly 2 types – primary batteries and secondary batteries.

ü  Primary batteries: Here the reaction occurs only once and after use over a period of time, they become dead and cannot be reused. E.g. Dry cell, mercury button cell etc.

                   (Primary cells cannot be recharged and reused).

v Dry Cell

v Mercury Cell

ü  Secondary cells: A secondary cell can be recharged and reused again and again. Here the cell reaction can be reversed by passing current through it in the opposite direction. The most important secondary cell is lead storage cell, which is used in automobiles and invertors.

v  Lead – Storage Battery

Fuel Cells: These are galvanic cells which convert the energy of combustion of fuels like hydrogen, methane, methanol, etc. directly into electrical energy.

One example for fuel cell is Hydrogen – Oxygen fuel cell, which is used in the Apollo space programme. Here hydrogen and oxygen are bubbled through porous carbon electrodes into concentrated aqueous sodium hydroxide solution. To increase the rate of electrode reactions, catalysts like finely divided platinum or palladium metal are filled into the electrodes.

Corrosion: It is a process of eating away of metals on their surfaces, it is an unwanted process as it results in loss of mass of metals. In this process metal surface reacts with atmospheric oxygen to form a layer of oxide. It is an electrochemical reaction.

Most familiar example for corrosion is rusting of iron. It occurs in presence of water and air. It is a redox reaction. At a particular spot of the metal, oxidation takes place and that spot behaves as anode. Here Fe is oxidized to Fe²⁺. 2 Fe(s)→2 Fe²⁺ +4 e^–

Electrons released at anodic spot move through the metal and go to another spot on the metal and reduce oxygen in presence of H⁺. This spot behaves as cathode. The reaction taking place at this spot is:

The overall reaction is:  2Fe(s)+O₂(g) + 4H⁺(aq) → 2Fe^{2 +}(aq)+ 2 H₂O (l )

The ferrous ions (Fe²⁺) are further oxidised to ferric ions (Fe³⁺) and finally to hydrated ferric oxide (Fe₂O₃. x H₂O), which is called rust.

Methods to prevent corrosion:

ü By coating the metal surface with paint, varnish etc.

ü By coating the metal surface with another electropositive metal like zinc, magnesium etc. The coating of metal with zinc is called galvanisation and the resulting iron is called galvanized iron.

ü By coating with anti-rust solution.

ü An electrochemical method is to provide a sacrificial electrode of another metal (like Mg, Zn, etc.) which corrodes itself but saves the object (sacrificial protection)

Related Post:

Electrochemistry - Part 4
Electrochemistry - Part 3
Electrochemistry - Part 2
Electrochemistry - Part 1