Corrosion slowly coats the surfaces of metallic objects with oxides or other salts of the metal. The rusting of iron, tarnishing of silver, development of green coating on copper and bronze are some of the examples of corrosion. In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides. Corrosion of iron (commonly known as rusting) occurs in presence of water and air. The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. At a particular spot of an object made of iron, oxidation takes place and that spot behaves as anode:
Anode: 2Fe(s) → 2Fe2+ + 4e−; E°(Fe2+/Fe) = −0.44 V
Electrons released at the anodic spot move through the metal and go to another spot on the metal and reduce oxygen in the presence of H+ (which is believed to be available from H2CO3 formed due to dissolution of carbon dioxide from air into water). This spot behaves as cathode:
Cathode: O2(g) + 4H+(aq) + 4e− → 2H2O(l); E° = 1.23 V
The overall reaction is:
2Fe(s) + O2(g) + 4H+(aq) → 2Fe2+(aq) + 2H2O(l); E°cell = 1.67 V
The ferrous ions are further oxidised by atmospheric oxygen to ferric ions, which come out as rust in the form of hydrated ferric oxide (Fe2O3.xH2O), with further production of hydrogen ions.
Fuel cells convert
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Chemical energy into electrical energy
A fuel cell is a galvanic cell to which the fuel (for example \( \ce{H2} \)) and oxygen are fed continuously. The energy of combustion of the fuel is turned directly into electrical energy, without first making heat and steam as a thermal plant does. In the hydrogen–oxygen fuel cell the cell reaction is:
\( \displaystyle \ce{2H2(g) + O2(g) -> 2H2O(l)} \)
Why the other options are incorrect
Option (1) Electrical energy into chemical energy — Incorrect. This is what an electrolytic cell does: it uses electrical energy to carry out a non-spontaneous reaction.
Option (2) Mechanical energy into chemical energy — Incorrect. No mechanical work goes into a fuel cell; its input is the chemical energy of the fuel.
Option (4) Electrical energy into heat — Incorrect. A fuel cell produces electrical energy; it does not consume it.
NCERT (2026-27 edition):
2.7 Fuel Cells – “… We know that a galvanic cell directly converts chemical energy into electricity and is highly efficient. It is now possible to make such cells in which reactants are fed continuously to the electrodes and products are removed continuously from the electrolyte compartment. Galvanic cells that are designed to convert the energy of combustion of fuels like hydrogen, methane, methanol, etc. directly into electrical energy are called fuel cells.”