Question

$Al$ is more reactive than iron but it is less easily corroded than iron because:
(A) It is a noble metal
(B) Oxygen forms a protective oxide layer
(C) Iron undergoes easily reaction with water
(D) Iron forms mono and divalent ion

Answer 1

Hint : Aluminium metal is highly reactive with oxygen and forms aluminium oxide which is responsible for the protection of metallic aluminium. Iron also forms a layer of metal oxide but it does not have any protective qualities.

Complete Step By Step Answer:
Aluminium is more reactive than iron. It easily reacts with oxygen in the presence of moist air and forms a layer of non-porous aluminium oxide on the surface.
$4Al + 3{O_2} \to 2A{l_2}{O_3}$
The aluminium oxide layer acts as a protection layer and does not allow the moist air to react further with aluminium and thus prevents further corrosion of aluminium. The protective layer of aluminium oxide is very thin and hence it might get damaged or fall off. When this happens, the aluminium metal surface again reacts with oxygen to recreate the aluminium oxide layer. The thickness of the oxide layer can be further enhanced by the process of anodising.
However, when iron reacts with oxygen in moist air(water), it forms a red-brown hydrated metal oxide called rust.
$4Fe + 3{O_2} + 6{H_2}O \to 4Fe{(OH)_3}$
$Fe{(OH)_3} \to F{e_2}{O_3}.3{H_2}O({\text{Rust}})$
Rust does not have any protective qualities and thus cannot protect the iron metal. Rust does not remain attached to the surface and falls off easily. This allows the further corrosion of iron by oxygen and moist air(water). Corrosion of iron can be prevented by painting it. Paint layers work as walls between iron and oxygen. Iron can also be protected by galvanizing it. In this process, iron is covered with another metal such as zinc to form a protective layer.
Hence, option B is correct.

Note :
Aluminium oxide can be used as an amphoteric substance as it can react with both acids and bases. It is also used in various chemical and industrial applications and in the extraction of chromium and manganese from their oxides.