Question

Aluminothermy used for the spot welding of large iron structure is based on the fact that-
(A)- reaction between iron and oxygen is endothermic
(B)- as compared to Al, Fe has greater affinity for oxygen
(C)- as compared to iron, Al has great affinity for oxygen
(D)- reaction between Al and oxygen is endothermic

Answer 1

The method used to produce elemental metal from its oxides using the powdered aluminium metal, which on reduction to its molten form, seeps into the cracks of the iron structures and binds it together. This is known as the thermite welding.

Complete step by step solution:
In the aluminothermy process, it is the extraction process of the metal from its oxides, by its reduction with the aluminium, in powder form acting as the reducing agent.
In the reaction of the metal oxide with the aluminium powder, on application of heat, it undergoes redox reaction. The aluminium being highly electropositive than the metal oxide, it acts as a reducing agent in the reaction.
So, in the reaction of iron (III) oxide mixed with aluminium powder. On ignition, a superheated liquid iron is produced with the evolution of heat. Thus, it is an exothermic reaction. The reaction is as follows:
$F{{e}_{2}}{{O}_{3}}+2Al\to 2Fe+A{{l}_{2}}{{O}_{3}}+\Delta$
where the electropositive aluminium atom $(A{{l}^{3+}})$, due to higher reduction potential, it has higher affinity for the oxygen $({{O}^{2-}})$ than the iron. So, $F{{e}_{2}}{{O}_{3}}\,to\,Fe$.
This molten elemental iron sinks into and seals the gap during the welding process.

Therefore, aluminothermy is based on the fact that option (C)- as compared to iron, Al has great affinity for oxygen.

Note: Even though the aluminothermy process is an exothermic reaction, the activation energy of aluminium is high, so the process takes place at high temperature. But it is possible for the aluminium to reduce most of the metallic oxides due to its high boiling point except for magnesium oxide.