Question

Which of the following is not an example of thermite reaction?
(a) $\text{C}{{\text{r}}_{2}}{{\text{O}}_{3}}+2\text{Al}\to \text{A}{{\text{l}}_{2}}{{\text{O}}_{3}}+2\text{Cr}$
(b) $\text{3M}{{\text{n}}_{3}}{{\text{O}}_{4}}+8\text{Al}\to \text{4A}{{\text{l}}_{2}}{{\text{O}}_{3}}+9\text{Mn}$
(c) $2\text{HgO+HgS}\to \text{3Hg+S}{{\text{O}}_{2}}$
(d) $2\text{C}{{\text{u}}_{2}}\text{S+3}{{\text{O}}_{2}}\to 2\text{C}{{\text{u}}_{2}}\text{O+S}{{\text{O}}_{2}}$

Answer 1

Hint: Thermite reaction is also called alumino-thermic reaction. It is a highly exothermic reaction often used to join railway tracks.

Complete step by step answer:
A thermite reaction is an aluminothermic reaction. It is the one in which aluminum metal is oxidized by an oxide of another metal. One of the most commonly used oxide is iron oxide.
This reaction is stable at room temperature. But when they are exposed to sufficient heat i.e their ignition temperature, they burn with an extremely intense exothermic reaction. It does not require any external source (such as air) as it contains enough oxygen to burn.
As a result of this it cannot smother ignite in any environment until and unless given sufficient initial heat. Due to the extremely exothermic nature of the reaction, it is used for quickly cutting or welding metal such as railway tracks. This reaction is also used to purify ores of some metals.
Thermite reaction is more often considered for iron oxide (rust) with aluminium to produce molten iron. The products are aluminum oxide, elemental iron and large amounts of heat. The reaction of iron (III) oxide and aluminum can be represented by the equation:
\[\text{F}{{\text{e}}_{2}}{{\text{O}}_{3}}(s)+2Al(s)\to A{{l}_{2}}{{\text{O}}_{3}}(\text{s)}+2\text{Fe(s)}\]
Similarly, other thermite reactions are-
$\text{3M}{{\text{n}}_{3}}{{\text{O}}_{4}}+8\text{Al}\to \text{4A}{{\text{l}}_{2}}{{\text{O}}_{3}}+9\text{Mn}$
$\text{C}{{\text{r}}_{2}}{{\text{O}}_{3}}+2\text{Al}\to \text{A}{{\text{l}}_{2}}{{\text{O}}_{3}}+2\text{Cr}$
Therefore, option (a) and option (b) are correct.

Note: The reaction is one of a class of reactions known as the “thermite” process, which has been used industrially for welding, the preparation of metals from their oxides. The process is initiated by heat, but then becomes self-sustaining.