Question

No chemical reaction takes place when granules of a rusty-brown solid A are mixed with the powder of another solid B. However, when the mixture is heated, a reaction takes place between its components. One of the products C is a metal and settles down in the molten state while the other product D floats over it. It was observed that the reaction is highly exothermic.
(a) What could the solids A and B be?
(b) What are the products C and D most likely to be?
(c) Write the chemical equation for the reaction between A and B leading to the formation of C and D. Mention the physical states of all the reactants and products in this equation and indicate the heat change which takes place.
(d) What is the special name of such a reaction? State one use of such a reaction.
(e) Name any two types of chemical reactions under which the above reaction can be classified.

Answer 1

(a) The rusty-brown solid A is iron (III) oxide and another solid B is aluminium. Mixing of iron granules with aluminium powder will have no reaction.

(b) The product are likely to be C (iron) and D (aluminium oxide). Once the mixture of iron (III) oxide granules and aluminium powder is heated, aluminium acts as reducing agent. Aluminium reduces iron (III) oxide to iron and aluminium powder oxidises to aluminium oxide with the liberation of excess heat.

(c)
${\mathrm{Fe}}_2{\mathrm{O}}_3\left(s\right)+2\mathrm{Al}\left(s\right)\stackrel{\mathrm{heat}}{\to }2\mathrm{Fe}\left(l\right)+{\mathrm{Al}}_2{\mathrm{O}}_3\left(s\right)+\mathrm{Heat}$
When A (iron (III) oxide) and B (aluminium powder) are heated, iron and aluminium oxide get produced with the liberation of excess heat. Thus, the product C is iron and product D is aluminium oxide. As excess heat is liberated in the reaction, the product C, i.e., iron metal, is in the molten state.

(d) The special name given to this reaction is thermite reaction. This reaction is widely used to weld broken hard substances like iron in railway tracks. This process of welding is called thermite welding.

(e) The above said reaction can be classified into displacement reaction and oxidation-reduction reaction. Aluminium, being more reactive than iron, displaces iron in iron (III) oxide, undergoing displacement reaction. Aluminium also acts as a reducing agent and reduces iron (III) oxide to iron metal and gets oxidised to aluminium oxide, undergoing oxidation-reduction reaction.