Manganese is extracted by heating manganese dioxide with aluminium powder. Which one of the following statements with regard to the reaction is correct?
- (a)The reaction is exothermic.
- (b)The reaction is endothermic.
- (c)Manganese is produced as a solid.
- (d)Manganese is more reactive than aluminium.
Correct — A, The reaction is exothermic. Heating manganese dioxide with aluminium powder is an aluminothermic reduction: 3MnO2 + 4Al → 3Mn + 2Al2O3 + heat. Aluminium sits above manganese in the reactivity series, so it strips the oxygen away, and the aluminium-oxygen bond is so much stronger than the manganese-oxygen bond that a large surplus of energy is released. The same chemistry with iron oxide is the thermite reaction used to weld railway tracks, and the heat given out there is enough to melt iron on the spot.
- (b)The reaction is endothermic. — An endothermic reading gets the energy balance backwards. Once started, the reaction sustains itself and gives out heat rather than needing a continuous supply.
- (c)Manganese is produced as a solid. — The heat released is so great that the manganese is produced in the molten state, not as a solid. That is the point NCERT makes about these reductions, and it is what makes the thermite version useful for welding.
- (d)Manganese is more reactive than aluminium. — Reactivity runs the other way. Aluminium is more reactive than manganese, which is precisely why it can displace manganese from its oxide; if manganese were the more reactive of the two, no reaction would occur.
A displacement reaction between a metal and a metal oxide runs when the free metal is more reactive than the one locked up in the oxide. Aluminium is a powerful reducing agent and is used to obtain manganese, chromium and iron from their oxides. These aluminothermic reductions are strongly exothermic, and the metal is usually obtained molten.
Three separate facts are being tested through one reaction, and each wrong option attacks a different one: the energy sign, the physical state of the product, and the direction of the reactivity comparison. The last is the most useful to reason from, because it also explains the other two. Aluminium must be the more reactive metal, or the reaction would not go at all; the strength of the aluminium-oxygen bond is what releases the surplus energy; and that energy is what leaves the manganese molten.
- 3MnO2 + 4Al → 3Mn + 2Al2O3, with a large release of heat.
- Aluminium is more reactive than manganese, chromium and iron, so it reduces their oxides.
- The thermite reaction, Fe2O3 + 2Al → 2Fe + Al2O3, is used to weld railway tracks and broken machine parts.
- The metals produced in these reductions come out in the molten state.
- Aluminium acts as the reducing agent and is itself oxidised to aluminium oxide.
- Assuming a reaction that needs heating to start must be endothermic overall.
- Expecting the product metal to be solid because the reactants were solid.
- Reversing the reactivity comparison and concluding manganese displaced aluminium.
A single reaction used to test three separate ideas at once; the sibling papers ask the same chemistry through the iron oxide version, where the statements offered are almost identical.
Consider the following reaction : Fe₂O₃(s) + 2Al(s) → 2Fe(s) + Al₂O₃(s) Which of the following statements about the given reaction is NOT correct ?
- (a) It is an example of displacement reaction
- (b) It is highly endothermic reaction
- (c) Fe₂O₃ acts as oxidizing agent
- (d) This reaction is applicable to joining of railway tracks
Answer(b) It is highly endothermic reaction
Marks the endothermic claim as the false statement about this class of reaction, which is exactly the choice being made here.
Which of the following statements are true for the reaction of Fe₂O₃ with aluminium? 1. It is known as the ‘thermite reaction’. 2. The heat evolved is used for welding purpose. 3. Aluminium metal acts as an oxidizing agent. 4. Molten Fe and Al are formed at the end of the reaction. Select the correct answer using the code given below.
- (a) 1 and 2
- (b) 1 and 3
- (c) 2 and 4
- (d) 1 and 4
Answer(a) 1 and 2
The same reduction with iron in place of manganese, and it settles two of the printed options here — the heat is released and used, and the molten product is the displaced metal alone.
- practice — not a real PYQ
In the reaction Fe2O3 + 2Al → 2Fe + Al2O3, aluminium acts as
- (a)an oxidising agent
- (b)a reducing agent
- (c)a catalyst
- (d)a solvent
Answer(b) a reducing agent — it removes oxygen from iron oxide and is itself oxidised.
- practice — not a real PYQ
The thermite reaction is used mainly to
- (a)galvanise iron sheets
- (b)weld broken railway tracks
- (c)electroplate ornaments
- (d)purify copper
Answer(b) weld broken railway tracks — the molten iron produced fills the joint.