CDS (II) 2022 General Knowledge · Question 63 of 120

Correct — A, Aluminium. This is the thermit reaction, and NCERT names the application in the same breath as the chemistry: 'the reaction of iron(III) oxide with aluminium is used to join railway tracks or cracked machine parts. This reaction is known as the thermit reaction', written as Fe2O3(s) + 2Al(s) giving 2Fe(l) + Al2O3(s) plus heat. Aluminium is more reactive than iron, so it takes the oxygen away from iron oxide in a displacement reaction, and the enthalpy released is enormous — enough, in NCERT's words, that 'the metals are produced in the molten state'. A crucible of the mixture is ignited over the gap between two rail ends, the white-hot liquid iron runs into the joint and solidifies as a weld, and the lighter aluminium oxide floats off as slag.

• (b)Zinc — Zinc does sit above iron in the reactivity series, so a zinc and iron oxide mixture is thermodynamically possible, but the heat released is far smaller. It would not deliver iron in the molten state, which is the entire point of the technique — the weld is made by liquid metal filling the gap.
• (c)Copper — Copper is less reactive than iron. It cannot take oxygen away from iron oxide at all, so no displacement occurs and no heat is released; if anything the reaction runs the other way, with iron displacing copper from copper salts.
• (d)Tin — Tin is also below iron in the reactivity series and cannot reduce iron oxide. Tin's real association with iron is tin-plating, a protective coating on steel food cans, which has nothing to do with welding.

A displacement reaction is one in which a more reactive metal takes the place of a less reactive one in its compound. Where the two metals are far apart in the reactivity series, the reaction can be violently exothermic, and the thermit reaction between aluminium and iron(III) oxide is the classic case: aluminium's affinity for oxygen is so much greater than iron's that the surplus energy appears as heat, reaching temperatures of the order of two thousand degrees Celsius and leaving the iron molten.

Two things are being tested at once — the reactivity series and an industrial application — and the reactivity series alone settles it, because only one of the four options lies above iron and releases enough heat to matter. It is worth being precise about the roles, because a companion CDS question turns on exactly this: aluminium is the reducing agent, since it is the one being oxidised to aluminium oxide, and iron(III) oxide is the oxidising agent. It is also only the iron that ends up molten; the aluminium oxide forms as a solid slag. A candidate who remembers the reaction as 'both metals melt' will get that companion question wrong even after getting this one right.

• The thermit reaction is Fe2O3(s) + 2Al(s) giving 2Fe(l) + Al2O3(s) plus heat.
• It is a displacement reaction and is highly exothermic; the iron is produced in the molten state.
• It is used to join railway tracks and cracked machine parts.
• Aluminium acts as the reducing agent and is itself oxidised; iron(III) oxide is the oxidising agent.
• Zinc lies above iron in the reactivity series but releases far less heat; copper and tin lie below iron and cannot reduce its oxide.

• Naming aluminium as the oxidising agent; it is oxidised, so it is the reducing agent.
• Claiming both products come out molten; only the iron does, while aluminium oxide forms a solid slag.
• Assuming any metal above iron will do — zinc qualifies on reactivity but not on the heat needed to melt the iron.

As a name-the-metal item, as a statements item on which claims about the thermit reaction are true, or as a not-correct item about the reaction's energetics.

• practice — not a real PYQ

  In the thermit reaction, aluminium acts as

  • (a)an oxidising agent
  • (b)a reducing agent
  • (c)a catalyst
  • (d)an inert diluent

  Answer(b) a reducing agent — aluminium is oxidised to aluminium oxide while iron(III) oxide is reduced to iron.

• practice — not a real PYQ

  Which one of the following pairs will NOT undergo a displacement reaction?

  • (a)Aluminium and iron(III) oxide
  • (b)Zinc and copper sulphate solution
  • (c)Copper and iron(III) oxide
  • (d)Iron and copper sulphate solution

  Answer(c) Copper and iron(III) oxide — copper is less reactive than iron and cannot take oxygen from its oxide.