CDS (I) 2020 General Knowledge · Question 3 of 120

Correct — C, Cu + PbCl₂ → CuCl₂ + Pb. A displacement reaction only runs when the free metal is more reactive than the metal locked in the salt. Read the four lines against the activity series — potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, hydrogen, copper, mercury, silver, gold, in falling order of reactivity. Iron, zinc and magnesium all stand above copper, so each of them can turn copper out of copper sulphate and take its place. Copper, however, stands below lead. It cannot displace lead from lead chloride, and this reaction simply does not happen.

• (a)Fe + CuSO₄ → FeSO₄ + Cu — Feasible, and it is the classic school demonstration — an iron nail dipped in blue copper sulphate solution acquires a brown copper deposit while the solution fades. Iron sits above copper in the activity series.
• (b)Zn + CuSO₄ → ZnSO₄ + Cu — Feasible. Zinc is well above copper in the activity series, so zinc goes into solution and copper comes out as metal. This is the reaction behind the classic Daniell cell.
• (d)Mg + CuSO₄ → MgSO₄ + Cu — Feasible, and the most vigorous of the three. Magnesium is far above copper, so the displacement is fast and strongly exothermic.

The reactivity or activity series is a list of metals ranked by how readily they give up electrons. A metal higher in the list will displace a metal lower down from that lower metal's salt solution; the reverse never occurs. The same ordering explains where each metal is found in nature and how it is obtained — the most reactive metals are never found free and need electrolysis, the middle group occurs as oxides, sulphides and carbonates, and the least reactive turn up as the free metal.

Three of the four lines share a template — some metal plus copper sulphate giving that metal's sulphate plus copper — so the odd one out announces itself by its shape alone, before any chemistry is done. Option (c) is the only line where copper is the attacking metal rather than the one being displaced, and copper is the least reactive metal named anywhere in the question. That structural tell is worth noticing under time pressure. The chemistry then confirms it: lead sits above copper and above hydrogen, which is why lead reacts with dilute acids while copper does not.

• NCERT's activity series runs K, Na, Ca, Mg, Al, Zn, Fe, Pb, [H], Cu, Hg, Ag, Au from most to least reactive.
• Lead stands above copper in that list, so copper cannot displace lead from a lead salt.
• Metals below hydrogen in the series — copper, mercury, silver, gold — do not displace hydrogen from dilute acids.
• Gold, silver, platinum and copper occur in nature in the free state because they sit at the bottom of the series.
• The most reactive metals, potassium through aluminium, are obtained by electrolysis of their molten compounds.

• Assuming that any metal plus any salt solution will react; direction is fixed by the series.
• Forgetting that lead sits above copper — the two are often lumped together as 'heavy, unreactive metals'.
• Balancing the equation and calling it feasible; a correctly balanced equation can still describe a reaction that never occurs.

As 'which reaction is not feasible', or in the observational form — what happens when an iron nail is left in copper sulphate solution.

• practice — not a real PYQ

  Silver nitrate solution is stored in a copper vessel and in a silver vessel. In which vessel, if either, will a reaction occur?

  • (a)In the copper vessel only
  • (b)In the silver vessel only
  • (c)In both vessels
  • (d)In neither vessel

  Answer(a) In the copper vessel only — copper is above silver in the activity series, so it displaces silver from the nitrate; silver cannot displace itself.

• practice — not a real PYQ

  Which one of the following metals will not displace hydrogen from dilute hydrochloric acid?

  • (a)Zinc
  • (b)Iron
  • (c)Magnesium
  • (d)Copper

  Answer(d) Copper — it lies below hydrogen in the activity series, so it leaves dilute hydrochloric acid alone.