NDA (II) 2023 General Ability Test · Question 113 of 146

Correct — A, Zinc. Zinc sits in the middle of the reactivity series, and that is exactly the band of metals won by reducing their oxides with carbon. Zinc ores are sulphides and carbonates, so they are first converted to the oxide — the sulphide by roasting in excess air, the carbonate by calcination in limited air — and the oxide is then heated with carbon, which strips the oxygen away, in the reaction ZnO(s) + C(s) → Zn(s) + CO(g). The other three metals fall outside that band. Silver and gold are so unreactive that they occur largely as the metal itself, while aluminium holds oxygen more tightly than carbon does, so carbon cannot take it away.

• (b)Silver — Silver is near the bottom of the reactivity series and is one of the metals found in the free state in nature. Compounds of such low-reactivity metals give up their metal on heating alone, so there is no need to bring in carbon as a reducing agent.
• (c)Gold — Gold is the least reactive of the four and occurs in nature as the metal itself, which is why it was among the first metals humans used. With no oxide to reduce, the question of a reducing agent does not arise.
• (d)Aluminum — Aluminium sits near the top of the reactivity series and has a greater affinity for oxygen than carbon has, so carbon cannot reduce aluminium oxide. Aluminium is obtained instead by the electrolytic reduction of its oxide. Note that this paper prints the American spelling here while spelling the same metal 'Aluminium' elsewhere in the booklet.

How a metal is extracted is decided by where it stands in the reactivity series. Metals at the bottom — gold, silver, platinum, and to a large extent copper — are found free or come out of their compounds on simple heating. Metals in the middle, such as zinc, iron, lead and copper, are converted to the oxide and then reduced with carbon, the cheap industrial reducing agent. Metals at the top — potassium, sodium, calcium, magnesium and aluminium — cannot be reduced by carbon at all and are obtained by electrolysis.

The question looks like a chemistry fact but is really a placement test: put the four metals on the reactivity series and only one of them lands in the carbon-reduction band. The distractors are chosen to cover the two ways of being outside that band — too unreactive to need carbon (silver, gold) and too reactive for carbon to work (aluminium). A related step worth remembering is that the ore is almost never an oxide to begin with; roasting a sulphide and calcining a carbonate are the preparatory moves that make carbon reduction possible. On the industrial scale the reducing agent is coke, which in a blast furnace does double duty, providing the carbon that takes the oxygen away and the fuel that supplies the heat.

• Zinc oxide heated with carbon gives zinc and carbon monoxide, ZnO(s) + C(s) → Zn(s) + CO(g).
• Sulphide ores are turned into oxides by roasting in excess air; carbonate ores by calcination in limited air.
• Gold, silver, platinum and copper are among the metals found in the free state in nature.
• Carbon cannot reduce the oxides of sodium, magnesium, calcium or aluminium because those metals have a greater affinity for oxygen than carbon does; they are obtained by electrolytic reduction.
• The same reduction is used industrially with coke, which in a blast furnace both supplies the heat and acts as the reducing agent that turns iron oxide into iron.

• Reading 'reducing agent' loosely and picking any metal that can be extracted at all, rather than the one carbon can extract.
• Assuming gold and silver need elaborate reduction; their problem is the opposite, they are already metals.
• Forgetting that aluminium's own use as a reducing agent in the thermit reaction is evidence that carbon cannot reduce it.

Asked as a one-from-four placement question — locate each metal on the reactivity series and pick the one in the carbon-reduction band.

• practice — not a real PYQ

  Which one of the following pairs of processes converts an ore into the oxide before reduction?

  • (a)Roasting and calcination
  • (b)Smelting and refining
  • (c)Leaching and electrolysis
  • (d)Froth flotation and magnetic separation

  Answer(a) Roasting and calcination — roasting oxidises a sulphide ore in excess air and calcination decomposes a carbonate ore in limited air, both giving the metal oxide.

• practice — not a real PYQ

  Carbon reduction cannot be used to obtain aluminium from its ore because

  • (a)aluminium oxide is too cheap to bother reducing
  • (b)aluminium has a greater affinity for oxygen than carbon has
  • (c)aluminium oxide does not exist in nature
  • (d)carbon reacts with aluminium to form a carbide that cannot be broken

  Answer(b) aluminium has a greater affinity for oxygen than carbon has — so carbon cannot pull the oxygen away, and electrolytic reduction is used instead.