Which one of the following metals is widely extracted by the electrolysis of its molten compound?

Correct — D, Na. Sodium sits at the top of the reactivity series, and a metal that reactive holds on to its compound too tightly for an ordinary reducing agent to take it back. Carbon cannot do the job: sodium has a greater affinity for oxygen than carbon has, so roasting its ore with coke leaves the sodium exactly where it was. What works is to melt the salt and drive the reaction with an electric current. NCERT's Class 10 chemistry puts it in a single sentence — sodium, magnesium and calcium are obtained by the electrolysis of their molten chlorides, the metal depositing at the cathode, the negatively charged electrode, and chlorine coming off at the anode. The word carrying this question is 'molten'. Copper also meets an electrolysis cell, but only at the refining stage and in a water-based bath, never as a melt; and a melt is what the stem specifies. The general rule behind the item is that the position of a metal in the reactivity series decides how it is obtained: the top band needs electricity, the middle band yields to carbon, and the bottom band is often lying there as the free metal already.

• (a)Cu — Copper is low in the reactivity series and is won from its sulphide ore by roasting and smelting. Electricity enters only afterwards, to purify what has already been produced: the impure metal is made the anode, a thin strip of pure copper the cathode, and the electrolyte is a solution of acidified copper sulphate. That is refining, from an aqueous bath, not obtaining a metal from a molten compound.
• (b)Au — Gold is at the very bottom of the reactivity series and is one of the metals NCERT names as occurring in the free state. There is nothing to reduce and no compound to melt — the work is separating the metal from the rock around it, not chemistry.
• (c)Sn — Tin comes from cassiterite, an oxide ore, and tin sits in the middle band of the series. Middle-band oxides give up their metal to carbon: heat the roasted ore with coke and the metal runs out. No melt is electrolysed.

Where a metal stands in the reactivity series decides how it is obtained from its ore. The most reactive metals — potassium, sodium, calcium, magnesium and aluminium — are never found free in nature and are so firmly bound to oxygen or chlorine that carbon cannot displace them; their compounds are melted and split by an electric current, a process called electrolytic reduction. Metals in the middle of the series, such as zinc, iron and tin, occur as oxides, sulphides or carbonates and can be reduced by heating with carbon. Metals at the bottom — gold, silver, platinum and, in part, copper — occur as the free metal and need no reduction at all.

The examiner has put one metal from each band of the reactivity series in front of you, so the item is really testing whether you can place the four names on that ladder. Sodium is the only one from the top band, and the top band is the electrolysis band. The tempting wrong answer is copper, because copper and electrolysis do belong in the same sentence in every Class 10 syllabus — but there the cell is refining an impure metal in an aqueous solution of acidified copper sulphate. Reading the word 'molten' carefully is what separates the two. It is worth noticing that the same passage of NCERT supplies both halves of the trap, one paragraph apart.

• NCERT's activity series runs K, Na, Ca, Mg, Al, Zn, Fe, Pb, then hydrogen, then Cu, Hg, Ag, Au — most reactive at the top.
• Carbon cannot reduce the oxides of sodium, magnesium, calcium or aluminium because these metals have a greater affinity for oxygen than carbon does.
• In the electrolysis of a molten chloride the metal is deposited at the cathode and chlorine is liberated at the anode.
• Electrolytic refining of copper uses impure copper as the anode, a strip of pure copper as the cathode, and a solution of acidified copper sulphate as the electrolyte.
• Gold, silver, platinum and copper are found in the free state; the metals at the top of the series are never found in nature as free elements.

• Choosing copper because copper and electrolysis are linked in the textbook — that link is refining an already-obtained metal in an aqueous bath.
• Ignoring the word 'molten' in the stem; the melt is the whole distinction being tested.
• Assuming a metal with a well-known ore must be reduced chemically; for the top band there is no chemical reducing agent strong enough.

Either as a which-metal item like this one, or in reverse — naming a method and asking which metal it suits, or asking what is deposited at the cathode and what is liberated at the anode.

• practice — not a real PYQ

  In the electrolysis of molten sodium chloride, the product liberated at the anode is

  • (a)Sodium
  • (b)Chlorine
  • (c)Hydrogen
  • (d)Oxygen

  Answer(b) Chlorine — the metal is deposited at the negatively charged cathode, while chlorine is liberated at the positively charged anode.

• practice — not a real PYQ

  Which one of the following metals occurs in nature largely in the free state?

  • (a)Aluminium
  • (b)Calcium
  • (c)Gold
  • (d)Magnesium

  Answer(c) Gold — it lies at the bottom of the reactivity series; aluminium, calcium and magnesium are so reactive that they are never found in nature as free elements.