NDA (II) 2016 General Ability Test · Question 95 of 150

Correct — D, the perchlorate ion. In a disproportionation one and the same element is simultaneously oxidised and reduced, so the element must start in an intermediate oxidation state — it needs somewhere to go both up and down. Work out the state of chlorine in each ion, taking oxygen as minus two and the overall charge as minus one: hypochlorite gives plus one, chlorite plus three, chlorate plus five and perchlorate plus seven. Plus seven is the highest oxidation state chlorine can reach, because chlorine has seven electrons in its outermost shell and all of them are already committed. With no room left to be oxidised further, perchlorate can only be reduced, and a species that can move in one direction alone cannot disproportionate.

• (a)ClO⁻ — Chlorine here is at plus one, comfortably intermediate between minus one and plus seven, so it can go both ways. Hypochlorite does exactly that on warming, three hypochlorite ions giving two chloride ions and one chlorate ion — which is why a bleach solution slowly loses strength.
• (b)ClO₂⁻ — Chlorine is at plus three in chlorite, again an intermediate value, so the ion can be pushed up towards chlorate and perchlorate and down towards chloride at the same time. It is a known disproportionating species.
• (c)ClO₃⁻ — The most tempting wrong answer, because plus five looks close to the maximum. It is not the maximum, though, and chlorate disproportionates readily on heating — four chlorate ions give three perchlorate ions and one chloride ion, the very reaction by which perchlorates are made.

A disproportionation reaction, sometimes called an auto-redox reaction, is one in which a single element in a single species is both oxidised and reduced, ending up in two different products at higher and lower oxidation states. The necessary condition is that the element starts in an intermediate oxidation state. If it is already at its maximum it can only be reduced, and if it is already at its minimum it can only be oxidised; either way, disproportionation is impossible.

The item can be answered without recalling a single reaction, simply by computing four oxidation states and asking which one is at the ceiling. The arithmetic is the same each time: each oxygen counts minus two, the sum must equal the ion's charge of minus one, so chlorine comes out at plus one, plus three, plus five and plus seven for the four ions in order. Chlorine's group position, group 17 with seven outer electrons, fixes plus seven as its ceiling. Two familiar examples of the ceiling rule in other elements are worth carrying: the permanganate ion, with manganese at plus seven, and the sulphate ion, with sulfur at plus six, are both stuck at their maxima and cannot disproportionate either. The complementary case is an element at its floor, such as chloride at minus one, which cannot be reduced any further and so cannot disproportionate for the opposite reason.

• Chlorine is at plus one in hypochlorite, plus three in chlorite, plus five in chlorate and plus seven in perchlorate.
• Plus seven is chlorine's maximum oxidation state, matching its seven outermost electrons in group 17.
• Hypochlorite disproportionates to chloride and chlorate; chlorate disproportionates on heating to perchlorate and chloride.
• Disproportionation requires an intermediate oxidation state, so a species at its highest or its lowest state cannot undergo it.

• Assuming plus five is already the maximum for chlorine; the ceiling is plus seven, which is why chlorate still disproportionates.
• Forgetting that a species at its minimum oxidation state is equally incapable of disproportionation, for the mirror-image reason.

NDA tests redox mostly through oxidation-number arithmetic dressed up as a concept question, so practise assigning states quickly for oxoanions before worrying about named reactions.

• practice — not a real PYQ

  The oxidation state of chlorine in the perchlorate ion is

  • (a)+1
  • (b)+3
  • (c)+5
  • (d)+7

  Answer(d) +7 — four oxygens at minus two each total minus eight, and the ion's charge of minus one leaves chlorine at plus seven.

• practice — not a real PYQ

  In a disproportionation reaction, the same element is

  • (a)only oxidised
  • (b)only reduced
  • (c)simultaneously oxidised and reduced
  • (d)neither oxidised nor reduced

  Answer(c) simultaneously oxidised and reduced — which is why the element must begin in an intermediate oxidation state.