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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2018 · Paper I

Q7(c)(i). Why +3 is the common oxidation state of lanthanoids

  • 5 marks
  • Answered
Question

The commonly observed oxidation state of lanthanoids in aqueous solution is +3. – Comment.

Answer in brief

Removing the two 6s and one 4f (or 5d) electron costs moderate energy, and this is more than repaid by the large hydration enthalpy of the small, highly charged Ln³⁺. A fourth electron would come from the contracted, core-like 4f shell and costs too much. So +3 is the stable state in water; +2 and +4 appear only where they give f⁰, f⁷ or f¹⁴.

Key points

  • Lanthanoids have [Xe]4fⁿ6s² (or 4fⁿ⁻¹5d¹6s²). The 6s² and one 5d or 4f electron are lost easily: I₁+I₂+I₃ is relatively low and similar along the series.
  • Ln³⁺ ions are small and highly charged, so their hydration enthalpy (and lattice enthalpy) is very large and negative. It more than pays for the third ionisation.
  • The 4f orbitals lie inside the 5s²5p⁶ shell; once three electrons are gone they are strongly held, and I₄ is too large to be recovered by hydration. So +4 is rare.
  • The 4f electrons take hardly any part in bonding. All Ln³⁺ have the same charge and similar radii, so their chemistry is very similar (hence the difficulty of separation).

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

  1. Lanthanoids have [Xe]4fⁿ6s² (or 4fⁿ⁻¹5d¹6s²). The 6s² and one 5d or 4f electron are lost easily: I₁+I₂+I₃ is relatively low and similar along the series.
  2. Ln³⁺ ions are small and highly charged, so their hydration enthalpy (and lattice enthalpy) is very large and negative. It more than pays for the third ionisation.
  3. The 4f orbitals lie inside the 5s²5p⁶ shell; once three electrons are gone they are strongly held, and I₄ is too large to be recovered by hydration. So +4 is rare.
  4. The 4f electrons take hardly any part in bonding. All Ln³⁺ have the same charge and similar radii, so their chemistry is very similar (hence the difficulty of separation).
  5. Exceptions: ions with an empty, half-filled or filled f shell: Ce⁴⁺ (4f⁰), a strong oxidant in water (ceric sulfate titrations); Tb⁴⁺ (4f⁷); Eu²⁺ (4f⁷) and Yb²⁺ (4f¹⁴), which are reducing and readily oxidised to +3 in water.

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Model answer to Q7(c)(i), 2018 Paper I: solution page 45
Solution page 45 of 55
Read the whole 2018 solution2018 question paper

All parts of Question 7