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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 · 2019 · Paper I

Q2(c). Preparation of XeO₃; reactions of HXeO₄⁻ and Ba₂XeO₆

  • 10 marks
  • Answered
Question

Suggest a method for the preparation of Xenontrioxide. Complete the following reactions by balancing of atoms.
H×eO₄⁻ + OH⁻
Ba₂×eO₆ + H₂SO₄

Answer in brief

XeF₆ + 3H₂O → XeO₃ + 6HF; 2HXeO₄⁻ + 2OH⁻ → XeO₆⁴- + Xe + O₂ + 2H₂O; Ba₂XeO₆ + 2H₂SO₄ → XeO₄ + 2BaSO₄ + 2H₂O

Key points

  • xenon oxides are endergonic and cannot be made from the elements; they come from hydrolysis of xenon fluorides: 3pt] XeF₆(s) + 3H₂O(l) → XeO₃(s) + 6HF(aq) 3pt] or, with disproportionation of Xe(IV): 6XeF₄ + 12H₂O → 2XeO₃ + 4Xe…
  • XeO₃ + OH⁻ ⇌ HXeO₄⁻ (xenate). The xenate ion slowly disproportionates, Xe(VI) going to Xe(VIII) and Xe(0), with water oxidised to O₂: 3pt] 2HXeO₄⁻ + 2OH⁻ → XeO₆⁴- + Xe + O₂ + 2H₂O 3pt] Check: Xe 2 = 2; O 10 = 6 + 2 + 2; H 4 =…
  • concentrated sulfuric acid at low temperature releases xenon tetroxide; insoluble BaSO₄ drives the reaction: 3pt] Ba₂XeO₆ + 2H₂SO₄ →XeO₄ + 2BaSO₄ + 2H₂O 3pt] XeO₄ is a tetrahedral, explosively unstable gas (Xe(VIII), no lone…

The full model answer

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

The printed “×e” is read as Xe: the species are HXeO₄⁻ and Ba₂XeO₆.

  1. Preparation of XeO₃: xenon oxides are endergonic and cannot be made from the elements; they come from hydrolysis of xenon fluorides: 3pt] XeF₆(s) + 3H₂O(l) → XeO₃(s) + 6HF(aq) 3pt] or, with disproportionation of Xe(IV): 6XeF₄ + 12H₂O → 2XeO₃ + 4Xe + 3O₂ + 24HF. Evaporating the solution leaves white XeO₃, a dangerously explosive solid; it is pyramidal (one lone pair on Xe).
  2. HXeO₄⁻ in base: XeO₃ + OH⁻ ⇌ HXeO₄⁻ (xenate). The xenate ion slowly disproportionates, Xe(VI) going to Xe(VIII) and Xe(0), with water oxidised to O₂: 3pt] 2HXeO₄⁻ + 2OH⁻ → XeO₆⁴- + Xe + O₂ + 2H₂O 3pt] Check: Xe 2 = 2; O 10 = 6 + 2 + 2; H 4 = 4; charge -4=-4.
  3. Barium perxenate with acid: concentrated sulfuric acid at low temperature releases xenon tetroxide; insoluble BaSO₄ drives the reaction: 3pt] Ba₂XeO₆ + 2H₂SO₄ →XeO₄ + 2BaSO₄ + 2H₂O 3pt] XeO₄ is a tetrahedral, explosively unstable gas (Xe(VIII), no lone pair); the low temperature limits its decomposition.

Application. perxenates are among the strongest oxidising agents known in water: XeO₆⁴- oxidises Mn²+ to MnO₄⁻ in acid, and XeO₃ (E°=+2.10 V) leaves only xenon gas as a by-product.

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Model answer to Q2(c), 2019 Paper I: solution page 10
Solution page 10 of 40
Read the whole 2019 solution2019 question paper

All parts of Question 2