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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 · 2020 · Paper I

Q8(c). IUPAC names of five compounds

  • 10 marks
  • Answered
Question

Write the name of the following complexes using the IUPAC nomenclature:
(i) [Co(NH₃)₆]Cl₃, (ii) Na₂[ZnCl₄], (iii) [Pt(Py)₄][PtCl₄],
(iv) [(NH₃)₅Co.NH₃.Co(NH₃)₅](NO₃)₅, (v) AlK(SO₄)₂.12H₂O

Answer in brief

Names in the table: ligands alphabetically, then the metal with its oxidation number; -ate for a complex anion. (v) is a double salt (potash alum).

Key points

  • printed with an NH₃ bridge. A neutral bridge with five nitrate ions would need 2x=+5, an impossible average. The bridge is the amido ion, NH₂⁻: then both Co are +3.
  • cation before anion; inside the complex, ligands alphabetically (prefixes di-, tetra-, bis- ignored), then the metal and its oxidation number.
  • Anionic ligands end in -ido in the current rules (chlorido, amido from azanide); older books write tetrachlorozincate(II). bis-, tetrakis- are used for ligands with complex names (pyridine, the bracketed unit in (iv)).
  • (v) is not a complex: in water it gives K⁺, Al³⁺ (as [Al(H₂O)₆]³+) and SO₄²-, the tests of the separate ions. The water is named as a hydrate.

The full model answer

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

CompoundIUPAC nameOxidation state and point to note
(i)[Co(NH₃)₆]Cl₃(III) chloridex+0=+3; ammine (NH₃) has two m
(ii)Na₂[ZnCl₄]tetrachloridozincate(II)Zn(II); complex anion ends in -ate
(iii)[Pt(py)₄][PtCl₄](pyridine)platinum(II) tetrachloridoplatinate(II)both Pt(II); cation named first
(iv)[(NH₃)₅Co(μ-NH₂)(NH₃)₅](NO₃)₅μ-amido-bis[pentaamminecobalt(III)] nitrate2x-1=+5, so both Co(III); μ marks the bridge
(v)AlK(SO₄)₂.12H₂Opotassium bis(sulfate) dodecahydratedouble salt (potash alum); cations in alphabetical order

READING(iv) the bridge must be NH₂⁻ (amido) for five nitratesPypyridine

  1. Reading of (iv): printed with an NH₃ bridge. A neutral bridge with five nitrate ions would need 2x=+5, an impossible average. The bridge is the amido ion, NH₂⁻: then both Co are +3.
  2. Order of naming: cation before anion; inside the complex, ligands alphabetically (prefixes di-, tetra-, bis- ignored), then the metal and its oxidation number.
  3. Anionic ligands end in -ido in the current rules (chlorido, amido from azanide); older books write tetrachlorozincate(II). bis-, tetrakis- are used for ligands with complex names (pyridine, the bracketed unit in (iv)).
  4. (v) is not a complex: in water it gives K⁺, Al³⁺ (as [Al(H₂O)₆]³+) and SO₄²-, the tests of the separate ions. The water is named as a hydrate.

Application. (iii) is a polymerisation isomer of [PtCl₂(py)₂] (same empirical formula), and potash alum is used in water purification: Al³⁺ hydrolyses to gelatinous Al(OH)₃, which coagulates suspended clay.

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Model answer to Q8(c), 2020 Paper I: solution page 41
Solution page 41 of 45
Read the whole 2020 solution2020 question paper

All parts of Question 8