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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 · 2026 · Paper I

Q5(h). Ionisation isomerism, and three pairs of isomers

  • 5 marks
  • Answered
Question

What is ionization isomerism? Mention the type of isomerism in the given pairs of complexes:
(i) [Cr(H₂O)₆Cl₃] and [CrCl(H₂O)₅]Cl₂.H₂O
(ii) [Co(NH₃)₅SO₄]NO₃ and [Co(NH₃)₅NO₃]SO₄
(iii) [Pt(NH₃)₄][PtCl₆] and [Pt(NH₃)₄Cl₂][PtCl₄]

Answer in brief

Same formula, different ions in solution: a ligand and a counter-ion exchange places. (i) hydrate (ii) ionisation (iii) coordination isomerism

Key points

  • (i) Hydrate (solvate) isomerism. Water is a ligand in one and water of crystallisation in the other: [Cr(H₂O)₆]Cl₃ (violet) gives three ionic Cl⁻, [CrCl(H₂O)₅]Cl₂.H₂O (blue-green) only two. Both are CrCl₃.6H₂O.
  • (ii) Ionisation isomerism (different ions on dissociation): the first gives NO₃⁻ and no precipitate with BaCl₂; the second gives SO₄²- ions and white BaSO₄.
  • (iii) Coordination isomerism. Cation and anion are both complexes; two Cl move between them: Pt(II) cation + Pt(IV) anion becomes Pt(IV) cation + Pt(II) anion. The ligands are distributed differently between the two spheres.

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 first formula of (i) is printed [Cr(H₂O)₆Cl₃]; it is read as [Cr(H₂O)₆]Cl₃, with the three chlorides outside the bracket.

  1. (i) Hydrate (solvate) isomerism. Water is a ligand in one and water of crystallisation in the other: [Cr(H₂O)₆]Cl₃ (violet) gives three ionic Cl⁻, [CrCl(H₂O)₅]Cl₂.H₂O (blue-green) only two. Both are CrCl₃.6H₂O.
  2. (ii) Ionisation isomerism (different ions on dissociation): the first gives NO₃⁻ and no precipitate with BaCl₂; the second gives SO₄²- ions and white BaSO₄.
  3. (iii) Coordination isomerism. Cation and anion are both complexes; two Cl move between them: Pt(II) cation + Pt(IV) anion becomes Pt(IV) cation + Pt(II) anion. The ligands are distributed differently between the two spheres.

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Model answer to Q5(h), 2026 Paper I: solution page 32
Solution page 32 of 55
Read the whole 2026 solution2026 question paper

All parts of Question 5