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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 · 2017 · Paper I

Q7(a)(ii). Sodium nitroprusside from K₄[Fe(CN)₆]; reaction with S²-

  • 10 marks
  • Answered
Question

How is K₄[Fe(CN)₆] is converted into sodium nitroprusside? Write reaction of S²- ion with sodium nitroprusside.

Answer in brief

Heat K₄[Fe(CN)₆] with concentrated nitric acid: one CN⁻ is replaced by NO⁺; neutralise with Na₂CO₃ to obtain Na₂[Fe(CN)₅(NO)] * 2H₂O. Sulfide gives the violet [Fe(CN)₅(NOS)]⁴-.

Key points

  • Preparation. Potassium ferrocyanide is warmed with concentrated HNO₃; nitric acid oxidises one cyanide to CO₂ and supplies NO: K₄[Fe(CN)₆] + 6HNO₃ → H₂[Fe(CN)₅(NO)] + CO₂ + NH₄NO₃ + 4KNO₃ (ionic form: [Fe(CN)₆]⁴- + 4H⁺ + NO₃⁻…
  • H₂[Fe(CN)₅(NO)] + Na₂CO₃ → Na₂[Fe(CN)₅(NO)] + CO₂ + H₂O. Ruby-red crystals of Na₂[Fe(CN)₅(NO)] * 2H₂O separate after KNO₃ is removed.
  • sodium pentacyanidonitrosylferrate(II). NO is bound as NO⁺ (linear Fe–N–O, ν(NO) 1939 cm⁻¹), so iron is Fe(II), low-spin d⁶, diamagnetic.
  • the nucleophile S²- attacks the electrophilic N of the coordinated NO⁺: Na₂[Fe(CN)₅(NO)] + Na₂S → Na₄[Fe(CN)₅(NOS)] The thionitrosyl adduct is violet (purple).

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. Preparation. Potassium ferrocyanide is warmed with concentrated HNO₃; nitric acid oxidises one cyanide to CO₂ and supplies NO: K₄[Fe(CN)₆] + 6HNO₃ → H₂[Fe(CN)₅(NO)] + CO₂ + NH₄NO₃ + 4KNO₃ (ionic form: [Fe(CN)₆]⁴- + 4H⁺ + NO₃⁻ → [Fe(CN)₅(NO)]²- + CO₂ + NH₄⁺).
  2. Neutralisation: H₂[Fe(CN)₅(NO)] + Na₂CO₃ → Na₂[Fe(CN)₅(NO)] + CO₂ + H₂O. Ruby-red crystals of Na₂[Fe(CN)₅(NO)] * 2H₂O separate after KNO₃ is removed.
  3. Nature: sodium pentacyanidonitrosylferrate(II). NO is bound as NO⁺ (linear Fe–N–O, ν(NO) 1939 cm⁻¹), so iron is Fe(II), low-spin d⁶, diamagnetic.
  4. Reaction with sulfide: the nucleophile S²- attacks the electrophilic N of the coordinated NO⁺: Na₂[Fe(CN)₅(NO)] + Na₂S → Na₄[Fe(CN)₅(NOS)] The thionitrosyl adduct is violet (purple).
  5. The colour appears with S²- in alkaline solution but not with H₂S itself, so the solution is made alkaline for the test.

Application. this violet colour is the test for sulfur in Lassaigne's sodium fusion extract and for sulfide ions in qualitative analysis; nitroprusside itself releases NO and is used as a vasodilator in hypertensive emergencies.

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Model answer to Q7(a)(ii), 2017 Paper I: solution page 29
Solution page 29 of 38
Read the whole 2017 solution2017 question paper

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