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VKC Notes · Non-aqueous solvents · IFoS P-I · Civil Services P-I

14.2 Non-Aqueous Solvents: Liquid NH₃, HF, H₂SO₄, SO₂, BrF₃ and Superacids

★★★ 2021–2025: 11 parts, 92 marks (IFoS 10 parts / 82 m · Civil Services 1 / 10 m). A named unit of the IFoS syllabus, asked every year as “predict the products” or “explain the blue colour”.

Syllabus lines. IFoS: “Reactions in liquid NH₃, HF, SO₂ and H₂SO₄. Failure of solvent system concept, coordination model of non-aqueous solvents. Some highly acidic media, fluorosulphuric acid and super acids”. Civil Services asks it under “interhalogen compounds” (main group chemistry).

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CSE 2024 Q7(a) = asked there; tap to open the solved answerSN = line kept in the short notes

14.2.1 The solvent-system concept and levelling ★★★

  • Solvent-system definition (Cady and Elsey): a solvent that self-ionises gives a characteristic cation and anion. An acid is any solute that raises the concentration of the solvent's cation; a base raises the solvent's anion. It covers protic and aprotic solvents alike.SN
  • Protic solvents (H₂O, NH₃, HF, H₂SO₄) transfer H⁺; aprotic ones (SO₂, BrF₃, N₂O₄) transfer another ion (O²⁻, F⁻, NO⁺) or none. Polar solvents with high dielectric constant (HF 84, H₂SO₄ ≈ 100) dissolve salts; NH₃ (22) and SO₂ (15) less so.SN
Solvent (bp)Self-ionisationAcid (raises cation)Base (raises anion)
NH₃ (−33 °C)NH₄ClKNH₂
HF (19.5 °C)SbF₅, BF₃KF, RCOOH
H₂SO₄ (≈ 300 °C, dec.)H₂S₂O₇, HSO₃FKHSO₄, H₂O, HNO₃
SO₂ (−10 °C) formalSOCl₂Cs₂SO₃
BrF₃ (126 °C)SbF₅KF
  • Levelling: every acid stronger than the solvent's cation is converted completely into it, so all look equally strong (HCl, HNO₃, HClO₄ are all “H₃O⁺” in water). A basic solvent levels more acids; an acidic solvent turns weak acids into bases.SN
  • Failure of the solvent-system concept: it needs self-ionisation that may not really occur (SO₂: no measurable SO²⁺; ³⁵SO₂ exchange with SOCl₂ is too slow), and it ignores solvation. The coordination model (Drago) explains the same reactions by the Lewis acid or base coordinating the solvent, e.g. SOCl₂ ionising only when a chloride acceptor binds Cl⁻.SN
liquid NH3basic solventCH3COOH + NH3→ NH4+ + CH3COO−complete: a strong acidacids stronger than NH4+are all levelledwaterreferenceCH3COOH + H2O⇌ H3O+ + CH3COO−partial: weak acid (pKₐ 4.76)acids stronger than H3O+are levelled100% H2SO4acidic solventCH3COOH + H2SO4→ CH3C(OH)2+ + HSO4−acetic acid is a baseHNO3 is a base too:→ NO2+ + H3O+ + 2HSO4−proton affinity of the solvent falls →
Fig. 14.2.1 One acid in three solvents.
Draw it in 60 s: three boxes NH3 | H2O | H2SO4 with acetic acid strong, weak, a base; an arrow under them: proton affinity of the solvent falls.

Solved PYQ IFoS 2021 Q5(e) 8m

Acetic acid is weak and nitric acid strong in water, but both behave as bases in sulfuric acid. Why?

  1. Acid strength is relative to the solvent. H₂SO₄ is a much stronger proton donor than either, so it protonates them.
  2. ; . Both raise [HSO₄⁻], the solvent anion: they are bases in H₂SO₄. Cryoscopy shows i = 2 for acetic acid and i = 4 for nitric acid.SN

14.2.2 Reactions in liquid ammonia ★★★

  • bp −33 °C, ε ≈ 22, K(self-ionisation) ≈ 10⁻³³ at −50 °C. A weaker H-bonding, more basic solvent than water: dissolves fewer ionic salts but more organic compounds and alkali metals; acetic acid is a strong acid in it.SN
TypeExampleAnalogy in water
Acid–base (neutralisation)HCl + KOH → KCl + H₂O
Ammonolysis; ; hydrolysis
Amphoterism; Zn(OH)₂ in OH⁻ and in H⁺
Precipitation (reversed solubility)runs the other way in water
Imide/nitride formation IFoS 2025 Q5(e) 8mlike oxide from hydroxide
Redox (by dissolved metals) (Pt(0)) IFoS 2024 Q6(c) 10m—

14.2.3 Solutions of alkali metals in liquid ammonia ★★★ IFoS favourite

(14.2.1)
  • Dilute (< 0.05 M): deep blue, paramagnetic (one unpaired electron per metal atom), high molar conductivity (the solvated electron is very mobile). The colour is the broad absorption of the solvated electron near 1500 nm whose tail covers the red: so Li, Na, K, Rb, Cs all give the same blue, because the colour belongs to the electron, not the metal IFoS 2025 Q8(c)(ii) 5m.SN
  • Rising concentration: molar conductivity first falls (ion pairs M⁺·e⁻, then spin-paired species e₂²⁻ and M⁻ reduce the number of free carriers) and the paramagnetism falls (electrons pair). Above ≈ 3 M the solution is bronze, metallic: conductivity rises steeply to metal-like values (electrons in a conduction band, an “expanded metal”) IFoS 2025 Q8(c)(i) 5m IFoS 2022 Q8(b)(i) 10m.SN
  • The solutions are strong reducing agents; they slowly decompose (faster with Fe catalysts): .SN
e−HNH2HNH2HNH2HNH2HNH2HNH2e−(NH3)ₙ in a cavity, H pointing inabsorbs ≈ 1500 nm, tail in the red → blueconcentration of Na in NH3 (log scale) →Λ (molar conductivity)χ per mol (paramagnetism)ion pairs, e− pairbronze, metallicdilute: blue, paramagnetic> 3 M: bronze
Fig. 14.2.2 The solvated electron, and how Na/NH₃ solutions change with concentration.
Draw it in 60 s: a dashed circle with e− inside and NH3 around it, H pointing in; beside it two curves: Λ high → dip → rises steeply; χ high → falls.
  • Reactions of Na/NH₃ IFoS 2022 Q8(b)(ii) 5m: with O₂, reduction to superoxide or peroxide (; ); with metal carbonyls, carbonylate anions: ; Birch reduction of arenes; [Ni(CN)₄]²⁻ → [Ni(CN)₄]⁴⁻ (Ni(0)).SN

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