Q3(c). Lanthanides with aqueous acid; anomalous oxidation states
(i) Give a balanced equation for the reaction of lanthanides with aqueous acid. Justify your answer. (ii) Name the two lanthanides that have greatest tendency to deviate from the usual positive oxidation state and correlate this deviation with electronic structure.
Answer in brief
(i) 2Ln + 6H⁺ → 2Ln³⁺ + 3H₂ ↑; E^°(Ln³⁺/Ln)≈-2.3 V. (ii) Cerium (+4, 4f⁰) and europium (+2, 4f⁷); ytterbium (+2, 4f¹⁴) and terbium (+4, 4f⁷) follow.
Key points
- lanthanides are strongly electropositive. E^°(Ln³⁺/Ln) lies between -2.0 V (Lu) and -2.5 V (La), far below E^°(H+/H₂)=0: 2Ln + 6H⁺ → 2Ln³⁺ + 3H₂ (also 2Ln + 6H₂O → 2Ln(OH)₃ + 3H₂ slowly with water)
- The reaction is vigorous because Δ G^°=-nFE^°_(cell)≈-6(96.5)(2.3)≈-1.3×10³ kJ per 2 mol Ln. The three electrons removed (6s² 5d¹ or 4f 6s²) leave the stable Ln³⁺ with 4fⁿ core, so +3 is the common state.
- the empty 4f shell (xenon core) is stable: E^°(Ce⁴⁺/Ce³⁺)=+1.72 V (in HClO₄); Ce⁴⁺ is a strong oxidant.
- half-filled shell, exchange energy gain: E^°(Eu³⁺/Eu²⁺)=-0.35 V; Eu²⁺ is a mild reductant and resembles Sr²⁺ (precipitates as EuSO₄).
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- Reaction with acid: lanthanides are strongly electropositive. E^°(Ln³⁺/Ln) lies between -2.0 V (Lu) and -2.5 V (La), far below E^°(H+/H₂)=0: 2Ln + 6H⁺ → 2Ln³⁺ + 3H₂ (also 2Ln + 6H₂O → 2Ln(OH)₃ + 3H₂ slowly with water)
- The reaction is vigorous because Δ G^°=-nFE^°_(cell)≈-6(96.5)(2.3)≈-1.3×10³ kJ per 2 mol Ln. The three electrons removed (6s² 5d¹ or 4f 6s²) leave the stable Ln³⁺ with 4fⁿ core, so +3 is the common state.
- Cerium, Ce⁴⁺ (4f⁰): the empty 4f shell (xenon core) is stable: E^°(Ce⁴⁺/Ce³⁺)=+1.72 V (in HClO₄); Ce⁴⁺ is a strong oxidant.
- Europium, Eu²⁺ (4f⁷): half-filled shell, exchange energy gain: E^°(Eu³⁺/Eu²⁺)=-0.35 V; Eu²⁺ is a mild reductant and resembles Sr²⁺ (precipitates as EuSO₄).
- Others: Yb²⁺ (4f¹⁴, -1.05 V) and Tb⁴⁺ (4f⁷). The deviation shows a tendency to reach f⁰, f⁷ or f¹⁴.
Ce⁴⁺ (ceric ammonium sulphate) is a standard oxidant in redox titrations; Eu²⁺ separates Eu from the other Ln by reduction with Zn amalgam, then precipitation as sulphate.
Coversredox chemistry of lanthanidesLanthanides with aqueous acidanomalous oxidation states
All parts of Question 3
- 3(a) NiCr₂O₄ and NiCo₂O₄: normal or inverse spinel10 marks
- 3(b) cis–trans isomerisation of PtCl₂(PEt₃)₂10 marks
- 3(c) Lanthanides with aqueous acid; anomalous oxidation states10 marks