LANTHANIDES & ACTINIDES

Problems Set

1. Compare and contrast the chemistry of the lanthanides with those of the first row transition metals and pre-transition metals.

2. Explain how you would prepare pure samples of the lanthanide elements, and indicate how you would assess the purity of your samples.

3. (a) Explain why YbS and YbI₂ are semiconducting solids, but LaS and LaI₂ show metallic behaviour.

(b) Compare and contrast the structures adopted by LaCl₃, FeCl₃ and AuCl₃.

(c) Comment on the following statement: "Aqueous Eu²⁺ is thermodynamically less susceptible to oxidation than any other aqueous M²⁺ lanthanide cations because of the stability of the half-filled 4f⁷ shell".

(d) Comment in the variation of the exothermic enthalpies of formation of the lanthanide trichlorides, MCl₃, given below (&endash;DH in kJmol^-1)

La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb
1075	1058	1061	1045	1030	939.5	1012	1001	991	1009	1002	990	949

(e) Predict the magnetic moments you would expect for Pr³⁺ and Gd³⁺ salts. Would you expect U⁴⁺ salts to have the same magnetic moments as Pr³⁺ salts?

4. Comment on points of interest in respect of the following compounds:-

(i) U(C₈H₈)₂

(ii) (C₅Me₅)₂LuMe

(iii) La_1.85Sr_0.15CuO₄

(iv) Ce(NO₃)₆^3&endash;

(v) UO₂²⁺

(vi) Gd₂Cl₃

(vii) UF₆

5. Traditionally uranium was assigned a place below tungsten as a 4^th Row transition element. In 1945 Seaborg re-assigned it as a member of the Actinide 5f series. With this in mind, compare the chemistry of uranium with those of tungsten and neodymium.

6. When sodium carbonate and uranium dioxide are heated in equimolar proportions, at 800°C under a flow of oxygen, a solid orange product A is formed. The infra-red spectrum of A shows strong absorption bands near 900 cm^-1, similar to those observed in aqueous solutions of uranyl salts. When A is heated with a further equimolar quantity of UO₂, a chocolate brown compound B is obtained; B contains 77% uranium by weight. There are no strong absorption bands near 900 cm^-1 in the infra-red spectrum of B. If UO₂ and sodium carbonate are heated in 2:1 molar proportions under oxygen at 800°C, an orange compound C is formed. C is reduced by hydrogen to compound B.

Describe the reactions involved in the above scheme.

 Identify A, B and C.

 [Atomic masses: U = 238, Na = 23, O = 16, C = 12]

7. (a) State how ²³⁸U₉₂ can be converted into ²³⁹U₉₄ and indicate which other elements may be formed in the process. Suggest how these elements may be separated from Pu.

(b) Suggest what happens in the following and, where possible, give equations and comment on the species formed:

(i) Metallic plutonium dissolves in dilute HNO₃.

(ii) The nature of the solid product (A) formed when HF is added to the solution obtained in (i).

(iii) When A is heated with oxygen a mixture of two solids is obtained, B and C. B is involatile and has the fluorite structure, whereas C is volatile and has a relative molecular mass of 353. The IR and Raman spectra of gaseous C have no band in common.

(iv) Hydrolysis of C results in the formation of a species D which has a strong band at 710 cm^-1 in its Raman spectrum.

(v) Reaction of C with two molar equivalents of CsF at 400° leads to formation of a solid (E) and the evolution of 1 mol equivalent of fluorine gas.