Pyromorphite from Chaillac Mine, Centre, France (© Didier Descouens) ... solution was was neutralize — Physical Chemistry — Thermodynamics Chemistry Question
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Pyromorphite from Chaillac Mine, Centre, France (© Didier Descouens)
... solution was was neutralized with potassium hydroxide up to pH ≈ 5. An addition of 1.224 g of KI was needed to form 1.700 g of a bright yellow precipitate.
... impurity should be pre-oxidized into C(+m). For this purpose, the solution of C(+n) in H2SO4 was heated in the presence of Ag2S2O8 (Ag+ was used as a catalyst). The solution was then transferred to a 100.0 mL volumetric flask and brought up to volume with distilled ...
Data at 298 K:
E°(MnO4 ‒/Mn2+) = 1.51 V /SHE
E°(Fe3+/Fe2+) = 0.77 V /SHE
E°(C(+m)/C(+n)) = 1.33 V /SHE
Determine the formula of pyromorphite.
Hint 1: The volume of a hexagonal unit cell is V = abc × sin β.
Hint 2: The mass m of 2 A5(PO4)3B in one unit cell is: m = 2M / NA, where M is the molar mass of the mineral and NA Avogadro’s constant. Also, m = ρV, where ρ is the density and V the volume of a unit cell.
Model Answer
To determine the formula of pyromorphite, its molar mass needs first to be
Write an equation for a reaction that could occur if the KI was added in excess.
Identify the impurity C. Write an equation for each reaction mentioned in the text.
Model Answer
M(C) = 207 /3.98 = 52 g mol‒1. Therefore, C = Cr.
Pb2+(aq) + SO42‒(aq) = PbSO4(s)
Cr3+(aq) + 3 NH3(aq) + 3 H2O(l) = Cr(OH)3(s) + 3 NH4+(aq)
Cr(OH)3(s) + 3 H+(aq) = Cr3+(aq) + 3 H2O(l)
2 Cr3+(aq) + 3 S2O82‒(aq) + 7 H2O(l) = Cr2O72‒(aq) + 6 SO42‒(aq) + 14 H+(aq)
Cr2O72‒(aq) + 6 Fe2+(aq) + 14 H+(aq) = 2 Cr3+(aq) + 6 Fe3+(aq) + 7 H2O(l)
5 Fe2+(aq) + MnO4‒(aq) + 8 H+(aq) = Mn2+(aq) + 5 Fe3+(aq) + 4 H2O(l)
[Question text missing in source excerpts]
Model Answer
The method described here is a back titration. Indeed, after the following reaction:
Cr2O7 2‒(aq) + 6 Fe2+(aq) + 14 H+(aq)= 2 Cr3+(aq) + 6 Fe3+(aq) + 7 H2O(l)
Calculate the equilibrium constant of the titration reaction, for one equivalent of permanganate ions, at 298 K.
Model Answer
5 Fe2+(aq) + MnO4‒(aq) + 8 H+(aq) = Mn2+(aq) + 5 Fe3+(aq) + 4 H2O(l)
ΔrG° = ‒5 × F E°(MnO4‒/Mn2+) ‒ 5 × (‒F E°(Fe3+/Fe2+))
K° = exp(‒ΔrG° / RT) = exp(‒5F(E°(Fe3+/Fe2+) ‒ E°(MnO4‒/Mn2+)) / RT)) = 3.8·10^62
Mn2+ can be added to the solution to indicate the completeness of the C(+n) pre-oxidation reaction. Write the equation of the reaction that indicates the completeness of the C(+n) pre-oxidation reaction. Underline the species that allows the detection of the completeness of the reaction.
Model Answer
2 Mn2+(aq) + 5 S2O82‒(aq) + 8 H2O(l) = 2 MnO4‒(aq) + 10 SO42‒(aq) + 16 H+(aq)
Mn2+ is oxidized only after the complete oxidation of Cr3+ (E°(MnO4‒/Mn2+) > E°(C(+m)/C(+n))).
The solution will then turn pink.
Fe(NH4)2(SO4)2 is more soluble than FeSO4.
Fe(NH4)2(SO4)2 is a cheaper reagent than FeSO4.
Model Answer
Correct answer:
FeSO4 is not stable and get quickly oxidized by the oxygen in the air.
4 Fe2+(aq) + O2(aq) + 4 H+(aq) = 4 Fe3+(aq) + 2 H2O(l)