A-LevelChemistryChemical periodicity9701/42 · Feb/Mar 2024 · Q314 Marks

Iron is a transition metal in Group 8 of the Periodic Table.

(a)(i)Explain why iron has variable oxidation states.[1]
(a)(ii)Complete the shorthand electronic configurations of Fe and Fe3+. Fe [Ar] Fe3+ [Ar][1]
(b)An aqueous solution of Fe(NO3)3 contains the complex [Fe(H2O)6]3+. When solutions of KSCN(aq) and [Fe(H2O)6]3+(aq) are mixed, a colour change is observed. The red complex [Fe(H2O)5SCN]2+ forms.
(b)(i)Define complex.[1]
(b)(ii)State the coordination number of Fe in [Fe(H2O)6]3+.[1]
(b)(iii)The H—O—H bond angle in water is 104.5°. Suggest the H—O—H bond angle in [Fe(H2O)6]3+. Explain your answer.[1]
(b)(iv)Explain why iron complexes are coloured.[3]
(b)(v)Aqueous solutions of complexes [Fe(H2O)6]3+ and [Fe(H2O)5SCN]2+ are different colours. Explain why these complexes are different colours.[2]
(c)Table 3.1 gives values for the stability constants, Kstab, of different complexes of iron. Table 3.1 | complex | stability constant, Kstab | |:----------------------------|:--------------------------| | [Fe(H2O)5(H2PO4)]2+ | 5.90 × 101 | | [Fe(H2O)5SCN]2+ | 1.30 × 102 |
(c)(i)[Fe(H2O)5(H2PO4)]2+ can form when H3PO4 reacts with [Fe(H2O)6]3+. Write an equation for this reaction.[1]
(c)(ii)Write an expression for Kstab of [Fe(H2O)5SCN]2+ and give its units. Kstab = units =[2]
(c)(iii)Use the stability constant data in Table 3.1 to calculate the value of the equilibrium constant, Kc, for the following equilibrium. [Fe(H2O)5(H2PO4)]2+ + SCN¯ ⇌ [Fe(H2O)5SCN]2+ + H2PO4 value of Kc =[1]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2024). The mark or mark type is shown in brackets.

[1] energies / energy levels of the 3d and the 4s (sub-shells / orbitals) are similar OR the difference between the 3d and the 4s is small
[1] Fe = [Ar] 3d6 4s2 AND Fe3+ = [Ar] 3d5
[1] (a molecule or ion formed by a central) metal atom / ion surrounded / bonded by one or more ligands
[1] 6 / six
[1] bond angle 106-108° AND lone pair from O is donated (in a bond) OR one more of O's electron pairs is now a bond/bonding pair (so repels less)
[1] M1 (degenerate) d orbitals split (into two energy levels) OR (degenerate) d orbitals become non-degenerate
[1] M2 as an electron moves up / to a higher energy level
[1] M3 absorption of light energy / energy in the visible region AND colour seen is complementary (to colour absorbed)
[1] (d-d) energy gap / ∆E/ ∆oct is different
[1] different frequency / wavelength (of light) absorbed / transmitted / reflected
[1] [Fe(H2O)6]3+ + H3PO4 → [Fe(H2O)5(H2PO4)]2+ + H3O+
[1] Kstab = [[Fe(H2O)5(SCN)]2+] / ([[Fe(H2O)6]3+] [SCN¯])
[1] mol-1 dm³ u/c
[1] 1.30 x 102/59.0 = 2.2(03) min 2sf

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