A-LevelChemistryChemical periodicity9701/43 · Oct/Nov 2025 · Q28 Marks

(a) Iron can form stable ions in the +2 and +3 oxidation states. Explain why transition elements have variable oxidation states. [1] (b) Aqueous solutions of iron(II) salts contain the complex ion [Fe(H2O)6]2+. Define complex ion. [1] (c) [Fe(H2O)6]2+ can be converted into [Fe(H2O)4(OH)2]. (i) Suggest a suitable reagent for this conversion. State the type of reaction. reagent type of reaction [1] (ii) [Fe(H2O)4(OH)2] is a green precipitate that turns brown on standing in air. Table 2.1 shows electrode potentials for some electrode reactions. Table 2.1 electrode reaction E° / V Fe(H2O)3(OH)3 + H2O + e⁻ ⇌ Fe(H2O)4(OH)2 + OH⁻ -0.56 O2 + 2H2O + 4e⁻ ⇌ 4OH⁻ +0.40 Use the information in Table 2.1 to explain why [Fe(H2O)4(OH)2] turns brown on standing in air. Include an equation for this reaction. [3] (d) The complex [Co(NH3)6]2+ reacts with hydrogen peroxide as shown. reaction 1 2[Co(NH3)6]2+ + H2O2 → 2[Co(NH3)6]3+ + 2OH- Ecell = +1.67V Calculate ∆G°, in kJ mol⁻¹, for reaction 1. AG = .............................................. kJ mol⁻¹ [2]

(a)Iron can form stable ions in the +2 and +3 oxidation states. Explain why transition elements have variable oxidation states.[1]
(b)Aqueous solutions of iron(II) salts contain the complex ion [Fe(H2O)6]2+. Define complex ion.[1]
(c)(i)Suggest a suitable reagent for this conversion. State the type of reaction. reagent type of reaction[1]
(c)(ii)[Fe(H2O)4(OH)2] is a green precipitate that turns brown on standing in air. Table 2.1 shows electrode potentials for some electrode reactions. Table 2.1 electrode reaction E° / V Fe(H2O)3(OH)3 + H2O + e⁻ ⇌ Fe(H2O)4(OH)2 + OH⁻ -0.56 O2 + 2H2O + 4e⁻ ⇌ 4OH⁻ +0.40 Use the information in Table 2.1 to explain why [Fe(H2O)4(OH)2] turns brown on standing in air. Include an equation for this reaction.[3]
(d)The complex [Co(NH3)6]2+ reacts with hydrogen peroxide as shown. reaction 1 2[Co(NH3)6]2+ + H2O2 → 2[Co(NH3)6]3+ + 2OH- Ecell = +1.67V Calculate ∆G°, in kJ mol⁻¹, for reaction 1. AG = .............................................. kJ mol⁻¹[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

[1] (a) the (3)d and (4)s sub-shells / orbitals / electrons are close / similar in energy
[1] (b) species or ion formed by a central metal atom/ion AND surrounded by/bonded to (one or more) ligands
[1] (c)(i) NaOH / OH-(aq) AND precipitation / ligand exchange / deprotonation / acid-base
[3] (c)(ii) M1: Fe(H2O)4(OH)2 is oxidised (from + 2 to + 3 on standing) M2: E of Fe(H2O)3(OH)3 is more negative than E of O2 OR Ecell = 0.40 – (−0.56) = (+)0.96 V M3: 4Fe(H2O)4(OH)2 + O2 → 4Fe(H2O)3(OH)3 + 2H2O
[2] (d) AG = -2 x 1.67 × 96 500 = −322 310 J mol-1 M1: AG = -nEcellF AND n = 2 OR -2 x 1.67 × 96 500 M2: AG = −322.3 kJ mol-1

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