A-LevelChemistryChemical periodicity9701/42 · Oct/Nov 2024 · Q412 Marks

Transition metal atoms and transition metal ions form complexes by combining with species called ligands.

(a)(i)When NaOH(aq) is added to an aqueous solution containing [Co(H₂O)₆]²⁺ a precipitation reaction occurs accompanied by a colour change. In this reaction two of the water ligands each lose one H⁺ ion. The H⁺ ions are gained by OH⁻ ions from the NaOH(aq). State the colour change seen in this precipitation reaction. from to[1]
(a)(ii)Complete the ionic equation for this precipitation reaction. [Co(H₂O)₆]²⁺ +[1]
(a)(iii)This precipitation reaction can also be described as a different type of reaction. Name this type of reaction.[1]
(b)(i)L is an uncharged tridentate ligand. L donates three lone pairs to a metal atom or ion. Cobalt forms an octahedral complex ion, E, with L. Complex ion E has a 2+ charge. Give the formula of E.[1]
(b)(ii)Identify the oxidation state of cobalt in E.[1]
(b)(iii)The d-orbitals of the cobalt atom or ion present in E are split in energy. State the number of d-orbitals that are at a higher energy level and the number of d-orbitals that are at a lower energy level. number of d-orbitals at a higher energy level number of d-orbitals at a lower energy level[1]
(b)(iv)Define the term non-degenerate d-orbitals.[1]
(c)(i)The mineral chromite contains a compound which has the formula FeCrO₄. The oxidation state of iron in FeCrO₄ is +2. A sample of 4.18 g of FeCrO₄ is dissolved in an excess of sulfuric acid. The resulting solution is made up to 250 cm³. This is solution F. All the Fe²⁺ ions in 25.0 cm³ of solution F are oxidised to Fe³⁺ ions by exactly 18.7 cm³ of 0.0200 mol dm⁻³ KMnO₄. One MnO₄⁻ ion reacts with five Fe²⁺ ions. Assume no other oxidation reaction occurs. Write an equation for the reaction of Fe²⁺ ions with MnO₄⁻ ions in acid solution.[1]
(c)(ii)Calculate the number of moles of Fe²⁺ ions in 25.0 cm³ of solution F. number of moles of Fe²⁺ ions =[2]
(c)(iii)Calculate the M_r of FeCrO₄ and use your answer to deduce the value of n. M_r of FeCrO₄ = value of n =[2]

Mark scheme

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

[1] red/pink blue
[1] [Co(H₂O)₆]²⁺ + 2OH⁻ → Co(H₂O)₄(OH)₂ + 2H₂O
[1] acid-base
[1] [CoL₂]²⁺
[1] +2
[1] 2; 3
[1] Not of the same energy / have different energy
[1] 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
[1] 0.02 × 18.7/1000 = 3.74 × 10⁻⁴
[1] 3.74 x 10⁻⁴ × 5 = 1.87 x 10⁻³
[1] 1.87 x 10⁻³ × 10 = 1.87 × 10⁻²; 4.18/1.87 × 10⁻² = 224 / 223.5(294)
[1] n = 2