A-LevelChemistryChemical periodicity9701/41 · Oct/Nov 2022 · Q512 Marks

A transition element is a d-block element which forms one or more stable ions with incomplete d-orbitals.

(a)Two of the 3d orbitals are the 3dxy orbital and the 3dz2 orbital. Sketch the shapes of these two orbitals. [Figure sketches of 3dxy and 3dz2 orbitals][1]
(b)(i)The Ni2+ ion forms many different complexes. A solution containing the [Ni(H2O)6]2+ complex ion is green. When an excess of 1,2-diaminoethane, en, H2NCH2CH2NH2, is added, the colour of the solution changes to blue. This is due to the formation of the [Ni(en)3]2+ complex ion. Explain why the two solutions are coloured, and why the colours are different. [4][4]
(b)(ii)The [Ni(en)3]2+ complex can exist as a mixture of two stereoisomers. Complete the three-dimensional diagram to show one of the stereoisomers. Each en ligand can be represented using N N [Figure incomplete diagram with Ni and two N-N links][1]
(b)(iii)Name the geometry of the complex ion drawn in (b)(ii) and the type of stereoisomerism shown by [Ni(en)3]2+. geometry stereoisomerism shown [1][1]
(c)Iron(II) carbonate, FeCO3, and nickel(II) carbonate, NiCO3, both decompose when heated. FeCO3 decomposes at a lower temperature than NiCO3. Suggest a possible reason for this difference. Explain your answer. [2][2]
(d)(i)A is a pale green salt containing Fe2+ ions. A sample of 2.62 g of A is dissolved in water and the solution is made up to exactly 100 cm³ with water. 25.0 cm³ samples of this solution are placed in conical flasks and titrated against 0.0100 mol dm-3 acidified potassium manganate(VII). The equation for the only reaction that occurs is shown. 5Fe2+ + MnO4- + 8H+ → 5Fe3+ + Mn2+ + 4H2O The average titre value is 35.0 cm³ of 0.0100 mol dm-3 acidified potassium manganate(VII). Describe the colour change that is seen in the conical flask at the end-point of this titration. The colour changes from ...................................... to ...................................... [1][1]
(d)(ii)Calculate the percentage by mass of iron in A. [Ar: Fe, 55.8] percentage by mass of iron = ...................................... % [2][2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

[1] 3dxy & 3dz2 sketches (as shown in the question paper)
[4] • d orbital(s) of different energy / d-d splitting occurs • electron(s) promoted / excited • wavelength of visible light absorbed AND complementary colour seen • different energy gap / different ΔΕ OR different frequency/wavelength of light is absorbed
[1] Diagram of one stereoisomer of [Ni(en)3]2+ (as shown in the mark scheme)
[1] octahedral AND optical isomerism
[2] Fe2+ is smaller / has a smaller radius OR Fe2+ greater charge density [1] polarises/distorts the anion / CO3(2)- more [1]
[1] colourless / (pale) green to pink / purple
[2] moles MnO4 = 0.0100 × 0.0350 OR 3.50 × 10-4 moles Fe2+ = 5 × 3.50 × 10–4 OR 1.75 × 10-3 [1] moles Fe2+ = 1.75 x 10-3 × 4 = 7.00 × 10-3 % of Fe = (7.00 × 10-3 × 55.8) = 0.3906 g = 0.3906/2.62 × 100 = 14.9 % by mass [1] ecf M1 min 2sf