A-LevelChemistryChemical periodicity9701/42 · May/June 2025 · Q217 Marks

When cobalt(II) sulfate, CoSO4, is dissolved in distilled water, solution A is formed. The reaction scheme in Fig. 2.1 shows some reactions of solution A. [Figure 2.1]

(a)(i)Complete Table 2.1 to show the formula and colour of each of the cobalt-containing species present in A, B and C. Identify the type of reaction forming each of B and C. Table 2.1[4]
(a)(ii)Suggest a suitable reagent for the formation of [CoCl4]2– from solution A.[1]
(b)(i)The complex ion [CoC14]2- has tetrahedral geometry. The 3d orbitals in an isolated Co2+ ion are degenerate. Complete Fig. 2.2 to show the relative energies of the 3d orbitals in an isolated Co2+ ion and in Co2+ in a tetrahedral complex. [Figure 2.2][2]
(b)(ii)Draw a three-dimensional diagram to show the structure of the complex ion [CoC14]2-.[1]
(c)H2O2 can act as an oxidising agent or a reducing agent when reacting with species that contain manganese. Table 2.2 shows electrode potentials, Eº, for some electrode reactions. Table 2.2 Use only the species listed in Table 2.2 to suggest: : one reaction in which H2O2 acts as an oxidising agent and one reaction in which H2O2 acts as a reducing agent. Include the value of the standard cell potential, Ecell', and an overall equation for each reaction. H2O2 acting as an oxidising agent H2O2 acting as a reducing agent[4]
(d)(i)Acidified manganate(VII) ions, MnO4¯, can be used to analyse the content of iron tablets by titration. Two identical iron tablets are crushed and dissolved in distilled water. The resulting solution is made up to 150.0cm³ with distilled water. 25.0 cm³ of this solution requires 18.60cm³ of 0.0500 moldm¯³ acidified MnO4 to reach the end-point. All the Fe2+ ions are oxidised. The relevant half-equations are shown. MnO4¯ + 8H+ + 5e® Mn2+ + 4H2O Fe2+ Fe3+ + e- Describe the colour change observed at the end-point of this titration. from ............................................ to ............................................[1]
(d)(ii)Calculate the mass, in mg, of iron in one tablet. Assume that all the iron in the tablets is Fe2+. Show your working. mass of iron in one tablet = ................................. mg [4][4]

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 (May/June 2025). The mark or mark type is shown in brackets.

[1] A = [Co(H2O)6]2+ pink
[1] B = [Co(H2O)4(OH)2] / Co(OH)2 blue
[1] C = [Co(NH3)6]2+ straw/yellow-brown
[1] B = precipitation / deprotonation / acid-base
[1] C = ligand exchange
Any two [1] any four [2] any six [3] all eight [4]
[1] concentrated HCl
[1] • five (3d) orbitals (lines, boxes) in the isolated Co2+ ion of same energy
[1] • splitting three higher and two lower d orbitals
[1] • energy of all d orbitals in the complex clearly higher than all d orbitals in isolated ion
any two [1] all three [2]
[1] (diagram showing Co at centre, 4 Cl atoms in tetrahedral arrangement, overall 2- charge)
[1] M1: H2O2 acting as oxidising agent with Mn(OH)2/Mn2+
[1] M2: Mn2+ + H2O2 → MnO2 + 2H+ AND E°cell = +0.56 OR Mn(OH)2 + H2O2 → MnO2 + 2H2O AND E°cell = +0.92
[1] M3: H2O2 acting as reducing agent with MnO2
[1] M4: MnO2 + 2H+ + H2O2 → Mn2+ + 2H2O + O2 AND Eºcell +0.54
[1] colourless to pale pink
[1] • moles MnO4 = 0.0500 × 18.60/1000 = 9.30 × 10-4 in 25.0 cm³
[1] • moles Fe2+ = 5 × 9.30 × 10-4 = 4.65 x 10-3 in 25.0 cm³
[1] • moles Fe2+ = 6 x 4.65 x 10-3 = 2.79 x 10-2 in 150.0 cm³
M1 / M2: any two of the above bullets [1] all three [2]
OR
• moles MnO4 = 0.0500 × 18.60/1000 = 9.30 × 10-4 in 25.0 cm³
• moles MnO4¯ = 6 × 9.30 × 10-4 = 5.58 × 10-3 in 150.0 cm³
• moles Fe2+ = 5 × 5.58 × 10-3 = 2.79 × 10-2 in 150.0 cm³
M1 / M2: any two of the above bullets [1] all three [2]
[1] M3: mass Fe = 55.8 × 2.79 × 10-2 = 1.55682 (g) in two tablets
[1] M4: mass Fe = 0.5 × 1.55682 = 0.77841 (g) in one tablet AND mass Fe = 7.7841 × 10-2 × 1000 = 778.4