A-LevelChemistryChemical periodicity9701/42 · May/June 2018 · Q517 Marks

Cobalt is a transition element with atomic number 27. Cobalt forms ions with charges of +2 and +3. Both these ions form complexes.

(a)Complete the electronic configurations of a Co atom and a Co2+ ion. Co atom 1s²2s²2p⁶ .............................................. Co2+ ion 1s²2s²2p⁶ ..............................................[2]
(b)(i)What is meant by the term complex ion?[1]
(b)(ii)Draw the tetrahedral complex ion formed by one Co2+ ion with four Cl- ions. Your drawing should clearly show three-dimensional shape, and should include the overall charge on the ion.[2]
(b)(iii)Explain why many transition metal complexes are coloured.[3]
(b)(iv)Using ideas from your answer to (iii) suggest why the colour of the complex formed by one Co2+ ion with four Cl- jons is blue.[1]
(c)Describe the colour change that occurs when water is added to blue crystals of the cobalt(II)-chloride complex. Name the type of reaction responsible for this colour change.[2]
(d)(i)Use the Data Booklet to suggest a suitable oxidising agent for this reaction.[1]
(d)(ii)Calculate the Ecell of this reaction.[1]
(d)(iii)Write an equation for the reaction between Co²+ and the oxidising agent you chose in (d)(i).[1]
(e)Cobalt(III) forms two isomeric octahedral complexes with the formula [Co(NH3)3(NO2)3]. The NO2⁻ ion is monodentate. Complete the diagrams to show the three-dimensional structures of the two isomers and suggest the type of isomerism shown here.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[1] 3s²3p⁶3d⁷4s² or 3s²3p⁶4s²3d⁷
[1] 3s²3p⁶3d⁷
[1] An ion formed by a central metal atom/ion surrounded by / bonded to (one or more) ligand(s)
[1] charge is 2-
[1] tetrahedral arrangement of four Cl around Co with two Cl pointing towards you (wedge), two Cl pointing away (dash) / or one Cl up, one Cl down, two in plane. Overall 2- charge.
[1] d-orbitals are split into two groups of orbitals
[1] absorption of light
[1] an electron is excited / (during) electron promotion (to a higher d-orbital)
[1] blue light is not absorbed or absorb (wavelength / frequency of) red and yellow / orange / green or absorbs least in the blue region
[1] (blue goes) pink
[1] ligand exchange / ligand substitution
[1] F2 or S2O8²⁻
[1] +1.05 or +0.19
[1] 2Co²⁺ + F2 → 2Co³⁺ + 2F⁻ or 2Co²⁺ + S2O8²⁻ → 2Co³⁺ + 2SO4²⁻
[1] trans 3D diagram – two O2N-Co-NO2 bond angles are 90° and the other is 180°
[1] cis 3D diagram – three O2N-Co-NO2 bond angles are 90° to each other
[1] cis-trans / geometric(al)

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