A-LevelChemistryChemical bonding9701/43 · Oct/Nov 2017 · Q414 Marks

Describe and explain how the density and melting point of cobalt compare to those of calcium.

(a)Describe and explain how the density and melting point of cobalt compare to those of calcium.[3]
(b)Transition metals can form complexes. What is meant by the term transition metal complex?[1]
(c)(i)Cobalt can form the compounds [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br. These two compounds are structural isomers. Define the term structural isomer.[1]
(c)(ii)Draw a three-dimensional diagram to show the structure of the ion [Co(NH₃)₅Br]²⁺. Name its shape.[1]
(c)(iii)State the type of bonding between the cobalt ion and NH₃ groups in the [Co(NH₃)₅Br]²⁺ ion.[1]
(c)(iv)State the oxidation number of cobalt in • [Co(NH₃)₅Br]²⁺ • [Co(NH₃)₅SO₄]⁺[1]
(d)Solutions of the compounds [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br can be distinguished from each other by simple chemical tests. Assume that any species bonded to the cobalt ion does not react in these tests. Complete the table with two different tests that could be used to positively identify each compound. Give the expected observation with each compound. test | observation with [Co(NH₃)₅Br]SO₄(aq) | observation with [Co(NH₃)₅SO₄]Br(aq) ---|---|---[2]
(e)The two compounds [Co(NH₃)₅Br]SO₄ and [Co(NH₃)₅SO₄]Br are different colours. Explain why the colours of the two compounds are different.[2]
(f)Some transition metals and their compounds act as catalysts. The catalysis can be classified as heterogeneous or homogeneous. Complete the table by placing one tick (✓) in each row to indicate the type of catalysis in each reaction. | | heterogeneous | homogeneous | |---|---|---| | Fe in the Haber process | | | | Fe²⁺ in the I⁻/S₂O₈²⁻ reaction | | | | NO₂ in the oxidation of SO₂ | | | | V₂O₅ in the Contact process | | |[2]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] density is higher and melting point is higher
[1] (density) due to A, being larger and smaller atomic radii or (Co) atoms/ions heavier and smaller
[1] (melting point) due to stronger attraction to cations as more delocalised electrons
[1] (a molecule or ion) formed by a central metal atom/ion surrounded by (one or more) ligands
[1] same number and type of atoms and different structural formula
[1] octahedral AND 3D structure of [Co(NH₃)₅Br]²⁺ e.g. [diagram]; name its shape: octahedral
[1] co-ordinate/dative covalent
[1] +3 for both
[1] (HNO₃) Ag⁺/AgNO₃ cream(-yellow) ppt. (of AgBr) and no reaction/white ppt. for other isomer
[1] Ba(OH)₂/Ba²⁺(aq)/BaCl₂ / Ba(NO₃)₂ white ppt. (of BaSO₄) and no reaction for other isomer
[1] (d-d) energy gap/ΔE is different
[1] absorb different wavelength/frequency (of light)
[2] Fe in the Haber process (heterogeneous), Fe²⁺ in the I⁻/S₂O₈²⁻ reaction (homogeneous), NO₂ in the oxidation of SO₂ (homogeneous), V₂O₅ in the Contact process (heterogeneous).