O-LevelChemistryMetals & Reactivity5070/21 · May/June 2019 · Q37 Marks

Molybdenum, Mo, is a transition element.

(a)Suggest one physical property of molybdenum that is typical of a transition element.[1]
(b)Suggest one chemical property of molybdenum compounds that is typical of compounds of transition elements.[1]
(c)Molybdenum steel is an extremely hard alloy. Suggest, using ideas about metallic structure, why molybdenum steel is much harder than pure iron. A labelled diagram may help you answer this question.[2]
(d)Molybdenum steel is made by reducing a mixture of MoO₃ and Fe₂O₃ with aluminium. MoO₃ + 2Al → Mo + Al₂O₃ Fe₂O₃ + 2Al → 2Fe + Al₂O₃ Molybdenum steel contains 20.0% by mass of molybdenum. Calculate the mass of MoO₃ needed to make 1000g of molybdenum steel. Give the answer to three significant figures. [The relative atomic mass of molybdenum, Mo, is 96.][3]

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

[1] high melting point/ high boiling point / high density / (good) conductor of electricity / (good) conductor of heat / malleable / ductile / hard / strong / sonorous (1)
[1] coloured / variable oxidation state / catalyst (1)
[2] idea that there are atoms or ions of different size in steel (1)
[2] in steel the layers (of atoms, ions or particles) cannot move as easily (1)
[3] process 1 - correct use of 20% in calculation e.g. need to make 200 g of molybdenum (1)
[3] process 2 - moles of molybdenum needed = 200/96 OR 2.083
[3] process 3 mass of MoO₃ = (moles of Mo × 144) = 300 (g)

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