O-LevelChemistryMetals & Reactivity5070/22 · May/June 2022 · Q810 Marks

Zinc is a metal.

(a)Coating iron with zinc prevents iron from rusting. Explain how a coating of zinc prevents iron from rusting when the coating is scratched.[2]
(b)(i)Zinc has metallic bonding. Describe, with the aid of a labelled diagram, the metallic bonding in solid zinc.[2]
(b)(ii)Explain why zinc is a good conductor of electricity.[1]
(c)One of the stages in the extraction of zinc involves electrolysis. At the cathode, zinc ions, Zn2+, are changed into zinc atoms. At the anode, hydroxide ions, OH¯, are changed into oxygen molecules and water molecules. Write the ionic equations for the reactions at the cathode and at the anode. cathode anode[2]
(d)A sample of 2.34 g of zinc is reacted with 50.0cm³ of 2.00 mol/dm³ hydrochloric acid. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) Show by calculation that the hydrochloric acid is in excess in this reaction.[3]

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Mark scheme

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

[1] zinc is more reactive than iron / zinc is above iron in the reactivity series
[1] zinc will react in preference to iron / zinc will oxidise instead of iron
[1] diagram showing closely packed positive ions surrounded by delocalised electrons
[1] strong attraction between electrons and positive ions
[1] electrons can move / has mobile electrons
[1] cathode: Zn2+ + 2e- → Zn
[1] anode: 4OH- – 4e- → O2 + 2H2O / 4OH- → O2 + 2H2O + 4e-
[1] moles of zinc = 0.036
[1] moles of HCl = 0.100
[1] 0.1 mole of acid would react with 0.05 mol of zinc / 0.036 mole of zinc would need 0.072 moles of HCl

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