O-LevelChemistryStoichiometry5070/21 · May/June 2016 · Q1010 Marks

Both zinc and iron react with dilute sulfuric acid. Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) Fe(s) + H2SO4(aq) → FeSO4(aq) + H2(g)

(a)(i)A 0.65 g sample of zinc is reacted with excess sulfuric acid. Calculate the volume of hydrogen, measured at room temperature and pressure, formed in this reaction.[2]
(a)(ii)Explain why a different volume of hydrogen, measured at room temperature and pressure, is formed when 0.65 g of iron is reacted with excess sulfuric acid.[1]
(b)A mixture of iron powder and zinc powder is added to excess sulfuric acid. When the reaction stops, aqueous sodium hydroxide is added drop by drop to the reaction mixture until it is in excess. Describe what you would observe during the addition of aqueous sodium hydroxide and explain the reactions taking place. observations explanations[4]
(c)(i)The products of heating iron(II) sulfate are iron(III) oxide, sulfur dioxide and sulfur trioxide. Explain how you can tell that the reaction involves an oxidation.[1]
(c)(ii)Describe the chemical test for sulfur dioxide. test observation[2]

Mark scheme

Marking points from the official mark scheme (May/June 2016). The mark or mark type is shown in brackets.

[2] Moles of Zn and H2 = 0.01 (1) Volume of H2 = 0.24 dm³ (1)
[1] The number of moles of iron is different (1)
[4] One mark each for any two observations from: Solution gets warmer/heat given off (1) White precipitate formed (1) which redissolves in excess (1) Green precipitate formed (1) With excess the green precipitate remains (1) One mark each for any two explanations from Acid is neutralised (1) Green precipitate is iron(II) hydroxide (1) White precipitate of zinc hydroxide (1) which will redissolve in excess (1)
[1] Oxidation number of iron increases/oxidation number of iron becomes more positive/iron(II) ions lose electrons (1)
[2] Use (filter paper dipped into acidified) potassium manganate(VII) (1) Purple colour changes to colourless (1)

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