O-LevelChemistryAcids, Bases & Salts5070/22 · Oct/Nov 2022 · Q410 Marks

This question is about ammonium sulfate, (NH4)2SO4.

(a)Ammonium sulfate is a fertiliser. Explain why farmers put fertilisers on soil where crops are grown.[1]
(b)Explain why farmers do not add calcium hydroxide to the soil immediately after adding ammonium sulfate.[2]
(c)Calculate the percentage by mass of nitrogen in ammonium sulfate. Give your answer to three significant figures. percentage by mass[3]
(d)Complete the equation for the reaction of ammonium sulfate with aqueous sodium hydroxide. (NH4)2SO4 + 2NaOH → ........................... + ........................... + ...........................[2]
(e)Ammonium sulfate decomposes when heated. The reaction is endothermic. 3(NH4)2SO4 → 4NH3 + 3SO2 + N2 + 6H2O Complete and label the energy profile diagram for this reaction to include: • the reactant and products • the enthalpy change of the reaction. [Figure 4.1 - Blank energy profile diagram][2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

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

[1] 4(a) to improve crop yield / to improve plant growth / to add minerals lost when plants are harvested
[2] 4(b) ammonia is produced (1); (ammonia) escapes (from the soil) / ammonia is a gas (1)
[3] 4(c) relative molecular mass of ammonium sulfate = 132 (1); 28/132 x 100 OR 21% /21.212% (1); 21.2 (1)
[2] 4(d) 2NH3 + Na2SO4 + 2H2O; correct formulae (1); correct balance (1)
[2] 4(e) reactants on the left and products on the right and reactant line below product line (1); arrow vertically upwards between reactants and products with ∆H or enthalpy change label (1)

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