O-LevelChemistryStoichiometry5070/21 · Oct/Nov 2021 · Q910 Marks

This question is about ammonia and nitrates.

(a)State the source of the hydrogen and nitrogen used in the manufacture of ammonia by the Haber process. source of hydrogen source of nitrogen[2]
(b)Ammonia is formed when aqueous ammonium sulfate is heated with sodium hydroxide. (NH₄)₂SO₄ + 2NaOH → 2NH₃ + Na₂SO₄ + 2H₂O A student adds 4.50 g of sodium hydroxide to 50.0cm³ of 1.25mol/dm³ aqueous ammonium sulfate. Show by calculation that the ammonium sulfate is in excess.[3]
(c)When dilute sodium hydroxide is electrolysed, the hydroxide ions are converted to oxygen and water at the anode. Construct the ionic equation for the reaction taking place at the anode.[1]
(d)The first stage in the test for nitrate ions is to heat aluminium and sodium hydroxide with a solution containing nitrate ions. State the type of chemical reaction which occurs when a solution containing nitrate ions is heated with aluminium and sodium hydroxide.[1]
(e)Nitrates from fertilisers cause eutrophication when they are leached from soils into rivers. Describe the process of eutrophication.[3]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[1] 9(a) hydrogen: (cracking) hydrocarbons
[1] 9(a) nitrogen: air
[1] 9(b) NaOH = 4.5 / 40 OR 0.1125 (mol)
[1] 9(b) (NH4)2SO4 = 50/1000 × 1.25 OR 0.0625 (mol)
[1] 9(b) ((NH4)2SO4 because 0.0625 × 2) = 0.125 OR ((NH4)2SO4 because 0.1125 ÷ 2) = 0.056
[1] 9(c) 4OH⁻ → O₂ + 2H₂O + 4e⁻/4OH⁻ - 4e⁻ → O₂ + 2H₂O
[1] 9(d) reduction
[1] 9(e) (nitrates) increase growth of algae / (nitrates) increase growth of water plants / algal bloom
[1] 9(e) plants (on surface) block sunlight and plants die
[1] 9(e) bacteria feed on dead plants and use up oxygen
[1] 9(e) (without oxygen) fish die / (without oxygen) water organisms die

On Oracle Prep you can write or photograph your own answer and have it marked by AI against this mark scheme, with feedback on what you missed.