Question

Ammonium sulfate is a fertiliser. The formula of ammonium sulfate is (NH₄)₂SO₄.

(a) Calculate the relative formula mass, M_r, of ammonium sulfate.
[A_r; H, 1; N, 14; O, 16; S, 32].

▶️Answer/Explanation

M_r = (2×N) + (8×H) + (1×S) + (4×O)
= (2×14) + (8×1) + (1×32) + (4×16)
= 28 + 8 + 32 + 64
= 132

(b) Ammonium sulfate is made by reacting ammonia with sulfuric acid. Write a balanced symbol equation for this reaction. 

▶️Answer/Explanation

2NH₃ + H₂SO₄ → (NH₄)₂SO₄

(c) Potassium sulfate, K₂SO₄, is another fertiliser. In an experiment, 22.4 g of potassium hydroxide, KOH, dissolved in distilled water, reacts with 19.6 g of sulfuric acid to make potassium sulfate.

(i) Calculate the number of moles of potassium hydroxide and the number of moles of sulfuric acid that react.
[A_r; H, 1; K, 39; O, 16; S, 32]

▶️Answer/Explanation

Moles of KOH = mass/M_r = 22.4 ÷ (39+16+1) = 22.4 ÷ 56 = 0.40 mol
Moles of H₂SO₄ = mass/M_r = 19.6 ÷ (2+32+64) = 19.6 ÷ 98 = 0.20 mol

(ii) Using your answers from part (c)(i), deduce the balanced symbol equation for the reaction. Show your working. 

▶️Answer/Explanation

From (i), mole ratio KOH:H₂SO₄ = 0.40:0.20 = 2:1
Therefore equation is:
2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

(d) Ammonia is used in the manufacture of some fertilisers. Ammonia is made in the Haber process.

nitrogen + hydrogen ⇌ ammonia

Fig. 2.1 shows the percentage of ammonia made using different conditions of temperature and pressure.

The highest percentage of ammonia is made at 200 °C and 300 atmospheres pressure. However, in an ammonia factory, a temperature of 450 °C and 200 atmospheres pressure are used. Explain why. Use ideas about the percentage of ammonia made and the rate of reaction. 

▶️Answer/Explanation

1. 450°C increases the rate of reaction (higher temperature gives particles more kinetic energy)
2. 200 atmospheres is safer and less expensive than 300 atmospheres
3. It’s a compromise – while 200°C gives higher yield, the rate is too slow for industrial production
4. At 450°C, the equilibrium yield is lower but is reached much faster