Fertilisers Exam Questions

• Nitrogen (N): for making proteins / amino acids / for leaf growth [1]
• Phosphorus (P): for root growth / making DNA / energy transfer [1]
• Potassium (K): for flower and fruit production / enzyme function / disease resistance [1]

Examiner note: The question asks for elements, not compounds. Writing “nitrate” instead of “nitrogen” can lose the mark. NPK is the shorthand — learn what each letter stands for and one function of each.

(a) ammonia + sulfuric acid → ammonium sulfate [1]

(b) 2NH3 + H2SO4 → (NH4)2SO4 [1]

(c) Heat the solution gently in an evaporating basin to evaporate some of the water [1]. Allow the remaining solution to cool slowly so that crystals form, then filter and dry the crystals [1]

Examiner note: This is crystallisation, not evaporation to dryness. Heating too strongly decomposes ammonium sulfate. The balanced equation is commonly tested — remember that ammonia is NH3, not NH4, until it reacts with the acid.

• Excess fertiliser is washed/leached from soil into rivers and lakes by rain [1]
• The fertiliser causes rapid growth of algae on the surface (algal bloom) [1]
• The algae block sunlight from reaching underwater plants, which die [1]
• Bacteria decompose the dead plant matter and algae [1]
• The bacteria use up dissolved oxygen in the water / the water becomes deoxygenated, causing fish and other aquatic organisms to die [1]

Examiner note: This process is called eutrophication. Learn the sequence as a chain — each step must logically follow the previous one. The most commonly missed step is the role of decomposing bacteria in using up oxygen. Simply saying “fish die because of fertiliser” without the mechanism earns at most 1 mark.

(a) The percentage by mass of nitrogen (N), phosphorus pentoxide (P2O5), and potassium oxide (K2O) in the fertiliser [1]

(b) The high phosphorus content (20) promotes root development and flowering / phosphorus is important for flower and fruit formation [1]

(c) Any one from: ammonium nitrate / ammonium sulfate / urea / ammonium phosphate [1]

Examiner note: NPK ratios are percentages, not actual masses. A 10-20-10 fertiliser contains twice as much phosphorus (as P2O5) as nitrogen or potassium by mass. Some candidates confuse the NPK numbers with molar ratios.

• Both provide essential minerals/nutrients (N, P, K) to the soil for plant growth [1]
• Advantage of natural fertiliser: improves soil structure / releases nutrients slowly / less likely to cause eutrophication / cheaper for small-scale use [1]
• Advantage of artificial fertiliser: contains precise/known concentrations of nutrients / faster acting / can be tailored to specific crop needs [1]

Examiner note: Natural fertilisers release nutrients more slowly as they must decompose first. Artificial fertilisers dissolve quickly and can be washed away more readily, increasing eutrophication risk. A balanced answer comparing both types scores best.

(a) Mr of NH4NO3 = 14 + (4 x 1) + 14 + (3 x 16) = 80 [1] Percentage of N = (28/80) x 100 = 35% [1]

(b) Ammonium nitrate contains nitrogen in two forms — the ammonium ion (NH4+) and the nitrate ion (NO3-) — so it has a high nitrogen content / both ions are directly usable by plants [1]

Examiner note: There are two nitrogen atoms in NH4NO3 (total mass 28), not one. This is a common error. The double nitrogen source is what makes it an efficient fertiliser compared to, say, ammonium sulfate.

• Ammonia is manufactured by the Haber process: N2 + 3H2 ⇌ 2NH3 [1]
• Nitrogen is obtained from the air (by fractional distillation of liquid air) and hydrogen from natural gas/methane [1]
• The ammonia is reacted with sulfuric acid: 2NH3 + H2SO4 → (NH4)2SO4 [1]
• The solution is evaporated and crystallised to produce solid ammonium sulfate [1]

Examiner note: This question links the Haber process to fertiliser production. The Haber process conditions (450 degrees C, 200 atm, iron catalyst) are not required here, but knowing the raw materials and the link to fertilisers is essential.