NPK — nitrogen, phosphorus, potassium
Plants need three elements from fertilisers. Nitrogen (N) is for protein synthesis and leaf/shoot growth; phosphorus (P) for root growth; potassium (K) for enzyme reactions and disease resistance. An NPK fertiliser supplies compounds of all three, and the N:P:K numbers on the bag give the percentage by mass of each. Common nitrogen fertilisers are ammonium salts (e.g. ), nitrates (e.g. ) and urea.
(Extended) Ostwald process — nitric acid from ammonia
(Extended) Nitric acid from ammonia, in three stages. Stage 1: ammonia oxidised on platinum at ~850 °C: . Stage 2: NO oxidised: . Stage 3: absorbed in water + oxygen → nitric acid. Neutralised by ammonia → ammonium nitrate: .
(Extended) Eutrophication — the oxygen-depletion chain
(Extended) Excess nitrate (and phosphate) fertiliser leaches into rivers and lakes. Nitrates act as nutrients, so algae multiply (an algal bloom). The algal layer blocks sunlight, so plants cannot photosynthesise and die. Aerobic bacteria decompose the dead matter, using up the dissolved oxygen. Fish can no longer respire and die from oxygen depletion, not nitrate toxicity. This process is eutrophication.
Drawn from real examiner reports.
Name the element, not "phosphates"
Asked to name the three elements plants need, candidates often give only nitrogen (in the stem) or write phosphates. Phosphate () is a compound ion, not an element — the element is phosphorus (P). The three answers are nitrogen (N), phosphorus (P) and potassium (K). Save compounds like nitrates for 'which compound' questions.
Poorly recalled across sittings: candidates re-state nitrogen or write 'phosphates'. (W22 P31 Q5(a); W22 P32 Q5(c)(i))
Match each role to N, P or K
Mark schemes need the right N–P–K pairing. Nitrogen for protein synthesis and leaf/shoot growth; phosphorus for root growth; potassium for enzyme reactions and disease resistance. A generic "it helps the plant grow" earns nothing. Hook: Phosphorus → roots Point down; Nitrogen → New green leaves.
Eutrophication — give the mechanism
"Excess nitrates are toxic and kill the fish" earns no marks — nitrates are not directly toxic; fish die indirectly. The full chain must appear: nitrates feed algae → algal bloom blocks light → plants die → aerobic bacteria decompose them → use up dissolved oxygen → fish cannot respire and die. Say "oxygen depletion", not "no oxygen".
Candidates write 'it kills fish' or 'it's toxic' instead of the chain to oxygen depletion. (S23 P31 Q3(b)(ii))
Nitrogen the element vs the nitrate ion
"Nitrogen" is the element N, one of the NPK elements. A nitrate is the ion — a compound containing nitrogen. Ammonium nitrate supplies nitrogen but is not "the element nitrogen". Both ammonium salts () and nitrates () are nitrogen fertilisers, since plants absorb nitrogen from either ion.
Count both nitrogen atoms in ammonium nitrate
Ammonium nitrate contains two nitrogen atoms — one in and one in . Using only one halves the answer. With , the nitrogen content is , not the a single nitrogen gives. Count every atom of the element in the whole formula before a percentage by mass.
(Extended) Ostwald uses platinum, Haber uses iron
(Extended) Do not swap the two catalysts. The Haber process (making ammonia, ) uses an iron catalyst at ~450 °C and 200 atm. The Ostwald process (oxidising ammonia to nitrogen monoxide) uses a platinum catalyst at ~850 °C. Naming iron for Ostwald, or platinum for Haber, loses the mark.
State N, P and K by name
State all three by name or symbol — nitrogen (N), phosphorus (P), potassium (K) — never "phosphates", "nitrates" or "potash" (compounds or trade names). If the stem names nitrogen, still supply phosphorus and potassium.
(Extended) Write the eutrophication chain in order
Write the chain, one mark per link: nitrates act as nutrients → algal bloom → block sunlight → plants die → aerobic bacteria decompose them → use up dissolved oxygen → fish die from oxygen depletion. Never skip oxygen depletion — the direct cause of death.
(Extended) Scaffold for making ammonium nitrate
Steps: ammonia from the Haber process; oxidised on hot platinum to NO; oxidised further to ; dissolves in water + oxygen → nitric acid; neutralised by ammonia: .
Fertiliser — a substance added to soil to supply essential elements that promote healthy plant growth.
NPK fertiliser — a fertiliser containing compounds of all three key elements: nitrogen (N), phosphorus (P) and potassium (K).
Eutrophication — the process by which excess nutrients (nitrates, phosphates) in a waterway lead to algal bloom, oxygen depletion and the death of aquatic life.
Ostwald process (Extended) — the industrial manufacture of nitric acid () by the catalytic oxidation of ammonia () over a platinum catalyst, followed by absorption of nitrogen dioxide in water.
State the three chemical elements that must be present in an NPK fertiliser.
Ammonium nitrate, , is used as a nitrogen fertiliser.
(a) Calculate the relative formula mass () of ammonium nitrate.
(: N = 14, H = 1, O = 16)
(b) Calculate the percentage by mass of nitrogen in ammonium nitrate.
