AQA GCSE Chemistry Foundation (8462), Foundation tier · Using resources › The Haber process and NPK fertilisers
Practise Production and uses of NPK fertilisers. 9 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What NPK fertilisers are, how they are made from ammonia, mined potassium compounds and phosphate rock, and how industrial production compares with making the same salts in the laboratory. This is in GCSE Chemistry only, not in Combined Science. Expect recall of salt names, balanced equations, percentage by mass calculations and a 'compare' question on Paper 2.
Key facts
NPK = nitrogen, phosphorus, potassium; fertilisers must be soluble
NH3 + HNO3 → NH4NO3 (ammonium nitrate)
2NH3 + H2SO4 → (NH4)2SO4 (ammonium sulfate)
Potassium chloride and potassium sulfate are mined
Phosphate rock + phosphoric acid → calcium phosphate (triple superphosphate)
% N by mass = (number of N atoms × 14) ÷ Mr × 100
Industry: continuous, large scale, concentrated; laboratory: batch, small scale, dilute
Notes
What NPK fertilisers are
Compounds of nitrogen (N), phosphorus (P) and potassium (K) are used as fertilisers to improve agricultural productivity. They replace elements removed from the soil by crops, so plants grow better and yields increase.
NPK fertilisers contain compounds of all three elements. They are formulations of various salts containing appropriate percentages of the elements.
Fertilisers must be soluble in water so that plants can absorb them through their roots.
Industrial production uses a variety of raw materials in several integrated processes.
Where the N, P and K come from
diagram
Nitrogen: ammonia from the Haber process is used to manufacture ammonium salts and nitric acid.
ammonia + phosphoric acid → ammonium phosphate (this supplies both N and P).
Potassium: potassium chloride and potassium sulfate are obtained by mining.
Phosphorus: phosphate rock is mined, but it cannot be used directly as a fertiliser because it is insoluble. It is treated with acid to produce soluble salts:
The compounds must be soluble so that plants can absorb them.
Laboratory preparation and industrial production
Laboratory (e.g. ammonium sulfate): titrate dilute ammonia solution with dilute sulfuric acid using an indicator, repeat with the same volumes but no indicator, then evaporate some water and leave the solution to crystallise. A few grams are made in a batch, from reactants bought ready-made.
Industry: thousands of tonnes are made in a continuous process using concentrated reactants. The ammonia and acid are made on site from raw materials (e.g. natural gas, air, sulfur), and the heat released by the reaction is used to evaporate the water.
How to answer each type of question
Identify the elements a fertiliser supplies
1 to 2 marksGrade 5
Look at each formula for N, P and K.
Ammonium (NH4) and nitrate (NO3) supply N; phosphate (PO4) supplies P; K supplies potassium.
Check that the mixture you choose contains all three elements.
Example. A gardener has three compounds: potassium nitrate, KNO3; ammonium phosphate, (NH4)3PO4; and ammonium nitrate, NH4NO3. Which two compounds should be mixed to make an NPK fertiliser? Explain your answer. (2 marks)
Show the model answerHide the model answer
Potassium nitrate and ammonium phosphate (1). Potassium nitrate supplies K and N, and ammonium phosphate supplies N and P, so together they contain all three elements (1).
Don’t lose marks
Writing 'ammonia nitrate' or 'ammonium nitrite'. The salt is ammonium nitrate.
Ignoring the bracket in (NH4)2SO4, so counting only one N or four H atoms.
Saying phosphate rock can be spread on fields directly. It is insoluble, so it must be treated with acid first.
Comparing laboratory and industrial methods with one-sided points such as 'industry is bigger'. Each comparison needs both sides.
Saying potassium comes from ammonia or phosphate rock. Potassium chloride and potassium sulfate are mined.
More tips
Memory tricks
N-P-K: Nitrogen, Phosphorus, and K for potassium (from its Latin name, kalium).
Phosphate rock + nitric acid gives an acid AND a salt; the other two acids give only salts.
S for sulfuric and single superphosphate; phosphoric acid gives triple superphosphate.
'Compare' needs matched points using 'whereas' or 'but'; a list about only one method scores few marks.
In percentage by mass questions show the Mr working, so you can earn method marks even if the final answer is wrong.
When naming salts, write the full name: 'calcium phosphate', not just 'phosphate'.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
State why farmers use fertilisersCompounds of nitrogen, phosphorus and potassium improve agricultural productivity (bigger crop yields).
Grade 4
Explain what an NPK fertiliser isA formulation of salts containing appropriate percentages of nitrogen, phosphorus and potassium.
Grade 4
Name the sources of the three elementsNitrogen from ammonia; potassium from mined potassium chloride and potassium sulfate; phosphorus from mined phosphate rock.
Grade 5
Name ammonium salts made from ammoniaWith nitric acid, ammonium nitrate; with sulfuric acid, ammonium sulfate; with phosphoric acid, ammonium phosphate.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Ammonia is used to manufacture ammonium salts and nitric acid. Name the salt formed when ammonia reacts with nitric acid.
ammonium nitrate
Name the salt formed when phosphate rock is treated with phosphoric acid.
calcium phosphate
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Name the three elements that NPK fertilisers provide.[1]
(b) Why do farmers use fertilisers?[1]
(c) Which substance is obtained by mining and is used as a source of potassium in fertilisers? Tick (✓) one box.[1]
Ammonium nitrate
Nitric acid
Phosphoric acid
Potassium chloride
(d) NPK fertilisers are formulations. What does this mean?[1]
Show the answer and mark scheme
(a)
nitrogen, phosphorus and potassium (all three needed)
(b)
to improve agricultural productivity / to increase crop yields
(c)Answer: Potassium chloride
(d)
they are mixtures of different salts containing the elements in appropriate percentages / designed as useful products
Question 2Medium5 marks
Ammonia is used to manufacture ammonium salts and nitric acid.
(a) Name the salt formed when ammonia reacts with nitric acid.[1]
(b) Balance the equation for the production of ammonium sulfate. ....NH3 + H2SO4 → (NH4)2SO4[1]
(c) Name the acid that reacts with ammonia to produce ammonium phosphate.[1]
(d) A fertiliser factory has an ammonia plant and a nitric acid plant on the same site. Suggest two advantages of this.[2]
Show the answer and mark scheme
(a)Answer: ammonium nitrate
ammonium nitrate
(b)Answer: 2NH3 + H2SO4 → (NH4)2SO4
2 (NH3)
(c)Answer: phosphoric acid
phosphoric acid
(d)
ammonia is needed to make nitric acid
the nitric acid can then be reacted with more ammonia to make ammonium nitrate
no transport costs / less transport between sites
energy released by one process can be used in another (integrated processes)