Haber process & NPK fertilisers (Chem only) – GCSE Chemistry exam questions with mark scheme

1Grade 6–7HigherTriple only5 marks

Ammonia is made by the Haber process.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) the forward reaction is exothermic

The process uses a temperature of about 450 °C.

Explain why 450 °C is used. [5]

a lower temperature would increase the (equilibrium) yield of ammonia; allow a higher temperature would decrease the yield(1)
because the forward reaction is exothermic, so (lowering the temperature) shifts the position of equilibrium to the right; allow the reverse reaction is endothermic so is favoured at higher temperature / equilibrium shifts left at higher temperature(1)
but a lower temperature would make the rate of reaction (too) slow; allow a higher temperature gives a faster rate(1)
so 450 °C is a compromise (temperature) giving an acceptable rate and a reasonable yield; must refer to both rate and yield(1)
any one from: an iron catalyst is used so that the rate is fast enough at this (relatively low) temperature; a higher temperature would need more energy / increase (energy) costs; do not credit a further statement that only repeats the rate or yield effects already credited above(1)
Answer: Lower temperature gives a higher yield (exothermic forward reaction) but too slow a rate; 450 °C is a compromise between rate and yield, with an iron catalyst to speed it up.
Examiner tip: Mention both rate and yield and call 450 °C a compromise — an answer only about yield scores at most 2 marks.

2Grade 4–5Triple only5 marks

NPK fertilisers contain compounds of nitrogen, phosphorus and potassium.

(a) Ammonium nitrate is made by reacting ammonia with an acid. Name the acid. [1]

(b) Calculate the percentage by mass of nitrogen in ammonium nitrate, NH₄NO₃.

Relative atomic masses (A_r): H = 1, N = 14, O = 16 [3]

(a) nitric acid; allow HNO₃(1)
(b) M_r = 14 + 4 + 14 + 48 = 80(1)
(b) (2 × 14) ÷ 80 × 100(1)
(b) 35 (%); correct answer with no working scores 3 marks; allow ECF from an incorrect M_r; 17.5 (using one N) scores 1 mark (for M_r)(1)
(c) potassium chloride / potassium sulfate; allow potassium nitrate / potassium phosphate / KCl / K₂SO₄ / KNO₃; do not accept potassium alone(1)
Answer: (a) nitric acid (b) 35% (c) potassium chloride (or potassium sulfate)
Examiner tip: Ammonium nitrate has two nitrogen atoms — using 14 instead of 28 gives 17.5%.

3Grade 1–3Triple only6 marks

Ammonia is made in industry by the Haber process.

$$\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}$$

(a) Most of the ammonia made is used to produce one type of product for farming. Name this type of product. [1]

(b) Give the raw material that the nitrogen is obtained from. [1]

(d) Complete the table to show the conditions used in the Haber process.

Choose answers from the box.

45 °C

450 °C

4500 °C

2 atm

200 atm

20 000 atm

copper

iron

platinum

Condition	Value used
Temperature
Pressure
Catalyst

[3]

(a) fertilisers; allow named fertiliser e.g. ammonium nitrate / ammonium sulfate / NPK fertiliser; ignore ‘for crops’ / ‘for plants’ on its own(1)
(b) (the) air; allow atmosphere; do not accept ammonia / nitric acid(1)
(c) natural gas; allow methane / CH₄; do not accept air; do not accept water (the question asks for the usual source); ignore ‘crude oil’ on its own(1)
(d) temperature: 450 °C(1)
(d) pressure: 200 atm(1)
(d) catalyst: iron; allow Fe(1)
Answer: (a) fertilisers (b) air (c) natural gas (methane) (d) 450 °C, 200 atm, iron
Examiner tip: Hydrogen comes from natural gas, not from ‘the air’ — only the nitrogen is taken from the air.

4Grade 6–7HigherTriple only7 marks

The Haber process makes ammonia from nitrogen and hydrogen.

$$\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}$$

The graph shows how the percentage yield of ammonia at equilibrium changes with pressure at three different temperatures.

(a) Determine the percentage yield of ammonia at 450 °C and 200 atmospheres. [1]

(b) Give the temperature and pressure on the graph that produce the highest percentage yield of ammonia. [1]

(d) A company wants the same percentage yield at 550 °C as it gets at 450 °C and 100 atmospheres.

Use the graph to determine the pressure the company would need to use at 550 °C. [2]

(e) Suggest one reason why a company might choose not to use this higher pressure. [1]

(a) 25 (%); allow any value from 24 to 26(1)
(b) 350 °C and 400 atm(ospheres); both needed(1)
(c) there are fewer molecules / moles of gas on the right-hand side (of the equation); allow 4 molecules (of gas) on the left and 2 on the right(1)
(c) so (increasing pressure) shifts the position of equilibrium to the right / towards ammonia; allow equilibrium shifts to the side with fewer molecules (of gas) / to reduce the pressure; do not accept ‘the rate increases’ as the explanation for yield(1)
(d) yield at 450 °C and 100 atm read as 16 (%); allow 15 to 17(1)
(d) 290 atm(ospheres); allow any value from 280 to 305; allow ECF from their yield read correctly across to the 550 °C curve(1)
(e) any one from: (higher pressure needs) more energy (for compressors) / greater energy costs; stronger / thicker-walled / more expensive equipment / pipes / reactor; greater risk of leaks / explosion / less safe; ignore ‘it is expensive’ unless linked to energy or equipment(1)
Answer: (a) 25% (b) 350 °C and 400 atm (c) fewer molecules of gas on the right, so higher pressure shifts equilibrium to the right (d) about 290 atm (e) higher energy / equipment costs or safety
Examiner tip: In (c) count gas molecules on each side (4 → 2); ‘particles are closer so they collide more’ is a rate answer and scores nothing here.

5Grade 6–7HigherTriple only7 marks

In the Haber process nitrogen reacts with hydrogen to make ammonia.

$$\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$$

In one hour, 840 kg of nitrogen is fed into a reactor with the correct amount of hydrogen.

Relative formula masses ($M_\text{r}$): N₂ = 28 NH₃ = 17

(a) Calculate the maximum mass of ammonia, in kg, that could be made from 840 kg of nitrogen. [3]

(b) On each pass through the reactor only 15% of the nitrogen reacts.

Calculate the mass of ammonia, in kg, made on one pass through the reactor. [1]

Explain why. [1]

(d) Calculate the volume of hydrogen gas, in dm³, measured at room temperature and pressure, needed to react with 840 kg of nitrogen.

The volume of one mole of any gas at room temperature and pressure is 24 dm³.

Give your answer in standard form. [2]

(a) moles of N₂ = 840 000 ÷ 28 = 30 000 (mol); allow 840 ÷ 28 = 30 (kmol)(1)
(a) moles of NH₃ = 2 × 30 000 = 60 000 (mol); allow ECF(1)
(a) mass = 60 000 × 17 = 1 020 000 g = 1020 (kg); correct answer with no working scores 3 marks; allow ECF; allow mass ratio method: 28 kg N₂ → 34 kg NH₃ so 840 × 34 ÷ 28 = 1020 kg; 510 kg (1:1 ratio used) scores 2 marks (M1 and ECF)(1)
(b) 153 (kg); allow ECF: 0.15 × their (a)(1)
(c) unreacted nitrogen and hydrogen are recycled / fed back into the reactor; allow unreacted gases are passed through the reactor again; ignore ‘a catalyst is used’(1)
(d) moles of H₂ = 3 × 30 000 = 90 000 (mol); allow ECF from moles of N₂ in (a)(1)
(d) volume = 90 000 × 24 = 2.16 × 10⁶ (dm³); allow 2.2 × 10⁶; correct answer with no working scores 2 marks; 2 160 000 not in standard form scores 1 mark(1)
Answer: (a) 1020 kg (b) 153 kg (c) unreacted gases are recycled (d) 2.16 × 10⁶ dm³
Examiner tip: Use the 1 : 2 mole ratio — assuming 840 kg of nitrogen makes 840 kg of ammonia, or using a 1 : 1 ratio, are the most common ways to lose marks.

6Grade 4–5Triple only5 marks

NPK fertilisers contain compounds of nitrogen, phosphorus and potassium.

Phosphate rock is mined to provide phosphorus. It cannot be put directly on to fields as a fertiliser.

(a) Give the reason why phosphate rock must be treated before it can be used as a fertiliser. [1]

(b) Phosphate rock is treated with different acids. Complete the table. [4]

Acid used to treat phosphate rock	Product(s)
nitric acid	(i) ____________ and (ii) ____________
sulfuric acid	(iii) ____________ (a mixture of calcium phosphate and calcium sulfate)
phosphoric acid	(iv) ____________ (calcium phosphate)

(a) phosphate rock is insoluble (in water) / does not dissolve, so plants cannot absorb it; allow converts it to a soluble form; ignore ‘to make it safe’(1)
(b)(i) phosphoric acid; allow H₃PO₄; (i) and (ii) in either order(1)
(b)(ii) calcium nitrate; allow Ca(NO₃)₂(1)
(b)(iii) single superphosphate; allow superphosphate; do not accept triple superphosphate(1)
(b)(iv) triple superphosphate; do not accept single superphosphate / superphosphate on its own(1)
Answer: (a) it is insoluble, so plants cannot take it up (b) phosphoric acid and calcium nitrate; single superphosphate; triple superphosphate
Examiner tip: Remember ‘triple’ goes with phosphoric acid — the acid that already contains phosphate adds the most phosphorus.

7Grade 6–7Triple only6 marks

Ammonium sulfate is used as a fertiliser. It can be made in the laboratory by reacting ammonia solution with dilute sulfuric acid.

$$2\text{NH}_3\text{(aq)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow (\text{NH}_4)_2\text{SO}_4\text{(aq)}$$

Both reactants are solutions, so you cannot see when the reaction is complete.

Ammonium sulfate breaks down if it is heated strongly.

Describe a method to make pure, dry crystals of ammonium sulfate from ammonia solution and dilute sulfuric acid.

You should name the apparatus you would use. [6]

use a (volumetric) pipette (and filler) to measure a known volume, e.g. 25.0 cm³, of ammonia solution into a conical flask(1)
add a few drops of indicator, e.g. methyl orange; allow named acid–base indicator such as phenolphthalein / litmus; do not accept universal indicator(1)
add dilute sulfuric acid from a burette (swirling the flask)(1)
until the indicator just changes colour (end point), e.g. methyl orange yellow to orange/red(1)
record the volume of acid added(1)
repeat the reaction using the same volumes but without the indicator; allow remove the indicator with (activated) charcoal and filter(1)
pour the solution into an evaporating basin and heat gently / use a water bath to evaporate some of the water(1)
do not evaporate to dryness / stop heating when crystals start to form (because ammonium sulfate decomposes on strong heating)(1)
leave the solution to cool so that crystals form(1)
filter to separate the crystals (from the remaining solution)(1)
dry the crystals between filter paper / in a warm oven / in a desiccator(1)
safety: wear eye protection; use a fume cupboard / well-ventilated room because ammonia solution gives off a pungent gas(1)

Level	Marks	What it looks like
Level 3	5–6	A coherent, logically ordered method that would produce pure, dry crystals: correct reagents measured accurately, the reacting volumes found with an indicator, the reaction repeated without indicator (or indicator removed), and a correct crystallisation, separation and drying sequence.
Level 2	3–4	A method that would produce crystals of ammonium sulfate, with most key stages in a sensible order, but with an important step missing or unclear (e.g. no way of finding the end point, or indicator left in, or drying not described).
Level 1	1–2	Some relevant, simple steps (e.g. mix the solutions, heat or evaporate) that are not linked into a workable method.
Level 0	0	No relevant content.

Answer: Pipette 25.0 cm³ ammonia solution into a conical flask, add methyl orange, titrate with sulfuric acid from a burette to the end point; repeat with the same volumes and no indicator; warm gently to evaporate some water, leave to crystallise, filter and dry the crystals between filter paper.
Examiner tip: Students lose marks for adding excess reactant and filtering it off — that only works with an insoluble solid; with two solutions you need a titration and then a repeat without indicator.

8Grade 4–5Triple only5 marks

Ammonium sulfate, (NH₄)₂SO₄, is made both in industry and in school laboratories.

The table gives information about the two methods.

	Industrial production	Laboratory preparation
Starting materials	Ammonia made from air and natural gas; sulfuric acid made from sulfur, air and water	Ammonia solution and dilute sulfuric acid bought from a chemical supplier
Number of stages	Several, carried out at one site	One reaction, then crystallisation
How the process runs	Continuous, 24 hours a day	One batch at a time
Concentration of reactants	Concentrated	Dilute
Mass of product	Hundreds of tonnes each day	A few grams

(a) Compare the industrial production of ammonium sulfate with its laboratory preparation.

Use the information in the table. [3]

(b) The reaction between ammonia and sulfuric acid is exothermic.

Suggest one reason why this matters more in the industrial process than in the laboratory. [1]

Relative atomic masses ($A_\text{r}$): H = 1 N = 14 O = 16 S = 32 [1]

(a) one valid comparison that refers to both methods, from: industry makes its own starting materials from raw materials (air, natural gas, sulfur, water) whereas the laboratory buys ready-made chemicals; industry uses several stages whereas the laboratory uses one reaction (then crystallisation); industry is continuous whereas the laboratory is a batch process; industry uses concentrated reactants whereas the laboratory uses dilute solutions; industry makes a much larger mass of product / is large scale whereas the laboratory makes a few grams(1)
(a) a second, different valid comparison from the list in the first mark point(1)
(a) a third, different valid comparison from the list; a statement about only one method scores 0; ignore copied table entries that are not linked to the other method(1)
(b) any one from: (with concentrated reactants on a large scale) the temperature rise is much greater / the mixture could overheat / boil; the heat (energy) released can be used / recovered to save energy (costs); cooling is needed; ignore ‘it is dangerous’ with no reason(1)
(c) 132(1)
Answer: (a) e.g. industry makes its own reactants from raw materials, uses several stages, runs continuously, uses concentrated reactants and makes far more product; the lab buys dilute solutions, uses one stage, makes one batch of a few grams (b) large heat release must be controlled / can be recovered (c) 132
Examiner tip: In a ‘compare’ question every mark needs both methods in the same sentence — use ‘whereas’ or ‘but’.

9Grade 4–5Triple only5 marks

The mixture of gases leaving the reactor in the Haber process contains ammonia, unreacted nitrogen and unreacted hydrogen.

The table shows the melting points and boiling points of the three substances.

Substance	Melting point in °C	Boiling point in °C
ammonia	−78	−33
nitrogen	−210	−196
hydrogen	−259	−253

(a) The mixture is cooled to −50 °C.

Give the state of each substance at −50 °C. [1]

(b) Explain how cooling the mixture separates the ammonia from the other gases. [2]

(d) After the ammonia is removed, the nitrogen and hydrogen are pumped back into the reactor.

Give one advantage of doing this. [1]

(a) ammonia: liquid; nitrogen: gas; hydrogen: gas; all three needed for the mark(1)
(b) ammonia has the highest boiling point / boiling point above the temperature it is cooled to, so ammonia condenses / liquefies(1)
(b) nitrogen and hydrogen (have lower boiling points so) stay as gases, so the liquid ammonia can be run off / separated from the gases; allow the liquid can be removed from the gases(1)
(c) nitrogen would also condense / liquefy (because −200 °C is below its boiling point) so it would not be separated from the ammonia; allow it would waste energy / be more expensive to cool that far as an alternative; ignore ‘too cold’ alone(1)
(d) any one from: raw materials are not wasted; saves money / reduces costs of raw materials; increases the overall yield (of ammonia); allow conserves natural gas (used to make hydrogen); ignore ‘better for the environment’ alone(1)
Answer: (a) ammonia liquid; nitrogen and hydrogen gases (b) ammonia has the highest boiling point so it condenses; nitrogen and hydrogen stay as gases and are separated (c) nitrogen would also condense (d) saves raw materials / costs, increases overall yield
Examiner tip: Say the ammonia condenses or liquefies — ‘ammonia is cooled down’ does not say what changes and scores nothing.

10Grade 8–9HigherTriple only7 marks

The Haber process is a reversible reaction. The forward reaction is exothermic.

$$\text{N}_2\text{(g)} + 3\text{H}_2\text{(g)} \rightleftharpoons 2\text{NH}_3\text{(g)}$$

(a) A student says: “The iron catalyst is used because it increases the yield of ammonia at equilibrium.”

Evaluate the student’s statement. [2]

(b) A pressure of 200 atmospheres is used. Some ammonia plants could be built to work at 1000 atmospheres.

Explain the advantages and disadvantages of using 1000 atmospheres instead of 200 atmospheres. [3]

Give one advantage and one disadvantage of making the hydrogen by electrolysis of water instead of from natural gas. [2]

(a) the statement is incorrect: a catalyst does not change the position of equilibrium / the (equilibrium) yield; allow it speeds up the forward and reverse reactions equally(1)
(a) the catalyst is used because it increases the rate (of reaction) / equilibrium is reached faster, so more ammonia is made per hour; allow so a lower temperature can be used with an acceptable rate (saving energy)(1)
(b) advantage: higher (equilibrium) yield because the equilibrium shifts to the right / to the side with fewer molecules of gas (2 molecules on the right, 4 on the left)(1)
(b) advantage: faster rate of reaction (because gas molecules are closer together so collide more frequently)(1)
(b) disadvantage: any one from: more energy needed to compress the gases / higher energy costs; stronger / thicker / more expensive reactor and pipes needed; greater risk of explosion / leaks; allow the extra yield is not worth the extra cost; a second, different disadvantage may score in place of the rate advantage (max 2 marks for advantages, max 2 for disadvantages)(1)
(c) advantage: any one from: water is a renewable / plentiful resource; does not use a finite resource / natural gas; no carbon dioxide produced (in making hydrogen); allow less carbon dioxide if renewable electricity is used(1)
(c) disadvantage: any one from: electrolysis needs a large amount of electricity (energy); electricity is expensive / so it costs more; if the electricity comes from fossil fuels carbon dioxide is still released; do not accept ‘water is expensive’(1)
Answer: (a) Wrong — a catalyst does not change the equilibrium yield; it is used to increase the rate so ammonia is made faster. (b) 1000 atm: higher yield (fewer gas molecules on the right) and faster rate, but much higher energy and equipment costs / safety risk. (c) Advantage: renewable / no finite natural gas used / no CO₂; disadvantage: needs a lot of expensive electricity.
Examiner tip: For (a) you must both reject the claim (no change to yield) and say what the catalyst does do (faster rate) to get both marks.

Still losing marks on this topic?

Book a free introductory session and we'll find exactly which step is costing you.

Book a free intro