The Haber process and the use of NPK fertilisers

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The Haber Process

The Haber process manufactures ammonia from nitrogen and hydrogen in a reversible reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g).
Nitrogen is obtained from the air (which is about 78% nitrogen) by fractional distillation.
Hydrogen is obtained from natural gas (methane) in a process called steam reforming.
The gases are passed over an iron catalyst at about 450°C and 200 atmospheres pressure.
The reaction is exothermic in the forward direction (ΔH = –92 kJ mol⁻¹).
On cooling, the ammonia liquefies and is removed; unreacted nitrogen and hydrogen are recycled back into the reactor.

Equilibrium and the Haber process

Dynamic Equilibrium

Dynamic equilibrium occurs in a closed system when the rate of the forward reaction equals the rate of the reverse reaction.
At equilibrium, the concentrations of reactants and products remain constant, but they are not necessarily equal.
The reaction is still occurring in both directions; it has not stopped.
Equilibrium is reached faster at higher pressure, higher temperature, higher concentration, or when a catalyst is used.
Le Chatelier's Principle: if a change is made to a system at equilibrium, the equilibrium shifts to oppose the change.

Choosing Conditions: Temperature

The forward reaction is exothermic, so a lower temperature would favour the forward reaction and give a higher yield of ammonia.
However, at low temperatures the rate of reaction is very slow.
A higher temperature favours the reverse (endothermic) reaction, giving a lower yield of ammonia.
450°C is a compromise temperature: it gives a reasonable yield at an acceptable rate.
The chosen temperature balances yield, rate and cost.

Choosing Conditions: Pressure

There are 4 moles of gas on the left (1 N₂ + 3 H₂) and 2 moles of gas on the right (2 NH₃).
A higher pressure favours the forward reaction (fewer gas molecules) and gives a higher yield of ammonia.
A lower pressure favours the reverse reaction (more gas molecules) and gives a lower yield.
Very high pressures are dangerous and require expensive equipment.
200 atmospheres is a compromise pressure between yield, safety and cost.

The Role of the Catalyst

An iron catalyst is used in the Haber process.
A catalyst does not affect the position of equilibrium; it increases the rate of both forward and reverse reactions equally.
It works by providing an alternative pathway with a lower activation energy.
The catalyst helps the reaction reach equilibrium faster.
It allows an acceptable yield at a lower temperature, reducing energy costs and preventing decomposition of ammonia.

Economic Considerations

Chemical industries aim to make a profit, so they must consider costs.
Raw materials for the Haber process are readily available and inexpensive to purify.
High temperatures and pressures are expensive to maintain.
The conditions chosen are a compromise between yield, rate and cost.
If raw materials become too expensive or unavailable, the process may no longer be economically viable.

NPK Fertilisers

NPK fertilisers contain compounds of nitrogen (N), phosphorus (P) and potassium (K).
They are formulations of various salts containing appropriate percentages of each element.
Nitrogen promotes healthy leaves.
Phosphorus promotes healthy roots.
Potassium promotes growth, healthy fruit and flowers.
Artificial fertilisers can be designed for specific needs, unlike natural fertilisers such as manure or seaweed.
Fertilisers must be water soluble so nutrients can be absorbed by plants.

Making Fertilisers from Ammonia

Ammonia is alkaline and neutralises acids to produce ammonium salts.
Ammonium nitrate is made by reacting ammonia with nitric acid: NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq).
Ammonium sulfate is made by reacting ammonia with sulfuric acid: 2NH₃(aq) + H₂SO₄(aq) → (NH₄)₂SO₄(aq).
Ammonia can be oxidised to produce nitric acid, which provides nitrate ions for fertilisers.
Common fertiliser ions include ammonium (NH₄⁺), nitrate (NO₃⁻), phosphate (PO₄³⁻) and potassium (K⁺).

Laboratory Preparation of Ammonium Sulfate

In the lab, ammonium sulfate is prepared by titration using dilute ammonia, dilute sulfuric acid and methyl orange indicator.
Method: add a known volume of ammonia to a conical flask, add indicator, then add acid drop by drop until the colour changes sharply.
Record the titre, then repeat without indicator (indicator is an impurity).
Evaporate some water by heating in a water bath until the volume is reduced to about one third.
Leave to crystallise; then filter to remove remaining water.

Industrial Production and Phosphate Rock

Industrial production of ammonium sulfate is a large-scale process with several stages.
Ammonia is made by the Haber process; sulfuric acid by the Contact process.
In industry, ammonia gas reacts with concentrated sulfuric acid at about 60°C.
Phosphate rock is mined but is insoluble in water, so it cannot be used directly as a fertiliser.
Treating phosphate rock with nitric acid produces phosphoric acid and calcium nitrate; the phosphoric acid can be neutralised with ammonia to form ammonium phosphate.
Treating phosphate rock with sulfuric acid produces a mixture of calcium phosphate and calcium sulfate, known as single superphosphate.
Treating phosphate rock with phosphoric acid produces calcium phosphate, known as triple superphosphate.
Potassium chloride and potassium sulfate are mined; they are easier to extract because potassium compounds are water soluble.

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1.Which statement about the Haber process is NOT true?
Easy
- APlatinum is used as a catalyst
- BThe process is carried out at a temperature of about 450 °C
- CThe process takes place at a pressure of about 200 atmospheres
- DNatural gas is a raw material
2.Which statement about the yield of ammonia in the Haber process is correct?
Easy
- ALess ammonia is produced at higher pressures
- BThe yield of ammonia decreases with increased temperature
- CThe optimum conditions are low pressure and high temperature
- DAt 500 atmospheres and 400 °C the yield of ammonia is 45%
3.Why is a lower temperature than 450 °C not used during the Haber process?
Easy
- AThe rate of reaction would be too slow
- BThe yield of ammonia would be lower
- CThe iron catalyst would not work at all
- DThe reaction would become endothermic
4.Which elements are found in an NPK fertiliser?
Easy
- APhosphorus, calcium, nitrogen
- BIron, nitrogen, magnesium
- CNitrogen, potassium, phosphorus
- DPotassium, calcium, nitrogen
5.Ammonia and sulfuric acid react in the laboratory to make ammonium sulfate fertiliser. What is the formula of the product?
Easy
- ANH₄SO₄
- B(NH₄)₂SO₄
- C(NH₃)₂SO₄
6.Ammonia is manufactured by the Haber process: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). A temperature of 450 °C is used instead of room temperature, and an iron catalyst can also be added. Which row shows how the temperature and the catalyst affect the rate of attainment of equilibrium?
Easy
- AIncreasing temperature to 450 °C decreases the rate; adding iron catalyst does not change the rate
- BIncreasing temperature to 450 °C decreases the rate; adding iron catalyst increases the rate
- CIncreasing temperature to 450 °C increases the rate; adding iron catalyst does not change the rate
- DIncreasing temperature to 450 °C increases the rate; adding iron catalyst increases the rate
7.Which of these statements is NOT a correct reason for the choice of pressure in the Haber process?
Easy
- AA higher pressure would give a greater equilibrium yield of ammonia
- BA lower pressure would favour the reverse reaction
- CHigh pressures are dangerous and require very expensive equipment
- DA higher pressure would favour the reverse reaction
8.Which acid is needed to manufacture ammonium sulfate by reacting it with ammonia?
Easy
- APhosphoric acid
- BSulfuric acid
- CHydrochloric acid
- DNitric acid

Plus de questions

9.Table 1 shows the percentages of nitrogen, phosphorus and potassium in four fertilisers. Which fertiliser contains the highest total percentage of all of the essential elements? Fertiliser | Nitrogen (%) | Phosphorus (%) | Potassium (%) W | 25 | 0 | 0 X | 12 | 0 | 37 Y | 19 | 22 | 0 Z | 23 | 18 | 21
Medium
- AFertiliser W
- BFertiliser X
- CFertiliser Y
- DFertiliser Z
10.Plants lacking in phosphorus can be slow to grow, and plants lacking in potassium can have wilted leaves. Which fertiliser in the table would be best to treat land where the plants are growing slowly and have wilted leaves? Fertiliser | Nitrogen (%) | Phosphorus (%) | Potassium (%) W | 25 | 0 | 0 X | 12 | 0 | 37 Y | 19 | 22 | 0 Z | 23 | 18 | 21
Medium
- AFertiliser W
- BFertiliser X
- CFertiliser Y
- DFertiliser Z
11.An iron catalyst is used in the Haber process. Why is a catalyst used?
Easy
- ATo produce more ammonia
- BTo use less energy
- CTo reduce the rate of reaction
- DTo use less hydrogen and nitrogen
12.How many atoms are there in one molecule of ammonia, NH₃?
Easy
- A2
- B3
- C4
- D5
13.What is meant by a closed system in the context of dynamic equilibrium?
Easy
- AA system where no reactants or products can escape
- BA system where the reaction has stopped
- CA system where the temperature is kept constant
- DA system where the pressure is kept constant
14.Which compound is produced when ammonia is neutralised with nitric acid?
Easy
- AAmmonium sulfate
- BAmmonium nitrate
- CAmmonium phosphate
- DAmmonium chloride
15.Ammonium phosphate, (NH₄)₃PO₄, is a fertiliser produced from ammonia. How many atoms of each element are there in one molecule of ammonium phosphate?
Medium
- ANitrogen = 3, Hydrogen = 12, Phosphorus = 1, Oxygen = 4
- BNitrogen = 1, Hydrogen = 4, Phosphorus = 1, Oxygen = 4
- CNitrogen = 3, Hydrogen = 12, Phosphorus = 4, Oxygen = 4
- DNitrogen = 3, Hydrogen = 4, Phosphorus = 1, Oxygen = 4
16.Which of the following is a disadvantage of making ammonium nitrate on an industrial scale?
Easy
- AThe process is fast
- BIt is an expensive process
- CLarge concentrations of fertiliser are produced
- DLow concentrations of fertiliser are produced
17.The conditions used in the Haber process are chosen to give the maximum possible yield of ammonia.
Easy
True or false?
18.Adding an iron catalyst to the Haber process shifts the position of equilibrium to produce more ammonia.
Easy
True or false?
19.Nitrogen for the Haber process is obtained from the air, and hydrogen is obtained from natural gas.
Easy
True or false?
20.In the Haber process, a higher pressure favours the forward reaction because there are fewer molecules of gas on the product side.
Easy
True or false?
21.Phosphate rock can be used directly as a fertiliser without any treatment.
Easy
True or false?
22.Which of the following statements about the Haber process are correct? (select all that apply)
Medium
- AThe reaction is reversible
- BThe forward reaction is exothermic
- CThe catalyst used is iron
- DThe process uses a temperature of about 450 °C
- EA low pressure is used to increase the yield of ammonia
23.Which of the following are sources of soluble nitrogen in fertilisers? (select all that apply)
Medium
- AAmmonium ions, NH₄⁺
- BNitrate ions, NO₃⁻
- CPhosphate ions, PO₄³⁻
- DPotassium ions, K⁺
24.Which of the following are true about the industrial production of ammonium sulfate compared with its laboratory preparation? (select all that apply)
Medium
- AIndustrial production uses high concentrations of reactants
- BIndustrial production uses simple equipment
- CThe industrial reaction is very exothermic
- DLaboratory preparation uses crystallisation to separate the product
- ELaboratory preparation is a fast process
25.Match each fertiliser compound to the essential element it supplies.
Medium
- Ammonium nitrate
- Calcium phosphate
- Potassium nitrate
- Potassium sulfate
- Nitrogen
- Phosphorus
- Potassium and nitrogen
- Potassium
26.Match each treatment of phosphate rock to the product(s) formed.
Medium
- Nitric acid
- Sulfuric acid
- Phosphoric acid
- Phosphoric acid and calcium nitrate
- Single superphosphate
- Triple superphosphate
27.Place the steps of the laboratory preparation of ammonium sulfate in the correct order.
Medium
- Add ammonia into a conical flask with some indicator
- Add sulfuric acid into a burette noting the starting volume
- Add the sulfuric acid to the ammonia slowly until the indicator changes colour
- Determine the volume of sulfuric acid added and repeat the titration without indicator
- Heat the solution to partially evaporate the water
- Leave the remaining solution to dry and crystallize
28.Place the stages of the Haber process in the correct order.
Medium
- Hydrogen and nitrogen gases are pumped into the compressor
- The gases are compressed to about 200 atmospheres
- The pressurised gases are passed over iron catalyst beds at 450 °C
- Ammonia is liquefied and removed
- Unreacted hydrogen and nitrogen are recycled
29.Which statement about the Haber process is correct?
Easy
- AThe reaction is irreversible
- BThe forward reaction is exothermic
- CThe catalyst used is platinum
- DThe process uses a low pressure
30.Which change would increase the equilibrium yield of ammonia?
Medium
- AIncreasing the temperature
- BDecreasing the pressure
- CIncreasing the pressure
- DAdding more catalyst
31.Match each raw material used in the Haber process to its source.
Medium
- Nitrogen
- Hydrogen
- Iron catalyst
- Obtained from the air by fractional distillation
- Derived from methane by steam reforming
- Mined metal used in the reactor beds
32.Which of the following statements about the effect of a catalyst in the Haber process are correct? (select all that apply)
Medium
- AIt increases the rate at which equilibrium is reached
- BIt increases the equilibrium yield of ammonia
- CIt speeds up the forward and reverse reactions by the same amount
- DIt provides an alternative pathway with lower activation energy
- EIt shifts the position of equilibrium to the right
33.Phosphate rock is treated with sulfuric acid. What is the mixture of products formed?
Medium
- ACalcium phosphate and calcium sulfate
- BPhosphoric acid and calcium nitrate
- CCalcium phosphate only
- DAmmonium phosphate and calcium sulfate
34.Which of the following are disadvantages of making ammonium nitrate on an industrial scale compared with a laboratory preparation? (select all that apply)
Medium
- AThe equipment needed is hugely expensive and complex
- BThe process is very fast
- CHigh concentrations of reactants release a lot of heat
- DLarge concentrations of fertiliser are produced
- EThe process involves several stages
35.NPK fertilisers are formulations containing appropriate ratios of nitrogen, phosphorus and potassium compounds.
Easy
True or false?
36.On a graph of ammonia yield against temperature at constant pressure, what happens to the yield as the temperature increases?
Hard
- AThe yield decreases
- BThe yield increases
- CThe yield stays the same
- DThe yield first decreases then increases
37.Which of the following correctly describes the role of nitrogen, phosphorus and potassium in plant growth?
Medium
- ANitrogen promotes healthy leaves, phosphorus promotes healthy roots, potassium promotes growth, healthy fruit and flowers
- BNitrogen promotes healthy roots, phosphorus promotes healthy leaves, potassium promotes growth, healthy fruit and flowers
- CNitrogen promotes growth, healthy fruit and flowers, phosphorus promotes healthy leaves, potassium promotes healthy roots
- DNitrogen promotes healthy leaves, phosphorus promotes growth, healthy fruit and flowers, potassium promotes healthy roots
38.A graph shows the percentage yield of ammonia from the Haber process plotted against pressure at several constant temperatures. At a constant pressure of 200 atmospheres, the curves show that the yield at 300 °C is about 60%, at 400 °C about 40% and at 500 °C about 25%. What does this show about the effect of temperature on the yield of ammonia?
Medium
- AThe yield of ammonia decreases as the temperature increases
- BThe yield of ammonia increases as the temperature increases
- CTemperature has no effect on the yield of ammonia
- DThe yield of ammonia is highest at 500 °C
39.A graph of ammonia yield against pressure shows that at 400 °C the yield rises from about 15% at 100 atm to about 40% at 300 atm and to about 55% at 500 atm. Which statement best explains this trend?
Medium
- AIncreasing the pressure shifts the equilibrium to the side with fewer gas molecules, so more ammonia is formed
- BIncreasing the pressure shifts the equilibrium to the side with more gas molecules, so more ammonia is formed
- CIncreasing the pressure has no effect on the position of equilibrium but increases the rate
- DIncreasing the pressure decomposes the ammonia back into nitrogen and hydrogen
40.A fertiliser is made by mixing ammonium nitrate, ammonium phosphate and potassium chloride. The mixture contains 20% nitrogen, 10% phosphorus and 15% potassium by mass. What is the total percentage of the three essential elements in this fertiliser?
Hard
- A45%
- B30%
- C15%
- D55%
41.Complete the sentences about the Haber process using the words provided.
Easy
The Haber process uses an ____ catalyst. The reaction is carried out at a temperature of about ____ °C and a pressure of about ____ atmospheres.
42.Complete the sentences about the raw materials for the Haber process.
Easy
Nitrogen is obtained from the ____. Hydrogen is obtained from ____.
43.Complete the sentences about the role of the three essential elements in NPK fertilisers.
Easy
Nitrogen promotes healthy ____. Potassium promotes growth, healthy fruit and ____. Phosphorus promotes healthy ____.
44.Complete the sentences about treating phosphate rock.
Medium
Phosphate rock is treated with nitric acid to produce phosphoric acid and ____. Phosphate rock treated with sulfuric acid produces a mixture known as ____.
45.Complete the sentences about the laboratory preparation of ammonium sulfate.
Medium
In the laboratory, ammonium sulfate is prepared by titrating ammonia with ____ acid. The indicator used is ____. After the reaction, the solution is heated to partially evaporate the water and then left to ____.
46.In the Haber process, the reaction is N₂ + 3H₂ ⇌ 2NH₃. How many molecules of ammonia are produced when 1 molecule of nitrogen reacts completely with 3 molecules of hydrogen?
Medium
47.Calculate the total percentage of the three essential elements in a fertiliser that contains 23% nitrogen, 18% phosphorus and 21% potassium.
Medium
48.How many atoms are there in total in one molecule of ammonium nitrate, NH₄NO₃?
Easy
49.Name the catalyst used in the Haber process.
Medium
50.Name the compound produced when ammonia reacts with nitric acid.
Medium
51.Name the process used to produce hydrogen from methane for the Haber process.
Medium
52.A fertiliser contains 20% nitrogen, 15% phosphorus and 25% potassium. Calculate the total percentage of these three essential elements in the fertiliser.
Medium
53.Explain why a temperature of 450 °C is used in the Haber process rather than a lower temperature.
Medium
54.Explain why a high pressure is used in the Haber process.
Medium
55.Explain how the industrial production of ammonium sulfate differs from its laboratory preparation in terms of equipment and separation of product.
Hard
56.Explain why a high pressure of about 200 atmospheres is used in the Haber process.
Hard
57.Explain why the Haber process is carried out at a pressure of about 200 atmospheres rather than a much higher pressure, even though a higher pressure would give a greater yield of ammonia.
Hard
58.In the Haber process, the reaction is N₂ + 3H₂ ⇌ 2NH₃. How many molecules of ammonia are produced when 1 molecule of nitrogen reacts completely with 3 molecules of hydrogen?
Medium
- A2 molecules
- B3 molecules
- C1 molecules
- D4 molecules
59.Calculate the total percentage of the three essential elements in a fertiliser that contains 23% nitrogen, 18% phosphorus and 21% potassium.
Medium
- A74 %
- B62 %
- C68 %
- D56 %
60.How many atoms are there in total in one molecule of ammonium nitrate, NH₄NO₃?
Easy
- A10 atoms
- B9 atoms
- C8 atoms
- D11 atoms
61.A fertiliser contains 20% nitrogen, 15% phosphorus and 25% potassium. Calculate the total percentage of these three essential elements in the fertiliser.
Medium
- A60 %
- B66 %
- C54 %
- D72 %

Cartes mémo

What is the word equation for the Haber process?

Nitrogen + hydrogen ⇌ ammonia

What is the symbol equation for the Haber process?

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

What are the two raw materials used in the Haber process?

Nitrogen and hydrogen.

Where does the nitrogen for the Haber process come from?

From the air (air is about 78% nitrogen, removed by fractional distillation).

Where does the hydrogen for the Haber process come from?

From natural gas (methane), in a process called steam reforming.

What catalyst is used in the Haber process?

Iron.

What temperature is used in the Haber process?

About 450°C.

What pressure is used in the Haber process?

About 200 atmospheres.

Why is a temperature of 450°C used rather than a lower temperature?

A lower temperature would give a higher yield, but the rate of reaction would be very slow. 450°C is a compromise between yield and rate.

Why is a temperature of 450°C used rather than a higher temperature?

A higher temperature favours the reverse (endothermic) reaction, so it would give a lower yield of ammonia.

Why is a pressure of 200 atmospheres used rather than a higher pressure?

A higher pressure would give a greater yield, but high pressures are dangerous and need very expensive equipment. 200 atm is a compromise between yield, safety and cost.

Why does a high pressure increase the yield of ammonia in the Haber process?

There are fewer molecules of gas on the product side (2) than the reactant side (4), so the equilibrium shifts to the right to reduce the pressure.

What is meant by a dynamic equilibrium?

The rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of reactants and products remain constant.

What is meant by a closed system?

A system where no reactants or products can escape.

Why is an iron catalyst used in the Haber process?

It speeds up the rate of reaction so equilibrium is reached faster, without changing the position of equilibrium or the yield.

Does adding a catalyst change the position of equilibrium in the Haber process?

No. It increases the rate of both the forward and reverse reactions equally, so the equilibrium concentrations are the same as without it.

What happens to the ammonia produced in the Haber process after it leaves the reactor?

It is cooled and liquefies, then is removed to pressurised storage vessels.

What happens to the unreacted nitrogen and hydrogen in the Haber process?

They are recycled back into the system.

Which three elements do NPK fertilisers contain?

Nitrogen, phosphorus and potassium.

What does nitrogen promote in plants?

Healthy leaves.

What does phosphorus promote in plants?

Healthy roots.

What does potassium promote in plants?

Growth, and healthy fruit and flowers.

Why must fertilisers be water soluble?

So the nutrients they provide can be absorbed and transported by the plant.

Which ions in fertilisers are the source of soluble nitrogen?

Ammonium ions (NH₄⁺) and nitrate ions (NO₃⁻).

Which ion in fertilisers is the source of soluble phosphorus?

Phosphate ions (PO₄³⁻).

What is the word equation for the production of ammonium nitrate?

Ammonia + nitric acid → ammonium nitrate

What is the symbol equation for the production of ammonium nitrate?

NH₃(aq) + HNO₃(aq) → NH₄NO₃(aq)

What is the word equation for making ammonium sulfate in the laboratory?

Ammonia + sulfuric acid → ammonium sulfate

Which acid is reacted with ammonia to make ammonium sulfate?

Sulfuric acid.

Which acid is reacted with ammonia to make ammonium nitrate?

Nitric acid.

Why is phosphate rock treated before it is used as a fertiliser?

Phosphate rock is insoluble in water, so it is reacted with acids to produce soluble salts that plants can absorb.

What salts are produced when phosphate rock is treated with nitric acid?

Phosphoric acid and calcium nitrate.

What is produced when phosphate rock is treated with sulfuric acid?

A mixture of calcium phosphate and calcium sulfate, known as single superphosphate.

What is produced when phosphate rock is treated with phosphoric acid?

Calcium phosphate, known as triple superphosphate.

How is ammonia used to make nitric acid?

Ammonia undergoes oxidation to produce nitric acid.

Why are the conditions used in the Haber process called compromise conditions?

They balance product yield, reaction rate and production cost rather than giving the maximum possible yield.

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