Learners' AlleyDesigned by Khow Willard

Ammonia

The Haber Process

1.1Making ammonia

Ammonia is made industrially by the Haber process. About 80% of it is used to make fertilisers. It is also used to make plastics, explosives, dyes and cleaning products.

Raw material	Obtained from
nitrogen, N₂	the fractional distillation of liquid air
hydrogen, H₂	the cracking of crude oil fractions

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Forward reaction

N₂(g) + 3H₂(g) → 2NH₃(g)

Backward reaction

2NH₃(g) → N₂(g) + 3H₂(g)

The reaction is reversible, so some of the ammonia made breaks down again into nitrogen and hydrogen.

NoteAt room temperature nitrogen is very unreactive. Recall from Chemical Bonding that a nitrogen molecule has a strong triple bond, N≡N. So the forward reaction needs a high pressure, a relatively high temperature and a catalyst.

1.2Choosing the conditions

Condition	Effect	Used
pressure	A higher pressure gives a higher yield and a faster reaction. But keeping a high pressure is costly: it needs expensive equipment and a large amount of electricity.	200 to 250 atm (a compromise)
temperature	A lower temperature gives a higher yield, because less ammonia decomposes. But the reaction would be too slow.	450 °C (a compromise)
catalyst	The reaction is still slow under these conditions, so a catalyst speeds it up. It does not change the yield.	finely divided iron

Exam tipWhen asked why a condition is a compromise, give both sides: what a change would do to the yield, and what it would do to the rate or the cost.

Worked exampleSome plants run the Haber process at 300 atm instead of 200 atm. Suggest one advantage and one disadvantage of the higher pressure.

Advantage: a higher yield of ammonia or a faster reaction.Disadvantage: keeping the higher pressure costs more, because more energy is needed.

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The Haber Process (continued)

1.3Inside the plant

Nitrogen and hydrogen are mixed in the ratio 1 : 3 by volume.
The mixture is compressed to 200 to 250 atm.
The gases flow over the iron catalyst at 450 °C. Only about 15% of the mixture leaving the converter is ammonia.
The mixture is cooled. Ammonia condenses into a liquid and is pumped into tanks.
The unreacted nitrogen and hydrogen are recycled back to the converter, so none is wasted.

1.4Reading yield data

The yield increases as the pressure increases.
The yield decreases as the temperature increases.
The conditions used are a compromise between a good yield, a fast enough rate and a reasonable cost.

Worked exampleIn the Haber process, a temperature of 450 °C is used instead of 150 °C or 650 °C. Explain why.

At 150 °C the yield would be higher, but the reaction would be too slow.
At 650 °C the reaction is faster, but more ammonia decomposes, so the yield is lower.
450 °C is a compromise: a reasonable yield at a fast enough rate.

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Learners' AlleyDesigned by Khow Willard

Reversible Reactions

2.1Reversible and irreversible reactions

DefinitionA reversible reaction can go both forward and backward. Its equation uses the sign ⇌.

	Reversible reaction	Irreversible reaction
What happens	The reactants change into the products, and the products can change back into the reactants.	The reactants change into the products, but the products cannot change back.
Examples	the Haber process, heating hydrated copper(II) sulfate, heating ammonium chloride	burning charcoal: the ash cannot be turned back into charcoal

NoteA reversible reaction does not go to completion. Even under the best conditions, the forward and backward reactions keep taking place at the same rate, so the yield is always less than 100%.

2.2Heating hydrated copper(II) sulfate

When heated, blue hydrated copper(II) sulfate gives off water and turns into white anhydrous copper(II) sulfate. The water is collected in test tube B.
When the water is added back, the white powder turns blue again.

CuSO₄•5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

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Reversible Reactions (continued)

2.3Heating ammonium chloride

When heated, solid ammonium chloride breaks down into ammonia and hydrogen chloride.
Where the tube is cooler, the two gases combine to form ammonium chloride again.
Ammonia is lighter, so it diffuses faster and turns damp red litmus paper blue before the hydrogen chloride arrives.

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

Worked exampleWhen solid ammonium chloride is heated, it forms ammonia and a gas X. On cooling, X combines with ammonia to form ammonium chloride again. (a) Identify X. (b) Write the equation with state symbols. (c) Ammonia reaches the end of the test tube first. What can you deduce? (d) What is the effect of X on damp blue litmus paper?

(a) X is hydrogen chloride, HCl.(b) NH₄Cl(s) ⇌ NH₃(g) + HCl(g)(c) Ammonia has a lower relative molecular mass (17) than hydrogen chloride (36.5), so it diffuses faster.(d) Hydrogen chloride turns damp blue litmus paper red.

2.4Yield from a reversible reaction

Worked example25 cm³ of nitrogen and 75 cm³ of hydrogen are passed forward and backward over heated iron wool until no further reaction is observed. Suggest the volume of ammonia produced. Explain your answer.

N₂ + 3H₂ ⇌ 2NH₃, so from the mole ratio, 25 cm³ of nitrogen and 75 cm³ of hydrogen could give at most 50 cm³ of ammonia.
The reaction is reversible, so it does not go to completion.
The volume of ammonia is less than 50 cm³.

NoteAt the same temperature and pressure, volumes of gases react in the same ratio as their moles. The iron wool is a catalyst.

2.5Other industrial processes

Worked exampleThe first stage of making nitric acid is a reversible reaction in which ammonia reacts with oxygen to form nitrogen monoxide, NO, and water. (a) Write the balanced equation. (b) The yield of NO is higher at lower temperatures, yet 600 °C is used. What would happen below 600 °C?

(a) 4NH₃ + 5O₂ ⇌ 4NO + 6H₂O(b) The yield of NO would be higher, but the reaction would be too slow.

Exam tipFor data on another industrial process, such as the Contact process, first describe the trend: how the yield changes as the pressure or the temperature increases, quoting values. Then weigh the yield against the rate and the cost, as for the Haber process.

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