Ammonia

IP Chemistry (6092), Sec 3

By the end of this topic, you can answer

1

Where do the nitrogen and hydrogen for making ammonia come from?

2

A lower temperature gives more ammonia. Why do factories use 450 °C?

3

What happens to the nitrogen and hydrogen that do not react?

4

What makes a reaction reversible, and why does it never give 100% of the product?

5

Why does blue copper(II) sulfate turn white when heated, and blue again with water?

6

Why does heated ammonium chloride form again at the top of the tube?

Starter

PredictAir is about 78% nitrogen, yet plants cannot use nitrogen gas directly. Farmers buy fertilisers made from ammonia instead.

If nitrogen is everywhere, why is it so hard to turn into ammonia?

NoteNitrogen is very unreactive, and the reaction that makes ammonia also runs backwards. This topic shows how industry gets a useful amount of ammonia anyway.

1

The Haber Process

1.1Making ammonia

Ammonia is made industrially by the Haber process. About 80% of it is used to make fertilisers.

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

RecallA nitrogen molecule has a strong triple bond, N≡N, so nitrogen is very unreactive at room temperature. The reaction needs a high pressure, a relatively high temperature and a catalyst.

1

The Haber Process

1.1A reversible reaction

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

The forward reaction makes ammonia. The backward reaction breaks it down into nitrogen and hydrogen.
Both happen at the same time, so the reaction never turns all the nitrogen and hydrogen into ammonia.

AnalogyA reversible reaction is like a busy two-way bridge: cars cross in both directions at once, so one side never fills up completely.

1

The Haber Process

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: expensive equipment and a lot of electricity.	200 to 250 atm
temperature	A lower temperature gives a higher yield, because less ammonia decomposes. But the reaction would be too slow.	450 °C
catalyst	The reaction is still slow, so a catalyst speeds it up. It does not change the yield.	finely divided iron

AnalogyChoosing the conditions is like choosing a delivery option: the fastest one costs the most, and the cheapest one takes too long, so most people pick something in between. 450 °C and 200 to 250 atm are that "in between".

1

The Haber Process

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

AdvantageA higher yield of ammonia or a faster reaction.

DisadvantageKeeping the higher pressure costs more, because more energy is needed.

Exam tipFor a compromise, give both sides: the effect on the yield, and the effect on the rate or the cost.

1

The Haber Process

Quick checkWhich statement about the iron catalyst in the Haber process is correct?

AIt increases the yield of ammonia.

BIt speeds up the reaction.

CIt stops ammonia breaking down.

DIt is used up in the reaction.

NoteB: a catalyst speeds up the reaction but does not change the yield, and it is not used up. The backward reaction still happens.

1

The Haber Process

1.3Inside the plant

Nitrogen and hydrogen are mixed in the ratio 1 : 3 by volume, then 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

The Haber Process

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.

1

The Haber Process

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

Step 1At 150 °C the yield would be higher, but the reaction would be too slow.

Step 2At 650 °C the reaction is faster, but more ammonia decomposes, so the yield is lower.

Step 3450 °C is a compromise: a reasonable yield at a fast enough rate.

1

The Haber Process

Quick checkWhich change increases the yield of ammonia in the Haber process?

Aincreasing the pressure

Bincreasing the temperature

Cadding more iron catalyst

Dremoving the cooler

NoteA: a higher pressure gives a higher yield. A higher temperature lowers the yield, a catalyst does not change the yield, and the cooler is needed to remove the ammonia.

2

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.	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. The forward and backward reactions keep taking place at the same rate, so the yield is always less than 100%.

2

Reversible Reactions

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.

PredictWhat will happen if you pour the water back onto the white powder?

2

Reversible Reactions

2.2Heating hydrated copper(II) sulfate

HeatWater is driven out of blue hydrated copper(II) sulfate, leaving white anhydrous copper(II) sulfate.

Add waterThe white powder turns blue again.

CuSO₄•5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

2

Reversible Reactions

● ammonia ● hydrogen chloride (particles not to scale)

2.3Heating ammonium chloride

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

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

2

Reversible Reactions

Worked exampleHeated ammonium chloride forms ammonia and a gas X, which recombine on cooling. (a) Identify X. (b) Write the equation. (c) Ammonia reaches the end of the tube first. What can you deduce? (d) What does X do to damp blue litmus paper?

(a)Ammonium chloride is NH₄Cl. Take away NH₃ and HCl is left, so X is hydrogen chloride.

(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)It turns damp blue litmus paper red.

2

Reversible Reactions

2.4Yield from a reversible reaction

Worked example25 cm³ of nitrogen and 75 cm³ of hydrogen are passed to and fro over heated iron wool until there is no further change. Suggest the volume of ammonia produced.

Step 1N₂ + 3H₂ ⇌ 2NH₃: from the mole ratio, at most 50 cm³ of ammonia.

Step 2The reaction is reversible, so it does not go to completion: less than 50 cm³.

2

Reversible Reactions

2.4Why the yield stops short

AnalogyFilling a bucket that has a hole in it: water pours in and leaks out at the same time. The fuller the bucket, the faster it leaks, so the level stops rising below the top. A reversible reaction stops short of 100% in the same way: the more product there is, the faster the backward reaction.

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

2

Reversible Reactions

2.5Other industrial processes

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

Worked exampleMaking nitric acid starts with a reversible reaction: ammonia and oxygen 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.

2

Reversible Reactions

Quick checkWhich reaction is irreversible?

Aheating hydrated copper(II) sulfate

Bheating ammonium chloride

Cburning charcoal

Dmaking ammonia by the Haber process

NoteC: the ash cannot be turned back into charcoal. The other three can go both forward and backward.

Exit ticket

1Write the equation for the Haber process with state symbols.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)[1]

2State the source of the hydrogen used.

The cracking of crude oil fractions.[1]

3Why are unreacted nitrogen and hydrogen recycled?

So that they are not wasted: they pass over the catalyst again to make more ammonia.[1]

4What is meant by a reversible reaction?

A reaction that can go both forward and backward.[1]