The Haber Process

Question 1

What are the raw materials used in the Haber process?

Answer 1

nitrogen - obtained from the air (which is 78% nitrogen) by fractional distillation, or by burning methane to remove the oxygen

hydrogen - obtained from methane reacting with steam to form carbon dioxide and hydrogen using a nickel catalyst

Question 2

How can nitrogen be obtained?

Answer 2

from the air (which is 78% nitrogen) - liquified air goes through fractional distillation

by burning methane to remove the oxygen

Question 3

How can hydrogen be obtained?

Answer 3

from methane reacting with steam to form carbon monoxide and hydrogen using a nickel catalyst

cracking hydrocarbons

Question 4

What type of reaction is the Haber Process?

Answer 4

a reversible reaction

Question 5

What is the product of the Haber Process?

Answer 5

ammonia (NH₃)

Question 6

What is the equation for the Haber Process?

Answer 6

nitrogen + hydrogen ⇌ ammonia

N_{2 (g)} + 3H_{2 (g)} ⇌ 2NH_{3 (g)}

Question 7

What happens to the unreacted nitrogen and hydrogen in the reaction?

Answer 7

they are recylced through the reactor so nothing is wasted

Question 8

What happens the ammonia in the reaction?

Answer 8

the reaction mixture is cooled and the ammonia condenses as a liquid and is removed

Question 9

What is the approximate percentage yield of the Haber Process?

Answer 9

abouyt 15%

Question 10

What are the conditions of the Haber Process?

Answer 10

temperature: 450^oC
pressure: 200 atmospheres
catalyst: iron

Question 11

What are the reasons for the proportions of nitrogen and hydrogen?

Answer 11

an excess of either nitrogen or hydrogen would clutter the reaction vessel with molecules which wouldn’t have anything to react with

Question 12

Explain the conditions: temperature

Answer 12

N_{2 (g)} + 3H_{2​ (g)} ⇌ 2NH_{3 (g)}

The exothermic reaction (foward reaction) is favoured by a low temperature because the, if you lower the temperature, the p.o.e. will move to counteract this (move to the right) to produce more heat (Le Chatelier’s Principle)

However:

If the temperature is too low, the rate of reaction becomes too slow; it is not economical

A compromise is chosen:

450°C is a compromise temperature producing a reasonably high proportion of ammonia reasonably quickly

Question 13

Explain the conditions: pressure

Answer 13

N_{2 (g)} + 3H_{2​ (g)} ⇌ 2NH_{3 (g)}

There are 4 moles of reactants to 2 moles of product

An increased pressure favours the foward reaction because, if you increase the pressure, the p.o.e. will move to counteract this (move to the right) to lower the pressure (Le Chatelier’s Principle)

High pressure also speeds up the reactions as a higher pressure pushes the same amount of gas particles into a smaller volume ensuring that there are more frequent, successful collisions in the same amount of time

However:

Generating high pressures and building plants that can withstand them are expensive

A compromise is chosen

200 atmopsheres gives the best percentage yield of ammonia at a reasonable expense

Question 14

Explain the conditions: catalyst

Answer 14

an iron catalsyt speeds up the rate of reaction by lowering the activation energy, but has no effect on the proportion of ammonia in the equilibrium mixture

the catalyst lowers operating temperatures

if the catalyst wasn’t used, the reaction would be so slow that virtually no ammonia would be produced

Question 15

How is ammonia extracted?

Answer 15

ammonia is separated as the reaction mixture is pumped to the condenser where it cools

ammonia has a higher boiling point than nitrogen and hydrogen so it condenses first and is collected as a liquid

the gaseous nitrogen and hydrogen are recycled and pumped back into the reaction chamber

Question 16

What are the uses of ammonia?

Answer 16

making fertilisers

making nitric acid

making nylon

Question 17

How is ammonia used to make fertlisers?

Answer 17

reacting ammonia with nitric acid to get ammonium nitrate:

NH_{3 (g)} + HNO_{3 (aq)} → NH₄NO_{3 (s)}

Question 18

Why is ammonium nitrate (NH₄NO₃) an especially good fertiliser?

Answer 18

it has nitrogen from two sources, the ammonia and the nitric acid

plants need nitrogen to make proteins

this is a much more effective fertiliser than organic alternatives so it helps farmers to produe crops from land taht otherwise wouldn;t have been fertile enough

Question 19

What is the main use of ammonia?

Answer 19

to make fertilisers

most of the ammonia used to make nitric acid eventually ends up in fertilisers as well

Question 20

What do plants convert the nitrogen in fertilisers into?

Answer 20

proteins (amino acids)

Question 21

Use the information form the graph, together with your own knowledge of the process, to explain why many industrial ammonia plants operate at 200 atmospheres and 450^oC

Answer 21

high pressure increases the yield but is also expensive

low temperature increases the yield but at low temperatures the rate of reaction is slow

a compromise is chosen: optimum conditions to balance percentage yield of ammonia and rate of reaction

Question 22

State which conditions of temperature and pressure would give the highest percentage yield of ammonia at equilibrium

Explain why

Answer 22

lower temperatures favour the foward reaction because it is exothermic

if the temperature is increased, the yield of product would decrease

higher pressures favour the foward reaction because their are four reactant moles but only two product moles