The Haber Process · 3 of 30 © Boardworks Ltd 2012 What is ammonia? It is made industrially by reacting nitrogen with hydrogen in the Haber process.The Haber process is a

1 of 30 © Boardworks Ltd 2012

The Haber Process



What is ammonia?

It is made industrially by reacting

nitrogen with hydrogen in the Haber

process. The Haber process is a

reversible reaction, so it never ends.

hydrogennitrogen + ammonia

N2 (g) 3H2 (g) 2NH3 (g)+

Ammonia is an important compound

in the manufacture of fertilizer and

other chemicals such as cleaning

fluids and floor waxes.

Why is this a problem for companies

making ammonia?


Feedstock

The hydrogen gas needed for

the Haber process comes from

methane (from natural gas).

CH4(g) H2O(g) CO(g)+ 3H2(g)+

The nitrogen gas needed for the Haber

process comes from the air.

The air is 78% nitrogen. Nitrogen can be

separated out by fractional distillation.

It is reacted with steam to

form hydrogen.


The Haber process


What is yield?

The amount of product made in a reaction is called the

yield and is usually expressed as a percentage.a

mm

on

ia y

ield

(%

)

pressure (atm)

The yield of ammonia produced by the Haber process

depends on the temperature and pressure of the reaction.


What is the Haber compromise?

In practice, these conditions

are not used. Why?

The highest yield of ammonia

is theoretically produced by

using a low temperature and

a high pressure.

A compromise is used to make an acceptable yield in a

reasonable timeframe while keeping costs down.

Lowering the temperature slows down the rate of reaction.

This means it takes longer for ammonia to be produced.

Increasing the pressure means stronger, more expensive

equipment is needed. This increases the cost of producing

the ammonia.


Temperature, pressure and yield


Changing the yield of ammonia


The Haber compromise

To produce a high yield of ammonia, but with a fast rate

of reaction and without the need for overly expensive

equipment, the Haber process is carried out at 450°C

and 200 atmospheres.

The most important factor in

deciding what conditions to use is

therefore not yield, but total cost.

raw materials

equipment

energy

wages.

What costs are involved

in the industrial

production of ammonia?


Maximizing productivity

What else can be done to maximize productivity in the

manufacture of ammonia?

An iron catalyst is used to increase the rate of

reaction. It speeds up both the forward and backward

reactions, so the position of equilibrium is not affected.

The ammonia is cooled, liquefied and then removed

as it is produced. This causes the equilibrium to shift to

the right to produce more ammonia.

Unreacted nitrogen and hydrogen are recycled and

given another chance to react.

Gases flow through the reactor too quickly for

equilibrium to be reached.


Temperature, pressure and yield


Stages of the Haber process



Environmental impact

Large industrial plants producing ammonia and its

feedstocks consume large amounts of power. This is

normally obtained from burning fossil fuels, which releases

carbon dioxide and contributes to global warming.

A large chemical

plant can also affect

the environment in

other ways.

Local residents may

experience noise, light

and sound pollution

from the plant.


Plants absorb nutrients through

their roots. Fertilizers must be

dissolved in water before they

can be absorbed by plants.

How do fertilizers work?

Fertilizers increase crop yield by

replacing the nutrients used up by

previous crops on an area of land.

This allows crops to be grown on

the same land year after year.

One of the main uses of ammonia is the production

of fertilizers.

Adding fertilizer to water systems

can cause environmental problems.


Eutrophication


Eutrophication: summary


Reasons for using fertilizers

Artificial nitrogen-based

fertilizers are important, as

they increase yield and

allow land to be farmed

more intensively.

It is estimated that around one-third of the world’s population is sustained by the use of artificial fertilizers.

This increases the amount of crops that can be grown.

Using fertilizers can also

allow crops to be grown

on poor quality soils.


Evaluating artificial fertilizers



Nitrates are important because they are a form of nitrogen that

plants can absorb. Nitrogen is used by plants to make protein

and is passed from plants to animals along food chains.

Some nitrogen compounds form during lightning

strikes and are washed into the soil by rain water.

Decomposers release ammonium

compounds from waste (such as

urea in urine) and dead matter.

Nitrifying bacteria then convert the

ammonium compounds into nitrates.

What processes add nitrates to the soil?

How are nitrates produced?


This rugged landscape is

in Iceland. Deforestation by the

original settlers and high levels

of volcanic activity have left

much of the country as bare

lava or sand – an environment

in which few plants grow.

In the 1960s, Iceland began to

manage its soil. It decided to

drop millions of lupin seeds

from the air.

Why might lupins survive where nothing else grows?

Can plants add nitrogen to the soil?


Most plants need nitrates from the soil because the nitrogen

in air is too unreactive. These plants rely on the presence of

nitrifying bacteria in the soil or artificial fertilizers.

Nitrogen-fixing bacteria living

in the root nodules of legumes

convert nitrogen gas into

nitrates, improving the fertility

of poor-quality soils.

By contrast, lupins and other

legumes, such as clover

and peas, are self-sufficient

in nitrates.

What are legumes?


Enzymes

Nitrogen-fixing bacteria use nitrogenase enzymes to

produce nitrates from the nitrogen in the air.

Nitrates could be produced without the high energy

requirements of the Haber process.

Why would scientists be interested in producing new

catalysts that mimic these natural enzymes?

The starting materials would come from the

air and not from non-renewable fossil fuels.

N2 8H+ 2NH3+ H2++ 8e–

The reaction goes to completion.


Comparing nitrogen fixation methods



Glossary


Anagrams


Multiple-choice quiz

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The Haber Process · 3 of 30 © Boardworks Ltd 2012 What is ammonia? It is made industrially by reacting nitrogen with hydrogen in the Haber process.The Haber process is a