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The Haber Process
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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?
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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.
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The Haber process
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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.
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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.
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Temperature, pressure and yield
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Changing the yield of ammonia
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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?
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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.
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Temperature, pressure and yield
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Stages of the Haber process
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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.
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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.
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Eutrophication
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Eutrophication: summary
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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.
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Evaluating artificial fertilizers
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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?
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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?
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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?
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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.
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Comparing nitrogen fixation methods
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Glossary
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Anagrams
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Multiple-choice quiz