Topic Page Numbers
Making Ammonia 2
Reversible reactions and the Manufacture of Ammonia 3
The Haber Process 4
Effect of Pressure and Temperature on Yield 5 - 6
Making Fertilisers and Advantages/Disadvantages 9 - 11
Testing for Ammonium Salts 14 - 15
By the end of the ammonia topic you should be able to:
Know how ammonia is made and write out the reversible reaction showing how it is produced.
Interpret data about the conditions required for the haber process
Know how ammonium fertilisers are made from ammonia and the advantages and disadvantages of using them
Describe the test used to identify an ammonium salt
Making Ammonia
Ammonia
· Ammonia is a colourless gas.
· It has the formula: NH3
· It is used to make fertilisers.
Making Ammonia
In industry, ammonia is made from nitrogen and hydrogen.
nitrogen + hydrogen ammonia
N2 + 3H2 2NH3
The nitrogen comes from air (atmospheric nitrogen)
The hydrogen is made from the reaction between methane and steam
This is a reversible reaction.
Reversible Reactions
Reversible reactions are reactions that can go either way
They can go left to right (reactants à products) OR
right to left (products à reactants)
A reversible reaction:
reactants products
We say that there is an equilibrium. That’s what the shows.
The Industrial Manufacture of Ammonia
Reversible reactions in industry
· Many important reactions in industry are reversible. This can be a problem for Chemists.
· Products can turn back into reactants so therefore less product is produced.
· This reduces the yield
· The yield of a chemical reaction tells us how much actual product we make.
· Chemists must choose conditions that will give a good yield of ammonia as quickly and economically as possible
How does the yield of ammonia depend on conditions?
The Haber Process is used in industry to produce ammonia:
nitrogen + hydrogen ammonia
N2 + 3H2 2NH3
· The raw materials are nitrogen gas and hydrogen gas
· These 2 gases are purified then mixed in a 1 to 3 ratio and passed over a hot iron catalyst.
· About 10 to 20% of the gases are converted to ammonia.
· The remaining 80 to 90 % of unreacted hydrogen and nitrogen are recycled back into the catalyst chamber.
· As soon as it forms, ammonia begins reacting back into hydrogen and nitrogen.
· Although the yield is not high, it is sufficient.
· A high pressure, moderate temperature and catalyst ensure a rapid conversion.
· The ammonia gas can be removed from the process by cooling it (under pressure) and thereby liquefying it.
The optimum conditions for the Haber process are:
· pressure of 200 atmospheres
· temperature of 450 oC
· iron catalyst
This ensures that a good yield is produced in a fairly short amount of time – it is the most economical.
A change in the pressure or temperature effects the yield of ammonia:
Pressure:
· If we increased the pressure, we would increase the yield
· This is not done in industry though, because it is too dangerous and expensive to use a pressure more than 200 atm
Temperature:
· If we increased the temperature then this would cause the yield to actually decrease!
· A decrease in temperature would give a higher yield
· This is not done in industry though, because although the yield would be greater, the reaction would be too slow (remember – decreasing temperature lowers the reaction rate)
You must be able to interpret data (e.g. a graph) that shows how the yield of ammonia depends on the conditions used.
e.g. This graph shows the effect of different pressures and temperatures on the yield of ammonia:
1) By varying the pressure & temperature we can increase the yield of ammonia.
2) Each line represents a fixed temperature (i.e. 350, 400, 450, 500, 550 deg C).
3) On the “x” axis is the pressure, and on the “y” axis” is the yield of ammonia.
4) Notice how the line rises from left to right.
5) This means that as pressure rises, so does the yield of ammonia.
6) Now focus on one pressure (e.g. 200 atmospheres).
7) As you change the temperature from 350 C to 550 C the yield decreases.
8) This means more ammonia is created at lower temperatures.
9) Hence, for maximum yield, the ideal conditions are high pressure & low temperature.
BUT, this creates 2 problems.
10) Firstly at low temperatures the reaction is too slow – so a compromise temperature is chosen (e.g. around 450 oC).
11) Secondly, high pressures require thicker pipes & stronger pumps to take the pressure.
12) So, again, a compromise pressure is chosen – usually around 200 atmospheres.
This gives roughly a 15% conversion but the unconverted nitrogen and hydrogen are recycled back into the reactor – so nothing is lost.
Try this typical GCSE question…
Answer these questions:
Making Fertilisers
Ammonia is soluble in water. It easily dissolves to give an alkali solution (this is sometimes known as ammonium hydroxide)
Making Fertilisers Using Ammonia
This is done by neutralising ammonia solution with an acid.
Making Ammonium nitrate:
Nitric acid is reacted with ammonia to make ammonium nitrate:
ammonia + nitric acid à ammonium nitrate
NH3 + HNO3 à NH4NO3
This is “Nitram”, the most popular synthetic fertiliser in the world.
Without it millions throughout the world would be starving since it increases crop yields hugely.
Making Ammonium sulphate:
Sulphuric acid is reacted with ammonia to make ammonium sulphate:
ammonia + sulphuric acid à ammonium sulphate
2NH3 + H2SO4 à (NH4)2SO4
These are both examples of neutralisation reactions.
Evaluating Nitrogenous Fertilisers – Advantages and Disadvantages
There are both advantages and disadvantages to using nitrogenous fertilisers, for individuals, communities and the environment.
Advantages:
· Intensive farming with high yield of crops – no need to rotate fields or “slash and burn”
· Production of large amounts of food
· Many deaths from starvation and malnutrition are prevented
· Prices will be relatively cheap
Disadvantages:
· Some people don’t like to use synthetic fertilisers
· Hydrogen is used to make ammonia (which makes the fertilisers) – this comes from methane from fossil fuels
· In some instances they can get into drinking water and cause health problems (e.g. blue baby syndrome)
· Synthetic fertilisers may damage the environment – particularly aquatic ecosystems
· Eutrophication can occur and lead to the death of organisms in lakes and rivers
Eutrophication
· Nitrogen based fertilisers such as “Nitram” can sometimes get washed into streams, rivers, lakes and even drinking water.
· This can cause EUTROPHICATION in which the fertiliser promotes the excessive growth of pond weed and algae.
· When these plants die, bacteria cause them to decay.
· The bacteria multiply and use up the dissolved oxygen in the water.
· This causes fish and other animals to die, as well as water based plants and the water becomes stagnant.
Here’s another typical GCSE question…
Ammonium Salts
Ammonium salts are usually made from ammonia.
They all contain positive ammonium ions joined to negative ions, forming an ionic compound.
Ammonium Ion : NH4+
Examples are:
ammonium nitrate NH4NO3
ammonium sulphate (NH4)2SO4
ammonium chloride NH4Cl
ammonium hydroxide NH4OH
………………..and many more!
Testing for the Presence of an Ammonium Salt
You must know how to test for the presence of ammonium ions in a compound.
Ammonium salts (e.g. ammonium chloride) react with sodium hydroxide solution to produce, amongst other things, ammonia gas.
NH4Cl + NaOH à NaCl + NH3 + H2O
Ammonium chloride + sodium hydroxide à sodium chloride + ammonia + water
Ammonia gas is an alkali because it reacts with water to give ammonium hydroxide which turns red litmus blue.
NH3 + H2O à NH4OH
ammonia + water à ammonium hydroxide
· Hence, you can test for the presence of an ammonium salt by warming the “unknown” salt with dilute sodium hydroxide solution in a test tube.
· Place a piece of damp red litmus paper (or damp U.I. paper) near the mouth of the tube and note if it turns blue.
· If so, ammonia is present, and this means that the “unknown” contains ammonium ions (i.e. it is an ammonium salt).
Testing for Ammonium Salts - Experiment
· Ask your teacher if you can carry out this test on 4 “unknown” powders labelled A, B, C & D.
· Place half a spatula full in a boiling tube, add 3 or 4 cm depth of dilute sodium hydroxide solution (2M), warm gently by wafting in & out of a blue flame.
· DO NOT BOIL AND WATCH FOR SPITTING.
· DO NOT POINT THE TUBE AT ANYONE.
· Wear safety specs and don’t sniff the gas given off.
· It forms corrosive solutions in water (ammonium hydroxide, an alkali)
Results
Sample / Does red litmus turn blue? / Is ammonia given off? / Does the unknown contain ammonium ions? / Other observations or comments?
Powder A
Powder B
Powder C
Powder D
Join up the boxes to summarise this topic…
Equation for Haber Process / Blue baby syndrome
Temperature of Haber Process / Eutrophication
NAME (not formula) of fertiliser made from ammonia and nitric acid / NH3
What would happen if we didn’t have artificial fertilisers / Ammonium nitrate
What happens in rivers and lakes if too much nitrogenous fertiliser is used / NH4NO3
Formula of ammonia / N2 + 3H2 à 2NH3
Fertiliser made from ammonia and sulphuric acid / The Atmosphere
Nitrogenous fertilisers are all types of… / 200 atmospheres
An excellent fertiliser (ammonium nitrate) / Ammonium sulphate
Where nitrogen is obtained from for Haber Process / 10 to 20%
General name to a substance that is added to soil to make plants grow better / 450 degrees celcius
Pressure of Haber Process / Salts
Percentage conversion in Haber Process / Fertiliser
Catalyst used in Haber Process / Starvation
Can happen to infants if their drinking water contains too much nitrate / Iron
Relevant Internet and School Network Links
In school, try these web links and click on the white arrow in the green square to view:
http://www.yenka.com/freecontent/item.action?quick=w8
http://www.yenka.com/freecontent/item.action?quick=v3
http://www.yenka.com/freecontent/item.action?quick=128
http://www.yenka.com/freecontent/item.action?quick=12a
http://www.yenka.com/freecontent/item.action?quick=w9
http://www.yenka.com/freecontent/item.action?quick=129
http://www.yenka.com/freecontent/item.action?quick=wa#
http://www.yenka.com/freecontent/item.action?quick=12e#
School network Links:
· SUBJECTS > SCIENCE > CHEMISTRY > ABSORB CHEMISTRY FOR GCSE > REVERSIBLE REACTIONS
· SUBJECTS > SCIENCE > CHEMISTRY > ABSORB CHEMISTRY FOR GCSE > THE MANUFACTURE OF AMMONIA
· SUBJECTS > SCIENCE > CHEMISTRY > ABSORB CHEMISTRY FOR GCSE > WATER POLLUTION
· subjects > science > multimedia science school > chemistry > haber process
· SUBJECTS > SCIENCE > FOOTPRINTS > MATERIALS & THEIR PROPERTIES > HABER PROCESS
Useful website:
http://www.woodrow.org/teachers/chemistry/institutes/1992/Haber.html
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