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9.4 Chemical monitoring and management: 2. maximise production
| Syllabus reference (October 2002 version) | ||
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2. Chemical processes in industry require
monitoring and management to maximise production |
Students learn to:
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Students: |
Extract from Chemistry Stage 6 Syllabus (Amended October
2002). © Board of Studies, NSW.
[Edit: 24Jun08]
Prior learning:Preliminary modules
8.2 (8.2.2) 8.3 (8.3.2,
8.3.5), 8.5 (8.5.5)
and HSC modules 9.2 (9.2.1),
9.3 (9.3.2, 9.3.4).
Background: There are some chemical reactions that have played a very important part in world history and human activity, allowing the development of industries and societies. The production of ammonia is one such example.
gather and process information from secondary sources to describe the conditions under which Haber developed the industrial synthesis of ammonia and evaluate its significance at that time in world history
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Gather information about Fritz Haber from
a range of resources including school textbooks,
CD–ROMs such as Encarta or Encyclopedia
Britannica, and the Internet. You should be able to
gather information like that provided below.
Fritz Haber
Nobel e-Museum - To process thesources you find, assess their reliability by comparing the information provided. Look for consistency of information. Check for consistency with the information provided below.
The significance of Fritz Haber’s work
World context: Nitrogen compounds were essential for the production of fertilisers and explosives. Much of the fertiliser originated in Chile, a long way from the industrial centres of Europe. It was known that the atmosphere contains large quantities of diatomic nitrogen. It would be advantageous to convert this readily available gas to usable compounds for agriculture and industry, cheaply and on a large scale.
Haber’s contribution: Fritz Haber had studied under Robert Bunsen, in Germany, and was interested in the effect of heat on the chemistry of gases. In the early 1900s, Haber reacted nitrogen with hydrogen, using an iron catalyst, to form ammonia. Ammonia can be readily converted to a range of valuable products. In 1908 he had improved the reaction and in 1911 he was rewarded with a directorship at a German institute. Carl Bosch developed the chemical engineering necessary for the large-scale production of ammonia by the Haber-Bosch process.
Significance: Germany used the process to make explosives in World War I after the British cut off supplies of nitrate from Chile. This lengthened the war.
identify and describe the industrial uses of ammonia
- Ammonia is used to make solid and liquid fertilisers, explosives, nitric acid, sodium carbonate, some pharmaceuticals and household cleaners. It is also used as a refrigerant.
- To make solid fertilizer industrially, ammonia, which is a weak base, is reacted with sulfuric acid to form ammonium sulfate fertiliser and with nitric acid to form ammonium nitrate fertiliser.
identify that ammonia can be synthesised from its component gases, nitrogen and hydrogen.
- Under pressure and heat, nitrogen and hydrogen react in
the ratio of 1 volume of nitrogen to 3 volumes of hydrogen to
produce 2 volumes of ammonia.
describe that synthesis of ammonia occurs as a reversible reaction that will reach equilibrium
- The synthesis of ammonia occurs as a reversible reaction.
This means that ammonia is formed from nitrogen and hydrogen
(the forward reaction) and once some ammonia is produced,
some nitrogen and hydrogen are formed from the ammonia (the
reverse reaction). When nitrogen and hydrogen are initially
added to a reaction vessel, the reaction is slow. Equilibrium
is reached when the rate of the forward reaction is the same
as the rate of the reverse reaction.
- To ensure that sufficient ammonia is produced, conditions
need to be established that shift the equilibrium position to
the right.
explain that the use of a catalyst will lower the reaction temperature required and identify the catalyst(s) used in the Haber process
- With the use of catalyst, the activation energy for the reaction is lowered. A finely ground iron catalyst, with large surface area, is used in the Haber process. The gaseous nitrogen and hydrogen molecules are adsorbed on to the solid catalyst surface and rearrange forming the ammonia molecules. By lowering the activation energy, a catalyst enables a more rapid reaction at lower temperatures.
identify the reaction of hydrogen with nitrogen as exothermic
The forward reaction, to produce ammonia, releases 46 kJ of energy for each mol of ammonia formed.
explain why the rate of reaction is increased by higher temperatures
- As the temperature rises, the particles move more quickly and have higher kinetic energy. This increases the frequency of collisions between particles that can react and also increases the amount of energy available for the reaction. Most of the increased rate of reaction comes from more of the colliding particles exceeding the activation energy needed for the reaction to occur. The increased frequency of collisions is less important in increasing the rate of reaction. The rate of both the forward and reverse reactions is increased.
explain why the yield of product in the Haber process is reduced at higher temperatures using Le Chatelier's principle
- The forward reaction in which ammonia is formed is exothermic. Le Chatelier's principle states that if a system in equilibrium is disturbed, the system will adjust itself to minimise the disturbance. In this case, Le Chatelier's principle indicates that with high temperature providing more heat, the reverse reaction is favoured and the decomposition of ammonia occurs.
Another way to view it
If the reaction is written like that following, heat is like a product.
As temperature, and therefore the heat available increases, the equilibrium position shifts to the left and the yield of ammonia is reduced.
analyse the impact of increased pressure on the system involved in the Haber process
- In accordance with Le Chatelier's principle, increasing the pressure favours the production of ammonia because two molecules of gaseous ammonia occupy a smaller volume than the four molecules of gaseous reactants.
- High pressure also increases the reaction rate because the gas molecules are closer and at higher concentrations. However, high-pressure equipment is expensive and requires considerable energy to operate.
- To achieve an economic yield of about 30%, a pressure of 35 000 kPa (35 MPa or 345 atm) is used.
explain why the Haber process is based on a delicate balancing act involving reaction energy, reaction rate and equilibrium
- As the temperature is increased more energy is available
to exceed the reaction activation energy and thus the
reaction rate between nitrogen and hydrogen to form ammonia
increases. However increasing temperature favours
decomposition of the ammonia product. A compromise
temperature providing a satisfactory reaction rate and
satisfactory yield of ammonia is selected.
- To achieve an economic yield of about 30%, the temperature is raised to 525°C and pressure of 35 000 kPa (35 MPa or 345 atm) is used.
Increasing yield
Liquefying and removing the ammonia as it is produced also increase the yield of ammonia.
explain why monitoring of the reaction vessel used in the Haber process is crucial and discuss the monitoring required
- The raw materials must be monitored to ensure they are clean. Any carbon dioxide detected must be removed. It is often separated and diverted to use for the production of urea at a nearby fertiliser manufacturing plant. Any oxygen present could cause an explosion with the hydrogen.
- A chemical engineer or technician monitoring the reaction vessel needs to ensure that the appropriate temperature and pressure conditions are maintained, within an acceptable range, so that about 30% yield is achieved.
- The quality of the catalyst surface needs to be monitored to ensure good adsorption of the nitrogen and hydrogen gases. The system must be kept free of contaminants to ensure maximum surface of the catalyst is available for adsorption of nitrogen and hydrogen.
- Temperature needs to be monitored, as too high a temperature can permanently damage the catalyst.
Haber
process for ammonia synthesis by Jayant M Modak, Department of Chemical
Engineering
Indian Institute of Science, Bangalore 560012, India, printed in Resonance,
2002
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