C3- making ammonia Flashcards
What is ammonia?
Ammonia, NH3, is a compound of nitrogen and hydrogen. It is a colourless gas with a choking smell, and a weak alkali that is very soluble in water.
What is ammonia used for?
Ammonia is used to make fertilisers (as a source of nitrogen for plants), explosives, dyes, household cleaners and nylon. It is also the most important raw material in the manufacture of nitric acid.
How is ammonia manufactured?
Ammonia is manufactured by combining nitrogen and hydrogen in an important industrial process called the Haber process.
How is hydrogen obtained for ammonia manufacture?
Hydrogen is obtained by reacting natural gas (mostly methane) with steam, or by cracking oil fractions.
How is nitrogen obtained for ammonia manufacture?
Nitrogen is obtained from the air. Air is 78 per cent nitrogen; nearly all the rest is oxygen. When hydrogen is burned in air, the oxygen combines with the hydrogen - leaving nitrogen behind.
Show the equation of the reaction between hydrogen and nitrogen?
The reaction between nitrogen and hydrogen is reversible:
nitrogen + hydrogen (double head arrow) ammonia
N2(g) + 3H2(g) (double head arrow) 2NH3(g)
In the Haber process, what conditions do hydrogen and nitrogen react under?
a high temperature - about 450°C
a high pressure - about 200 atmospheres (200 times normal pressure)
An iron catalyst is used to increase the rate of reaction.
What is the first step in the Haber process?
Having obtained the hydrogen and nitrogen gases (from natural gas and the air respectively), they are pumped into the compressor through pipes.
What is the second step in the Haber process?
The gases are pressurised to about 200 atmospheres of pressure inside the compressor.
What is the third step in the Haber process?
The pressurised gases are pumped into a tank containing beds of iron catalyst at about 450°C. In these conditions, some of the hydrogen and nitrogen will react to form ammonia.
What is the fourth step in the Haber process?
The unreacted nitrogen and hydrogen, together with the ammonia, pass into a cooling tank. The cooling tank liquefies the ammonia, which can be removed into pressurised storage vessels.
What is the fifth step in the Haber process?
The unreacted hydrogen and nitrogen gases are recycled by being fed back through pipes to pass through the hot iron catalyst beds again.
What is a closed system?
A closed system is a system in which no reactants are added and no products are removed.
What does equilibrium mean?
rate of the forward reaction is the same as the backward reaction and the concentrations remain the same.
What happens when a reversible reaction happens in a closed system?
When a reversible reaction happens in a closed system, equilibrium is reached in which the rate of the forward reaction is the same as the backward reaction.
For example, the production of ammonia is a reversible reaction:So while nitrogen and hydrogen continually combine to form ammonia, ammonia is continually breaking up to form nitrogen and hydrogen:
What does the percentage yield of ammonia depend on?
The percentage yield of ammonia at equilibrium depends on the balance between the forward and backward reactions.
What happens if you increase the pressure in a reversible reaction in a closed system?
If the pressure is increased during reactions involving gases, then the reaction that produces the least number of molecules of gas is favoured.
What happens if the temperature increases in a reversible reaction in a closed system?
If the temperature is increased, the yield from the endothermic reaction increases (and the yield from the exothermic reaction decreases).
What happens if the temperature decreases in a reversible reaction in a closed system?
If the temperature is decreased, the yield from the endothermic reaction decreases (and the yield from the exothermic reaction increases).
How do you change pressure to increase the equilibrium yield of ammonia?
There are fewer molecules of gas on the right-hand side of the chemical equation than there are on the left hand side:
N2(g) + 3H2(g) (double headed arrow) 2NH3(g)
If the pressure is increased, the reaction that produces the least number of molecules of gas is favoured. This means that the equilibrium yield of ammonia is increased if the pressure is increased.
What is the issue with increasing pressure in the Haber process?
achieving high pressures requires a lot of energy. It also requires stronger pipes and tanks to withstand that pressure. This is costly for companies.
What is the compromise which is made in the Haber process in terms of pressure?
Therefore, a compromise must be made between optimising the amount of product formed, and the cost remaining economically viable. The pressure used is around 200 atmospheres.
How do you change the temperature to increase the equilibrium yield of ammonia?
In the Haber process, the forward reaction is exothermic and the backward reaction is endothermic. If the temperature is decreased, the yield from the exothermic direction is increased. (ammonia yield increases)
What is the issue with decreasing temperature?
However, by decreasing the temperature the molecules move more slowly and collide less frequently. This lowers the rate of reaction.
What is the compromise which has to be made in the Haber process in terms of temperature?
Therefore, a compromise has to be made between achieving a reasonable rate of reaction and still achieving a reasonable percentage yield of ammonia. The optimum temperature for this compromise is around 450°C.
What are the different component that make up the cost of a new substance?
energy (gas and electricity) needed
the starting materials equipment(plant) labour (the wages of the people needed)
How does energy used affect cost?
The higher the temperature used, the higher the energy cost and the higher the impact on the environment. This impact has economic costs associated with it as companies are charged for the pollution they generate.
What is done to minimize the cost of raw materials?
Unused reactants are recycled to minimise the cost of raw materials.
How does equipment affect cost?
An industrial plant is expensive to build and maintain. When high pressures are used - as in the manufacture of ammonia - the cost is particularly high. Reaction vessels have to be very strong and there must be rigorous safety precautions.
How can labour costs be lowered?
Labour costs can be reduced by automating processes.
How do catalysts reduce cost in manufacture of new substances?
The longer a process takes, the more expensive it is likely to be. A large yield produced over a long period of time is more costly and less desirable than a reduced yield produced quickly. Catalysts are therefore often used to speed up reactions.
