SC15

Question 1

SC15a
1) Recall some compounds found in fertilisers.
2) Describe and compare small-scale and large-scale production of ammonium sulfate, including the use of titration

Answer 1

1) Fertilisers are made from compounds containing the elements nitrogen, potassium and phosphorus, which are essential for plant growth. The salt ammonium nitrate is a fertiliser which can be made by reacting ammonia with nitric acid.
2) The lab preparation of ammonium sulfate is a ‘batch’ process. Only a small amount of product is made slowly at any one time. The apparatus is then cleaned and made ready for another batch. The industrial production of ammonium sulfate is a ‘continuous’ process. The product is made quickly all the time, as long as raw materials are provided.
Titration may be another method used in the laboratory. However, this is not suitable on a large scale in industry because the volume of solution will be too large for the titration method. Large volumes of liquid will need to be heated, then allowed to crystallise.

Question 2

SC15b
1) Describe how changing the reaction conditions affects the relative amount of substances in an equilibrium mixture.
2) Predict how different conditions affect how quickly equilibrium is reached.

Answer 2

1) Altering the reaction conditions can result in the yield of products increasing, and the process being more profitable. Le Chatlelier’s principle states that if a system at equilibrium is subjected to any change, the system will adjust itself to counteract the applied change.
2) There are several factors like temperature, pressure and concentration of the system that affect equilibrium.

Question 3

SC15b
Explain how the conditions are chosen for the Haber process

Answer 3

The reaction conditions used are chosen to get an acceptable yield of product in an acceptable time. For example, there would be little profit in getting a very high yield if it took several weeks or months to achieve. High temperature and pressure produce the highest rate of reaction.
Temperature: as the reaction is endothermic, a low temperature is needed to favour the foreword reaction to have a high percentage yield. However, a high temperature is needed in order to have a high rate of reaction, because particles need a lot of kinetic energy in order to react. Therefore, 450C is chosen as a compromise. Also, generating heat is expensive, so using a higher temperature is too expensive.
Pressure: In order to achieve a high percentage yield, a high pressure is needed because there are fewer molecules of product than there are reactants. So a high pressure will push the equilibrium to the right. A high pressure will also mean that the particles collide more frequently, causing them to react more. Therefore, only things limiting the pressure is cost (maintaining high pressures is very expensive) and safety (high pressures can be very dangerous if something goes wrong), which means that 200 atmospheres is the best pressure.

Question 4

SC15b
Explain how the rate of reaction and equilibrium position determine the choice of reaction pathway.

Answer 4

Usually, there is more than one way to make a particular substance. A reaction pathway describes the sequence of reactions needed to produce a desired product. The pathway chosen for a product depends on factors such as:
- percentage yield
- atom economy
- cost of energy
- rate of reaction
- equilibrium position
- usefulness of by-products

Question 5

SC15a
1) Why is ammonium sulfate and ammonium nitrate used as fertilisers?
2) What is the balanced equation for the reaction of ammonia with nitric acid?
3) Describe how ammonium nitrate is made using ammonia produced by the Haber process.

Answer 5

1) Both are soluble in water, and they both include nitrogen which will increase the yield of crops.
2) NH3 + HNO3 → NH4NO3
3) Ammonium nitrate, NH4NO3, is a salt used as a fertiliser. It can be manufactured by reacting ammonia solution with nitric acid:
Ammonia + nitric acid → ammonium nitrate
Nitric acid is made from ammonia. Several stages are involved, but overall the process is:
Ammonia + oxygen → nitric acid + water
Ammonia is manufactured using the Haber process. This involves a reversible reaction between nitrogen and hydrogen:
Nitrogen + hydrogen ⇌ ammonia

Question 6

SC15b
Explain the effect that a pressure higher than 200 atmospheres would have on the rate of attainment of equilibrium and on the equilibrium yield of ammonia

Answer 6

The rate of attainment of equilibrium is faster. Equilibrium yield increases. Equilibrium shifts forward to the products side. This results in a decrease in the number of molecules.