The Haber Process and the Use of NPK Fertilisers

1. What does the Haber process make?

Answer: Ammonia is made in the Haber process.

2. What are the raw materials for the Haber process?

Answer: The raw materials for the Haber process are nitrogen and hydrogen.

3. What is the catalyst in the Haber process?

Answer: Iron is the catalyst used in the Haber process.

4. What elements are found in NPK fertilisers?

Answer: NPK fertilisers contain nitrogen, phosphorus and potassium.

5. What is phosphate rock treated with to make soluble salts?

Answer: Phosphate rock is treated with acid such as nitric acid, sulfuric acid or phosphoric acid to make soluble salts.

6. The figure below shows a flow chart to describe the Haber process.

a) Describe the processes happening at point A.

Answer: Gas mixture is cooled so that the ammonia condenses into a liquid, but the nitrogen and hydrogen do not condense.

b) Explain in terms of economics the importance of the process marked B on the flow chart.

Answer: It takes energy and time, and therefore money to make hydrogen and purify air to make nitrogen. The Haber process does not make a high yield and the unreacted hydrogen and nitrogen can be recycled, and used again to make ammonia. This is cheaper than creating nitrogen and hydrogen from raw materials (new reactants) and so money is saved.

c) Balance the symbol equation describing the Haber process.

Answer: N2 + 3H2  ⇋  2NH3

d) State the conditions in the reaction chamber during the Haber process.

Answer: high temperatures (450°C), high pressure (200 atmospheres) and iron based catalyst

7. The figure below shows a diagram of a bag of fertiliser. Fertilisers are important for farmers.

a) Name the three elements that are labelled on the fertiliser bag shown above.

Answer: This fertiliser contains nitrogen, phosphorus and potassium.

b) Calculate the percentage of the fertiliser not made of these three elements.

Answer: 100 - (20 + 30 + 10) = 40%

c) Describe an NPK fertiliser.

Answer: An NPK fertiliser is a formulation of various salts containing appropriate proportions (percentages) of the elements N, P, and K.

d) Write the word equation that shows how ammonium nitrate is made.

Answer: ammonia + nitric acid  →  ammonium nitrate

8. Phosphate rock and two other mined resources are used as raw material used to make fertilisers.

a) Give the name of one other raw material used to make fertilisers.

Answer: Potassium chloride or potassium sulfate are also raw materials used to make fertilisers.

b) State the name of the desired product when phosphate rock is treated with sulfuric acid.

Answer: Potassium sulfate is made when phosphate rock is reacted with sulfuric acid.

c) Describe how phosphate rock is processed to make NPK fertilisers.

Answer: Phosphate rock does not have all the elements in it that are needed to make fertilisers. The phosphate rock does not have soluble phosphate compounds in it. When the rock is treated with acids, such as nitric acid or sulfuric acid, then soluble phosphate compounds are made. These soluble phosphate compounds can be combined with other chemicals to make a formulation and an NPK fertiliser.

9. The figure below shows a titration set up to make a fertiliser.

a) State the chemical that would be in the burette.

Answer: sulfuric acid

b) Suggest the chemical name of the fertiliser made in this process.

Answer: ammonium sulfate

c) Outline how you would know that the reaction is complete.

Answer: Use a pH probe (with a data logger) and monitor the pH. Slowly add the acid and swirl. When it is pH = 7 then the neutralisation reaction is complete. Alternatively, add 1 cm3 amounts of the acid, and use a glass rod to take a small sample of the mixture onto indicator paper. Note the colour change and stop when the pH is 7 (colour indicates neutral).

10. The flow chart in the figure below shows how fertilisers are made in industry. Explain the stages of making an NPK fertiliser.

Answer: In industry, fertilisers are made on a much larger scale. In the neutraliser, the ammonia which contains nitrogen reacts with the phosphoric acid, which contains the phosphorus. Potassium chloride is added as a source of K. Water is removed in the evaporator to form a solid. The NPK formulation is turned into pellets in the granulator and coater to make the NPK fertiliser easier to store and use.

11. The graph in the figure below shows how the yield of ammonia changes in the Haber process under different temperatures and pressures.

a) 101 kPa is the same as 1 atmosphere in pressure. Convert 200 atmospheres into pascal. Give your answer to three significant figures and in standard form.

Answer: 101000 = 20200000 Pa = 2.02 × 107 Pa

b) Explain why pressures of greater than 200 atm are not used in the Haber process.

Answer: The Haber process has less moles of gas on the product side. By increasing the pressure the system will oppose the change and favour the products, shifting the position of equilibrium to the products and increasing the yield. The graph shows that after 200 atms pressure, increasing the pressure has very little effect on the yield. High pressures are more expensive due to high cost of energy and apparatus that can withstand high pressures, and also more dangerous than lower pressures. So, a compromise pressure of 200 atmospheres is used.

c) Use the graph to explain whether the Haber process is exothermic or endothermic.

Answer: As temperature increases, yield decreases. The equilibrium system will oppose the change and favour the endothermic reaction. Therefore, the backwards reaction is favoured, reducing the yield and therefore must be endothermic. So, the forward reaction must be opposite and exothermic.