10.4 Haber Process & NPK Fertilisers

Ammonia produced from the Haber process is used as a raw material to make fertilisers.

Ammonium sulfate is a common fertiliser that can be made in the laboratory by the titration of ammonia solution with sulfuric acid.

A student followed the method below to make ammonium sulfate.

Explain why the student used litmus paper instead of adding an indicator to the ammonia solution to find the end point.

The balanced symbol equation for the neutralisation of ammonia solution with sulfuric acid is:

   2NH₃ (aq)   +   H₂SO₄ (aq)   →    (NH₄)₂SO₄ (aq)

The student carried out the titration to make the fertiliser, ammonium sulfate.

They found that 23.45 cm³ of 1 mol/dm³ sulfuric acid was needed to neutralise 25.0 cm³ of ammonia solution.

Calculate the concentration of the ammonia solution.

Give your answer to three significant figures.

Urea is another compound used by farmers.

Urea is made from ammonia in the reaction below.

2NH₃ + CO₂  NH₂CONH₂ + H₂O

Compare the atom economy of urea for this reaction with the atom economy of ammonium sulfate in the reaction between sulfuric acid and ammonia.

Relative atomic masses (A_r): H = 1;  C = 12;  N = 14;  O = 16;  S = 32

Ammonium nitrate, NH₄NO₃ is another fertiliser made from ammonia. 

Fertilisers containing nitrogen are also known as nitrogenous fertilisers.

Calculate the percentage by mass of nitrogen for ammonium nitrate and ammonium sulfate, (NH₄)₂SO₄, to determine which fertiliser is the best nitrogenous fertiliser.

Relative atomic masses (A_r):  H = 1;  N = 14;  O = 16;  S = 32

Ammonia is made from nitrogen gas and hydrogen gas in the Haber process.

The equation for the reaction is:

N₂ (g)   +   3H₂ (g)      2NH₃ (g)

Figure 1 shows how the percentage yield of ammonia depends on the temperature and pressure of the reacting mixture.

Figure 1

What temperature and pressure would produce the maximum percentage yield of ammonia?

Typical conditions used in the Haber process are 450 ^oC and 200 atm.

Use Figure 1 to determine the percentage yield of ammonia using these conditions.

What is the effect of an increase in the temperature on the yield of ammonia?

Explain your answer.

A temperature of 100 ^oC at 200 atm gives a percentage yield of ammonia of over 98%.

Why is this temperature not used in the Haber process?

Farmers use fertilisers to improve agricultural productivity.

NPK fertilisers are one type of fertiliser that can be used.

Why can NPK fertilisers be described as formulations and what elements do they contain which are essential for healthy crops?

NPK fertilisers contain different compounds to provide the essential elements needed for healthy crop production.

Many of these compounds are made by neutralisation reactions.

Complete Table 1 which shows the acid and alkali required to make the fertilisers.

Table 1

Phosphate rock is obtained from mining. It cannot be used directly as a fertiliser so it has to be treated with acid to produce soluble salts that can be used as fertilisers.

Give the name of the salts produced when phosphate rock is treated with nitric acid and phosphoric acid.

Diammonium hydrogen phosphate, (NH₄)₂HPO₄ is a water soluble salt that can be used as a fertiliser.

Deduce the charge on the hydrogen phosphate ion.

Ammonia is used as a raw material for many fertilisers.

Figure 1 shows a flowchart for the manufacture of  fertiliser A.

Figure 1

What is the name of fertiliser A?

Some compounds that contain potassium are mined and can be used directly as a fertiliser.

Name one of these compounds.

Table 1 shows information about three compounds that are used as fertilisers.

A farmer uses fertilisers X and Z to fertilise a field.

Give an advantage and a disadvantage to using both fertilisers.

The farmer uses fertiliser Y on a different field.

400 kg of fertiliser Y covers an area of 0.1 km².

How much would it cost to fertilise a field with fertiliser Y which has an area of 0.03 km²?

Use Table 1.

Ammonia is produced in the Haber process and is used to make fertilisers.

Table 1 shows information about compounds used as fertilisers.

Table 1

Name the element in compound C which plants require to make amino acids for growth.

Explain why compounds B and C would not be mixed to make a fertiliser.

Phosphate rock is a source of phosphate that is used in the manufacture of fertilisers.

Triple superphosphate is made when phosphate rock is treated with phosphoric acid.

Describe how two other soluble salts can be produced from phosphate rock. 

NPK fertilisers are formulations of different salts.

Explain what is meant by the term ‘formulation’. 

This question is about the Haber process.

In the Haber process, the reaction of nitrogen with hydrogen to produce ammonia (NH₃) is reversible.

Reversible reactions can reach equilibrium in a closed system.

Explain what is meant by the term ‘closed system’.

Give the two sources of the reactants in the Haber process. 

Name the catalyst used in the Haber process and explain how it speeds up the reaction. 

When the reversible reaction in the Haber process reaches equilibrium, it appears to have stopped.

Explain why, referring to the substances involved. 

Suggest and explain the effect of increasing the pressure on the amount of ammonia produced if the reaction of the Haber process is at equilibrium.

You should refer to the reaction equation in your answer. 

In the Haber process, nitrogen and hydrogen react to produce ammonia.

The reaction between nitrogen and hydrogen is exothermic.

Draw a reaction profile for this reaction.

The reaction between nitrogen and hydrogen in the Haber process is reversible.

The graph in Figure 2 shows how the percentage yield of the product of this reaction, ammonia, varies at different temperatures and pressures.

Figure 2 

Suggest, using Figure 2,  the conditions that would produce the greatest yield of ammonia. 

In the Haber process, the conditions used to produce ammonia are not those that would give the greatest yield.

Using Figure 2, suggest and explain why. 

Purified hydrogen and nitrogen are also passed over powdered iron in the Haber process.

Explain why.