Ch. 2 - Ammonia

Question 1

Identify and describe industrial uses of ammonia

Answer 1

• The feedstock for fertilisers to grow food, in the form of urea, ammonium sulfate and ammonium nitrate
  NH3 (g) + HNO3 (aq) -> NH4NO3 (aq)
• Cleaning agents: ammonium hydroxide (NH4OH) is the active ingredient in the manufacture of a large variety of domestic cleaning products
• Production of nitric acid - an important industrial chemical, with application in the manufacture of fertilisers, explosives, and synthetic fibres

Question 2

Identify ammonia can be synthesised from its component gases

Answer 2

H2 (g) + 3N2 (g) 2NH3 (g)
Delta H = -92 kJ/mol
This is an exothermic reaction

Question 3

Describe that synthesis of ammonia occurs as a reversible reaction that will reach equilibrium.

Answer 3

Ammonia can be synthesised from its component gases, nitrogen and hydrogen. This reaction is exothermic and occurs as a reversible reaction that will reach equilibrium.

Question 4

Explain why the rate of reaction is increased by higher temperatures

Answer 4

• By the kinetic theory of gases, high temperatures favour an increase in reaction rate since the gas molecules can collide and react with higher kinetic energies.
• Furthermore, more molecules have sufficient energy to overcome the activation energy barrier, so there is a greater frequency of successful molecular collisions.

Question 5

Explain why the yield of product in the Haber process is reduced at higher temperatures using Le Chatelier’s principle

Answer 5

By Le Chatelier’s Principle, the system will counteract an increase in temperature by partially decreasing it, in order to minimise change. Hence the equilibrium will shift to the left to favour the endothermic reaction and use up more heat.
-> Therefore at higher temperatures, the equilibrium is more to the left and the ammonia yield is reduced.

Question 6

Explain the effect of using a catalyst

Answer 6

The use of a catalyst will lower the activation energy and reaction temperature required by providing
an alternate pathway for the reaction to occur. Hence the reaction is able to occur at an increased rate.

Question 7

Identify the catalyst used in the Haber process

Answer 7

The catalyst used is magnetite (Fe3O4) fused with K2O, CaO and Al2O3. The magnetite is then reduced to
porous iron.
-> ground to a fine powder to expose a high surface area for the gases to adsorb onto its surface.

Question 8

Analyse the impact of increased pressure on the system involved in the Haber process

Answer 8

 By Le Chatelier’s principle, the system will counteract an increase in pressure by partially decreasing it, so the equilibrium will shift to favour the reaction which produces less gaseous moles
 Since the molar ratio of gases for reactant to product is 4:2, the system will shift to the right. Hence
increasing the pressure of the system increases the yield of ammonia
 Also, increasing the pressure of the system brings gaseous particles closer together and increases the
frequency of successful molecular collisions
 Hence increasing the pressure of the system increases both the yield and the rate of reaction

Question 9

Explain why monitoring of the reaction vessel used in the Haber process is crucial and discuss the monitoring required

Answer 9

 Temperature and pressure – must be monitored to ensure it remains in the optimum range for yield and
rate of reaction. Excessive temperatures and pressures can increase the risk of explosion, and also damage the catalyst

 Ratio of H2 to N2 remains 3:1 – must be monitored to avoid build-up of one reactant which can lead to
excessive pressures and possible explosion

 Presence of sulfur compounds, O2, CO and CO2 –oxygen increases the risk of explosion. CO, CO2 and
sulfur compounds can poison the catalyst, thus lowering reaction rate

 Presence of argon and methane – must be minimised/removed since the build-up of these gases
increases the pressure in the reaction vessel, reducing efficiency and increasing the risk of explosion

 The catalyst – must be monitored to ensure it is in granulated form for a high surface area and a high
rate of reaction

 The ammonia – must be monitored to ensure the NH3 is continuously liquefied and removed to shift
equilibrium to the right (by Le Chatelier’s principle) and optimise yield. Its purity must be checked.

Question 10

Explain why the Haber process is based on a delicate balancing act involving reaction energy, reaction rate and equilibrium

Answer 10

 Kinetic theory of gases - high temperatures = increased reaction rate since the gas molecules can collide and react with higher kinetic energies; Also more molecules have sufficient energy to overcome the activation energy, giving a greater frequency of successful collisions.
 HOWEVER, by Le Chatelier’s Principle, higher temperatures also result in lower yields since the synthesis
of ammonia is an exothermic reaction. It also uses up more energy.
 Hence a compromise temperature of 400C-550C is used for a suitable yield and reaction rate

 Meanwhile, higher pressures favour higher yields (by Le Chatelier’s Principle) and reaction rates (by
kinetic theory), BUT are associated with costly equipment and safety hazards such as explosion.
 Hence a compromise pressure of 250 atm is used.

 Thus the Haber process is a delicate balancing act which requires compromise conditions involving
reaction rate, equilibrium yield, reaction energy and cost.