This is part of the HSC Chemistry course under Module 8 Section 3: Chemical Synthesis and Design.

HSC Chemistry Syllabus

Evaluate the factors that need to be considered when designing a chemical synthesis process, including but not limited to:

• availability of reagents

• reaction conditions (ACSCH133)

• yield and purity (ACSCH134)

• industrial uses (eg pharmaceutical, cosmetics, cleaning products, fuels) (ACSCH131)

• environmental, social and economic issues 

Case Studies: Haber Process and Contact Process

This video explores some of the factors discussed above in two specific examples of chemical synthesis: production of ammonia in the Haber process, and the production sulfuric acid in the Contact process. The aim of this video is to help you apply and reinforce the key factors that are considered when implementing a chemical synthesis process.

  

 Case Study 1: Haber Process

• Production of ammonia from nitrogen and hydrogen gas

 

$$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \hspace{1cm} \Delta H < 0$$ 

 

• Uses of ammonia: fertiliser (80%), textiles, explosives. Ammonia is an important chemical in agriculture and the mining industry.

 

Environmental, Social & Economical Considerations

• N₂ and H₂ are abundant → easily accessible and affordable → economical
• Reaction is reversible so reaction conditions can be changed to optimise reaction rate, yield and purity.
• NH₃ is the only product → no direct wastes (environmental)
• Unused N₂ and H₂ gases can be reused to produce more NH₃ → economical
• Exothermic reaction: energy produced can be used to power the production facility
• Heat released from reaction chambers is properly managed to minimise thermal pollution

 

Improving Reaction rate:

• Iron catalyst → faster reaction rate at a lower temperature
• ↑ pressure → increases collision rate between N₂ and H₂
• ↑ temperature → increases kinetic energy → increases collision rate

 

Improving Yield

Forward reaction is exothermic ↓ temperature to favour the forward reaction and increase yield However, ↓ temperature decreases reaction rate, so moderate temperature is used to achieve compromise between decent reaction rate and yield.  More gases on reactant side: ↑ pressure to shift equilibrium position to NH3 side Constant supply of N2 & H2 and removal of NH3 both shift the equilibrium position to the product side.

graph shows changes in yield of ammonia at various temperature and pressure settings

 

Case Study 2: Contact Process

• Production of concentrated H₂SO₄ from sulfur and oxygen as raw materials
• Many uses for sulfuric acid:
  • Fertiliser
  • Catalysts for chemical synthesis: dehydration, esterification
  • Anionic detergents
  • Pigments

 

• Contact process consists of four reactions:

1. S(s) + O₂(g) → SO₂(g) 
2. 2SO₂(g) + O₂(g) ⇌ 2SO₃(g)   ΔH = –197 kJ mol⁻¹
3. H₂SO₄(l) + SO₃(g) → H₂S₂O₇(l) (oleum)
4. H₂S₂O₇(l) + H₂O(l) → 2 H₂SO₄(l)

 

Environmental, Social & Economical Considerations

• Sulfur and oxygen gas are abundant raw materials that are easily accessible and relatively affordable. Production facility should be ideally situated close to a sulfur-rich mining site to allow for easy transportation.
• Sulfur dioxide and sulfur trioxide are pollutants (e.g. major contributor in formation of acid rain) so it is important to ensure they are confined in the reaction chambers.
• Second step of Contact process is a reversible reaction. Thus, reaction conditions can be changed to optimise rate and yield.
• Contact process uses existing sulfuric acid to produce a much larger quantity of sulfuric acid that is also higher in purity. Since sulfuric acid is highly corrosive, care must be taken when transporting it to the production facility. Besides improving safety, distance over which H₂SO₄ is transported can be minimised by building the production plant close to the market.

 

Improving Reaction Rate & Yield

• In general for complete irreversible reactions: high temperature, high pressure while minimising energy input.
• However, second reaction is reversible:

 

$$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g) \hspace{1cm} \Delta H = -197 \hspace{0.1cm}kJ \hspace{0.1cm} mol^{-1}$$

 

• V₂O₅ catalyst allows for a faster reaction at a lower temperature
• Since the forward reaction is exothermic, a lower temperature would increase its yield by shifting the equilibrium position to the product side.
  • However, a lower temperature decreases reaction rate
  • Moderate temperature is used to reach a compromise between good reaction rate and yield
• High pressure condition is used to shift the equilibrium position to the product side and increase yield of SO₃(g).
• Energy released can be reused to generate electricity to power the production process