2.6.7Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Industrial Compromises — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 4 parts Lesson 43 of 53 Unit quiz
Part 1 of 4Why a Compromise Is Needed
Conditions that give the best yield often give a slow rate, so industry chooses a balance.
1
For an exothermic reaction a low temperature gives a higher equilibrium yield but a very slow rate.
2
A high temperature gives a fast rate but a lower yield at equilibrium.
3
A high pressure raises the yield when fewer gas moles are on the product side, but costs energy and needs strong, expensive equipment.
4
A catalyst increases the rate without changing the yield, so it allows a lower, cheaper temperature.
5
The chosen conditions are a compromise that gives an acceptable yield at an economic rate and cost.
6
Engineers therefore look at the whole plant, including raw material costs and the energy needed for heating and compressing gases.
Part 2 of 4Case Studies
Haber process
- Haber process N₂ + 3H₂ ⇌ 2NH₃ uses about 450 °C, a high pressure of about 200 atm and an iron catalyst.
Contact process
- Contact process 2SO₂ + O₂ ⇌ 2SO₃ (ΔH = -196 kJ mol⁻¹) uses about 450 °C, near atmospheric pressure and a V₂O₅ catalyst.
Ethanol manufacture
- Ethanol manufacture C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g) uses about 300 °C, a high pressure and a phosphoric acid catalyst.
Recycling
- Recycling Unreacted gases are separated and recycled so that overall conversion is high.
Removing product
- Removing product Cooling the mixture to liquefy ammonia removes it, shifting the equilibrium towards more product.
Contact process pressure
- Contact process pressure Pressure stays close to atmospheric because the yield is already high with the catalyst,
- so extra pressure is not worth its cost.
| Process | Approx. temperature | Pressure | Catalyst |
|---|---|---|---|
| Haber | 450 °C | Around 200 atm | Iron |
| Contact | 450 °C | Close to 1-2 atm | V₂O₅ |
| Hydration of ethene | 300 °C | High, around 60-70 atm | H₃PO₄ |
Part 3 of 4Evaluating Conditions
Evaluating Conditions
Yield
- Yield State how temperature and pressure affect the position of equilibrium using Le Chatelier's principle.
Rate
- Rate State how conditions affect collision frequency and the proportion of molecules above Eₐ.
Cost and safety
- Cost and safety High pressure needs thicker vessels and more energy, which increases costs and safety risk.
Sustainability
- Sustainability A catalyst and recycled reactants reduce energy use and waste.
Part 4 of 4Exam Checklist & Tips
2.6.7 – Key Points for the Exam
- Compromise conditions Conditions chosen to balance equilibrium yield against reaction rate and cost.
- Recycling Returning unreacted starting materials to the reactor.
- Yield The amount of product obtained compared with the maximum possible.
- Catalyst A substance that increases the rate without being used up.
Common Mistakes
- Saying that high temperature is chosen because it gives the highest yield for an exothermic reaction.
- Forgetting that a catalyst does not affect the equilibrium yield.
- Ignoring the economic cost of high pressure.
- Mixing up the conditions used in the Contact and Haber processes.
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