Chemical Equilibria applied to Industry (A-Level Chemistry)
Chemical Equilibria applied to Industry
The Haber Process
Compromise in Industrial Reactions
Many industrial reactions that take place in industry have very expensive costs if optimal conditions are used such as high temperatures and pressures. Both the cost and yield of product need to be considered when considering the compromise to the optimal conditions.
Let’s look at the Haber Process: N2 (g) + 3H2 (g) 2NH3 (g) ∆H = – 82 kJmol-1
- Shifting the equilibrium. We need to consider the equilibrium conditions first. In order to get the maximum yield of ammonia, the position of the equilibrium needs to be shifted to the right. According to Le Chatelier’s Principle, the equilibrium will shift to the right if a lower temperature is used as the forward reaction is exothermic. The equilibrium will also shift to the right if a higher pressure is used as the right hand side of the equation has fewer gaseous molecules.
- Rate of reaction. We also need to consider the rate of reaction. Even though a low temperature favours the forward reaction, a low temperature also means a slower rate of reaction. For that reason we need to use a higher temperature of around 400 – 450°C as a compromise to ensure we still get a reasonable yield and a faster reaction
- Financial cost. The cost of the entire process also cannot be forgotten. A high pressure does increase the rate of reaction and favours the forward reaction in terms of the Haber Process. However using a very high pressure is very expensive as extremely strong pipes and containment vessels need to be built to withstand the very high pressure. For this reason we need to use a compromise of 200 atm (atmospheres).
- Overall. Therefore overall the compromise to the optimal conditions are: temperature of 400 – 450°C to ensure high rate of reaction and pressure of 200 atm as this is cheaper.
Summary
A temperature of 400 – 450°C is used to give a high rate, but it’s not too high otherwise the yield would be low and the cost would be high.
A pressure of 200atm is used to give a high rate and yield, but it’s not too high otherwise the cost would be high.
The Contact Process
The Contact Process refers to the industrial production of sulfuric acid:
2SO2 (g) + O2 (g) ↔ 2SO3 (g) ∆H = – 197 kJmol-1
Just as for the Haber process, temperature and pressure have to be compromised in order to maximize yield and minimise costs.
A temperature of around 450ºC is used to give a high rate, but it’s not too high otherwise the yield would be low (according to Le Chatelier’s Principle, the equilibrium will shift to the right if a lower temperature is used as the forward reaction is exothermic) and the cost would be high.
A pressure just over atmospheric pressure is used because the position of equilibrium already lies far over to the right and using very high pressures would be unnecessary and expensive.
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