How do industries choose conditions that balance yield, rate and cost in large-scale reactions?
The Haber process and the Contact process as industrial applications of rate and equilibrium, the choice of compromise conditions of temperature, pressure and catalyst, and the economic and environmental factors in industrial chemistry.
A CCEA Life and Health Sciences answer on industrial processes: the Haber and Contact processes as applications of rate and equilibrium, the choice of compromise conditions of temperature, pressure and catalyst, and the economic and environmental factors involved.
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What this dot point is asking
CCEA wants you to describe the Haber process and the Contact process as industrial applications of rate and equilibrium theory, explain why compromise conditions of temperature, pressure and catalyst are chosen, and discuss the economic and environmental factors that shape industrial chemistry. It is where the rate, energetics and equilibrium dot points come together to explain real manufacturing decisions.
The Haber process
The conditions are chosen as compromises. The forward reaction is exothermic, so a low temperature would give a higher equilibrium yield (Le Chatelier), but the rate would be far too slow; about 450 degrees Celsius balances a workable yield with a useful rate, and suits the catalyst. There are four moles of gas on the left and two on the right, so a high pressure shifts equilibrium towards ammonia and raises yield; but very high pressures need expensive, strong equipment and raise safety risks, so about 200 atmospheres is used. The iron catalyst speeds the approach to equilibrium without changing the yield. Ammonia is removed by cooling and liquefying it, and unreacted nitrogen and hydrogen are recycled, raising the overall conversion.
The Contact process
The Contact process shows the same reasoning as the Haber process: count the gas moles to predict the pressure effect, identify the exothermic direction to predict the temperature effect, and choose a catalyst to fix the rate. The difference is that the equilibrium for sulfur trioxide is so favourable that high pressure is unnecessary, so a near-atmospheric pressure is chosen to save money.
Economic and environmental factors
Designing a large-scale process means balancing several factors. Rate matters because a faster reaction makes more product per day, lowering fixed costs per tonne, which is why a catalyst and a compromise temperature are used. Energy cost is large (heating and compressing gases), so a catalyst that lets a lower temperature be used saves money and reduces carbon dioxide emissions. Equipment cost and safety limit the pressure that can be used. Recycling unreacted reactants reduces waste and cost. Environmental considerations include minimising energy use and emissions, preventing the escape of harmful gases (ammonia, sulfur dioxide), and using or safely disposing of by-products. The final choice of conditions is always a balance of maximum yield, acceptable rate and lowest overall cost.
Examples in context
Example 1. Ammonia for fertilisers. Most ammonia from the Haber process is used to make nitrogen fertilisers, which raise crop yields and help feed the world. This links industrial chemistry to food production, and the high energy demand of the process explains why fertiliser costs track energy prices.
Example 2. Sulfuric acid as a key chemical. Sulfuric acid from the Contact process is one of the most widely used industrial chemicals, in fertilisers, detergents and many manufacturing processes. Choosing near-atmospheric pressure because the yield is already high shows how cost drives condition choices once yield is satisfactory.
Try this
Q1. State the catalyst used in the Haber process and the catalyst used in the Contact process. [2 marks]
- Cue. Iron in the Haber process; vanadium(V) oxide in the Contact process.
Q2. Explain why a high pressure increases the yield of ammonia in the Haber process. [2 marks]
- Cue. There are fewer gas moles on the product side, so higher pressure shifts equilibrium towards ammonia to oppose the increase.
Q3. Give one reason unreacted gases are recycled in the Haber process. [1 mark]
- Cue. To increase the overall conversion and reduce waste and cost.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA AS 36 marksThe Haber process forms ammonia in the exothermic reaction N2(g) + 3H2(g) reversible 2NH3(g). Explain why a temperature of about 450 degrees Celsius and a pressure of about 200 atmospheres are used, even though these are not the conditions that give the maximum yield.Show worked answer →
The answer must explain the compromise for temperature and the practical limit on pressure.
Temperature: the forward reaction is exothermic, so by Le Chatelier's principle a low temperature would give a higher equilibrium yield of ammonia. However, a low temperature makes the rate very slow, so it would take too long to reach equilibrium. About 450 degrees Celsius is a compromise: a lower yield, but reached at a useful rate, and the catalyst also works well at this temperature.
Pressure: there are more gas moles on the left than the right, so a high pressure shifts equilibrium towards ammonia and increases yield. However, very high pressures need very strong, expensive equipment and are dangerous, so about 200 atmospheres is a compromise between a good yield and reasonable safety and cost.
A linking point: unreacted nitrogen and hydrogen are recycled, and ammonia is removed by cooling, both of which raise the overall conversion.
Markers reward the compromise for temperature (yield versus rate), the reason higher pressure raises yield but is limited by cost and safety, and ideally the recycling point.
CCEA AS 34 marksState two economic factors and one environmental factor that an industrial chemist must consider when designing a large-scale process such as the Haber process.Show worked answer →
Give genuine economic and environmental considerations.
Economic factors (any two): the cost of energy for heating and compressing gases; the cost of raw materials; the rate of production (a faster rate means more product per day and lower fixed costs per tonne); the cost of building and maintaining equipment that withstands high pressure; recycling unreacted gases to reduce waste and cost; and the use of a catalyst to lower the temperature and so save energy.
Environmental factor (any one): minimising energy use to reduce carbon dioxide emissions; recycling unreacted reactants to reduce waste; preventing the escape of ammonia or sulfur dioxide; and safe disposal or use of by-products.
Markers reward two valid economic factors (cost or rate based) and one valid environmental factor.
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Sources & how we know this
- CCEA GCE Life and Health Sciences specification — CCEA (2016)