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Science

Making ammonia

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The Haber process - Higher tier

The pressure chosen for the Haber process is a compromise. A high pressure increases the percentage yield of ammonia but very high pressures are expensive.

The temperature chosen is also a compromise. A high temperature gives a fast reaction but decreases the percentage yield of ammonia. 450°C gives a reasonably fast reaction with a sufficiently high percentage yield of ammonia.

The use of an iron catalyst increases the rate of the reaction, but it does not alter the percentage yield of ammonia.

Economic considerations

When a chemical is manufactured, the optimum conditions used are the ones that give the lowest cost. These conditions are not necessarily the ones that give the fastest reaction or highest percentage yield. For example:

  • the rate of reaction must be high enough to make enough product each day
  • the percentage yield must be high enough to make enough product each day.

Note that a low percentage yield could be accepted if unreacted started materials can be recycled (as they are in the Haber process).

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