Manufacturing Chemicals (Making Ammonia) – Revision Pack (C2)

The Haber Process:

The optimum (best) conditions for the Haber process are as follows:

Condition ReasoningIron Catalyst Speeds up the rate of reaction (or

successful collisions; does NOT increase the percentage yield.

Temperature of 450oC (quite low) Higher temperatures increase the reaction rate, BUT also break down the ammonia, reducing the percentage yield.

High Pressure of 200atm Higher pressure directly results in a higher percentage yield.

Any unreacted hydrogen or nitrogen is recycled and used again.

The optimum temperature is 450oC – this is quite low for an industrial process but is ‘the best of a bad balance’ because while the percentage yield suffers, the rate of reaction is very high. It is a compromise.

What affects the cost of chemical manufacturing?

Any chemical plant (where the chemicals are made) is NOT looking for the highest yield, but the most economic yield.

Costs will increase when more energy is used, or when the temperature is increased. This is why the temperature is kept quite low in the Haber process. If you increase the amount of pressure you use, you will increase the plant costs.

The world’s food production is dependent on nitrogen fertiliser. These fertilisers are made from ammonia, which is the product of the Haber process. The word equation for the Haber process is:

Nitrogen + Hydrogen Ammonia

Manufacturing Chemicals (Making Ammonia) – Revision Pack (C2)

Cost is decreased when you use an iron catalyst and recycle unreacted starting materials.

Automation is used to keep wage costs low; you don’t have to pay workers to do it, because it is all automated.

These factors are all taken into consideration for the Haber process:

- The temperature and pressure must be high enough to give a sufficient daily yield

- A low percentage yield can be accepted if the products can be recycled and the reaction repeated many times

The optimum temperatures used for the Haber process do NOT ensure that we get the highest percentage yield or rate of reaction, but actually give the lowest cost and as such, the most profit.

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