Grades 9 and 10 - Chemistry - High School - Production of Ammonia — Flashcards

The production of ammonia is one of the important industrial processes that you need to know about for your high school Chemistry exam. Ammonia is used to make many other materials such as nitric acid, explosives, nitrogen containing organic chemicals and fertilizers. Without the fertilizers made using ammonia, the world would probably not provide enough food to support the human population. It is used directly in many other products and processes including industrial fermentation and household cleaning products.

Fritz Haber invented the Haber process for production of ammonia.

The industrial production of ammonia from nitrogen and hydrogen requires high temperatures, high pressures, an iron catalyst and is a reversible reaction. The nitrogen comes from the air whilst the hydrogen is usually obtained by reacting steam with natural gas. The two gasses are continuously cycled and recycled through the reaction vessel. During the recycling, they are cooled to remove the ammonia and fresh nitrogen and hydrogen are added. The process is therefore continuous and you are expected to know the factors that affect the manufacturing costs. Since the equipment runs all day and every day, automating the process is a good way for manufacturers to save money. The recycling of the gasses is also a simple way of keeping costs down.

The pressures and temperatures used are always a compromise in order to make the process economically worthwhile and are the key factors that increase the costs. Using higher pressures would increase the amount of ammonia produced, however, the extra cost of building the equipment that could generate and withstand such pressures would not be balanced by the sale of the extra ammonia. Energy costs could be reduced by lowering the temperature but the reaction would slow dow too much and it would not be possible to satisfy the demands of customers. The iron catalyst is useful here as it speeds up the reaction, meaning that temperatures can be kept relatively low.

Plants require nitrogen for growth. This comes from the atmosphere but plants can't use it directly - it must be converted into nitrates that dissolve in groundwater which is then taken up by plants. This 'fixing' of nitrogen occurs naturally but only very slowly. For the natural world, that isn't a problem but for farmers growing crops, it is. A field of crops uses the nitrates faster than they are naturally replenished, which is why farmers use fertilizers.

Before the start of the 20th century, nitrates for use as fertilizers came mainly from South America where there were deposits of 'saltpeter' (sodium nitrate). Bird guano (droppings) from the sea birds in certain areas were also exported as fertilizers too. At the end of the 19th Century, it was realized that these deposits were unsustainable and so the race was on to find a method of making artificial fertilizers. This required nitric acid which is made from ammonia as well as ammonia itself, making the production of ammonia an extremely important process in the chemical industry.

The problem was that nitrogen is a very stable diatomic element. It is hard to get it to react with other elements without using extreme conditions.  In 1909, Fritz Haber used nitrogen and hydrogen, an iron catalyst, high temperature and high pressure to make a laboratory scale system that could produce about a liter of ammonia in something like 8 hours. 

Sometimes you will see it called the Haber-Bosch process.

N2 + 3H2 ⇌ 2NH3

The exact conditions for the production of ammonia vary but are usually in the range of 150 - 250 atmospheres of pressure, 400 to 500oC with an iron catalyst. The reaction is reversible so even as ammonia is being made, it is also decomposing back to the original hydrogen and nitrogen. The high pressure increases the amount of ammonia made but building equipment to contain these pressures is expensive. High temperatures decrease the yield of ammonia but increase the speed of reaction. The conditions that are chosen are a compromise that produces the ammonia economically. The actual conversion rate of the nitrogen and hydrogen to ammonia in the Haber process is about 15 percent. This doesn't sound much but the other 85 percent of unreacted gasses are recycled and the ammonia is continually removed from the apparatus.

Ammonia from the Haber process has many other uses - making nitric acid which is then converted to nitrates to make explosives being one of them.