Page 17 - Curriculum Visions Dynamic Book

In the main chamber nitrogen and hydrogen are passed over finely divided iron, which acts as a catalyst. The tower is designed to provide a very large surface area on which the gases can react in the presence of the catalyst. In this chamber much of the hydrogen and nitrogen is converted to ammonia.
catalyst: a substance that speeds up a chemical reaction but itself remains unaltered at the end of the reaction.
fixing: making solid and liquid nitrogen-containing compounds from nitrogen gas. The compounds that are formed can be used as fertilisers.
Also... Ammonia as a source of hydrogen
Hydrogen is a highly inflammable gas that is dangerous to transport. To solve this problem, hydrogen is often transported in the form of safe liquid ammonia. When it reaches its destination, the ammonia is
heated, and decomposes to one-quarter nitrogen and three-quarters hydrogen.
The gases are cooled, and ammonia is liquified and drawn off the base of the cooling chamber. The unreacted gases are recycled back to the catalytic chamber.
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 The catalytic towers in an ammonia plant.
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