The raw materials for the Haber process are nitrogen and hydrogen.

The idea

The two raw materials fed into the Haber process are nitrogen and hydrogen. Nitrogen is obtained from air, which is about four-fifths nitrogen, usually by fractional distillation of liquid air. Hydrogen is commonly obtained from natural gas or by electrolysis of water. The gases are mixed, compressed and passed over an iron catalyst, where they react reversibly to form ammonia. Knowing the sources matters because exam questions often ask where each gas comes from, not just their names. Air is a physical source of nitrogen, so no chemical reaction is needed to extract it; hydrogen needs to be manufactured. The word equation nitrogen + hydrogen ⇌ ammonia summarises the reaction and shows that the process is reversible.

What earns the marks

• Names nitrogen and hydrogen as the raw materials for the Haber process.
• States that nitrogen is obtained from air.
• States that hydrogen is obtained from natural gas or by electrolysis of water.
• Writes or recognises the word equation nitrogen + hydrogen ⇌ ammonia.
• Identifies the reaction as reversible and catalysed by iron.
• Distinguishes raw materials from the product ammonia.

Common mistakes and how to put them right

• Listing ammonia as a raw material.

  Put it right: Ammonia is the product, while nitrogen and hydrogen are the raw materials.

• Saying nitrogen comes from the soil or from fertiliser.

  Put it right: Nitrogen is obtained from air.

• Writing the equation with an equals sign.

  Put it right: Use the reversible arrow ⇌ to show the reaction can go both ways.

Exam tips

• Learn both the names and the sources of the raw materials, as either can be asked.
• Use the reversible arrow ⇌ in the word equation rather than an equals sign.
• Check whether the question asks for raw materials or products before writing your answer.