Ammonia and the Haber process

This page covers one Form 4 content standard from the Manufactured Substances in Industry chapter: ammonia and the Haber process. Ammonia, NH₃, is the starting material for nitrogenous fertilisers, so the process that makes it feeds much of the world. The exam asks for the uses of ammonia, the raw materials, equation and conditions of the Haber process, and the reason those conditions are chosen. This topic mirrors the Contact process, so learning them together saves time.

Uses of ammonia

Ammonia is used mainly to manufacture nitrogenous fertilisers such as ammonium sulfate, (NH₄)₂SO₄, ammonium nitrate, NH₄NO₃, and urea, CO(NH₂)₂, which supply nitrogen to crops. It is also used to make nitric acid, HNO₃, to manufacture synthetic fibres such as nylon, and in household cleaning agents. Because fertiliser demand is enormous, ammonia is produced on a very large industrial scale.

Raw materials

The two raw materials are nitrogen and hydrogen. Nitrogen is obtained from the fractional distillation of liquid air, which is about four-fifths nitrogen. Hydrogen is obtained mainly from natural gas (methane) reacting with steam. The two gases are purified and mixed in the correct ratio before entering the reactor.

The Haber process reaction and conditions

In the Haber process, nitrogen and hydrogen combine in a 1 : 3 ratio by volume to form ammonia. The reaction is reversible:

N₂ + 3H₂ ⇌ 2NH₃

The conditions used are:

• a catalyst of iron, which speeds up the reaction (it is not used up);
• a temperature of about 450–500 °C;
• a high pressure of about 200 atmospheres.

The mixture leaving the reactor contains ammonia together with unreacted nitrogen and hydrogen. The ammonia is cooled and liquefied and removed, while the unreacted gases are recycled back into the reactor so that little raw material is wasted.

Why the conditions are a compromise

Because the forward reaction is exothermic and reduces the number of gas molecules, the theoretical yield of ammonia is highest at low temperature and high pressure. But a low temperature makes the reaction too slow. The chosen temperature of about 450–500 °C is therefore a compromise that gives an acceptable yield at a reasonable rate, helped by the iron catalyst. A high pressure favours the yield, but very high pressures are expensive and dangerous, so about 200 atmospheres is chosen as a practical balance between yield and cost.

Worked example

Question. In the Haber process, nitrogen reacts with hydrogen according to N₂ + 3H₂ → 2NH₃. If 30 dm³ of nitrogen reacts completely with hydrogen, calculate (a) the volume of hydrogen that reacts and (b) the volume of ammonia produced. (All volumes measured at the same temperature and pressure.)

Step 1, Read the ratio from the equation. The volume ratio of gases equals the mole ratio, so N₂ : H₂ : NH₃ = 1 : 3 : 2.

Step 2, Find the volume of hydrogen. For every 1 volume of nitrogen, 3 volumes of hydrogen react. Volume of H₂ = 3 × 30 dm³ = 90 dm³.

Step 3, Find the volume of ammonia. For every 1 volume of nitrogen, 2 volumes of ammonia form. Volume of NH₃ = 2 × 30 dm³ = 60 dm³.

Answer. (a) The volume of hydrogen that reacts is 90 dm³. (b) The volume of ammonia produced is 60 dm³.

Practice question

Question. (a) Write the balanced equation for the Haber process. (b) State the catalyst used and the ratio of nitrogen to hydrogen by volume. (c) Explain why a temperature of about 450 °C is used rather than a much lower temperature, even though a lower temperature would give a higher yield.

Answer. (a) N₂ + 3H₂ ⇌ 2NH₃. (b) The catalyst is iron, and the ratio of nitrogen to hydrogen is 1 : 3 by volume. (c) A much lower temperature would give a higher yield of ammonia but the reaction would be far too slow to be economical. The temperature of about 450 °C is a compromise that gives a reasonable yield at an acceptable rate; the iron catalyst further increases the rate without changing the position of equilibrium.

Exam tip

The marks here cluster around three things: the balanced equation with the ⇌ sign, the 1 : 3 ratio and iron catalyst, and the compromise reasoning. Never write the equation with a single forward arrow, it is reversible. When asked to explain the temperature, always state both halves of the trade-off: lower temperature means higher yield but slower rate, so a moderate temperature is chosen. And remember the catalyst changes only the rate, never the position of equilibrium or the final yield, a very common trap.

Comparing the Haber and Contact processes

It helps to line the two processes up side by side, because they are tested in the same way. Both are reversible reactions run at a moderate, compromise temperature of about 450–500 °C with a catalyst. The Contact process uses a vanadium(V) oxide catalyst at about 1 atmosphere; the Haber process uses an iron catalyst at a high pressure of about 200 atmospheres, because pressure has a much bigger effect on the ammonia yield. In both, unreacted gases are recycled and the conditions balance rate, yield and cost. If you can explain that balance for one process, you can explain it for the other, and questions often ask you to do exactly that.

Where this fits

This is content standard 8.2 of the Manufactured Substances in Industry chapter, and it sits directly after sulfuric acid and the Contact process. Reinforce it with the chapter revision notes and practise the volume-ratio and condition questions in the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, with a paid one-hour trial, our teachers make the compromise-conditions argument second nature, because the same reasoning carries marks across 4541.

Quick recap

• Ammonia is used for fertilisers, nitric acid, nylon and cleaning agents.
• Raw materials: nitrogen (from air) and hydrogen (from natural gas + steam).
• Equation: N₂ + 3H₂ ⇌ 2NH₃, ratio 1 : 3 by volume.
• Conditions: iron catalyst, about 450–500 °C, about 200 atm.
• Conditions are a compromise between yield, rate and cost; unreacted gases are recycled.