What you'll learn
The Haber process and industrial conditions is one of the most important applied topics in AQA GCSE Chemistry, and it is where the theory of equilibrium meets the economics of manufacturing. Ammonia is the starting point for fertilisers, and fertilisers feed a substantial fraction of the world's population, so the process matters far beyond the examination. What makes it a demanding topic is that the conditions chosen industrially are deliberately not the conditions that give the highest yield: they are a compromise between yield, rate and cost. Explaining that compromise correctly is what separates a full-mark answer from a partial one. By the end of this guide you should be able to state the raw materials and their sources, write the equation and describe the conditions, explain the effect of temperature and pressure on yield and on rate, justify the compromise conditions, explain the role of the catalyst and of recycling, and describe how the ammonia is used.
Key terms and definitions
Haber process — the industrial manufacture of ammonia from nitrogen and hydrogen
Reversible reaction — a reaction in which the products can react to reform the reactants, written with a double arrow
Dynamic equilibrium — the state in a closed system when the forward and backward reactions occur at the same rate, so concentrations remain constant
Yield — the quantity of product obtained, often expressed as a percentage of the theoretical maximum
Compromise conditions — conditions chosen to balance yield, rate and cost rather than to maximise any one of them
Catalyst — a substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy, without being used up
Exothermic — transferring energy to the surroundings
Recycling — returning unreacted gases to the reaction vessel rather than discarding them
Feedstock — the raw material supplied to an industrial process
Core concepts
Raw materials and the reaction
The Haber process combines nitrogen and hydrogen to form ammonia.
The nitrogen is obtained from the air, which is about 78 per cent nitrogen, by fractional distillation of liquefied air. It is effectively free and unlimited.
The hydrogen is obtained mainly from natural gas, by reacting methane with steam. This is the expensive half of the feedstock, and it is also why the process has a substantial carbon footprint: the source of hydrogen is a fossil fuel.
The reaction is reversible, so it is written with a double arrow: nitrogen plus hydrogen giving ammonia, with one molecule of nitrogen reacting with three molecules of hydrogen to give two molecules of ammonia.
The forward reaction is exothermic, which means the backward reaction is endothermic. Knowing which direction is exothermic is the first step in every equilibrium question on this topic.
The industrial conditions
The conditions used in practice are a temperature of about 450 degrees Celsius, a pressure of about 200 atmospheres, and an iron catalyst.
Each of these is a deliberate choice, and each must be justified differently. The temperature is a compromise; the pressure is limited by cost and safety; the catalyst affects only rate.
The effect of pressure
The equation has four molecules of gas on the left — one of nitrogen and three of hydrogen — and two molecules of gas on the right.
Increasing the pressure shifts the position of equilibrium towards the side with fewer gas molecules, because fewer molecules exert less pressure and the system responds so as to counteract the change. That side is the right, so a higher pressure increases the yield of ammonia.
Higher pressure also increases the rate, because the particles are closer together and collide more frequently.
Since high pressure improves both yield and rate, why not use a much higher pressure? The answer is cost and safety. Very high pressures require extremely strong pipes and reaction vessels, which are expensive to build and maintain, and the energy to compress the gases is itself costly. There is also a greater risk associated with operating equipment at very high pressure. Two hundred atmospheres represents the point at which the additional yield no longer justifies the additional expense.
The effect of temperature
The forward reaction is exothermic. Increasing the temperature shifts the position of equilibrium in the endothermic direction, which here is the backward reaction, so a higher temperature decreases the yield of ammonia.
On yield alone, therefore, a low temperature would be preferable.
However, a lower temperature decreases the rate of reaction, because the particles have less energy, collide less frequently and fewer collisions have the activation energy required. At a low temperature the equilibrium yield would be high but it would take so long to be reached that very little ammonia would be produced per day.
The chosen temperature of 450 degrees Celsius is therefore a compromise: it accepts a lower percentage yield in exchange for reaching equilibrium quickly enough for the process to be productive.
This is the single most examined explanation in the topic, and a full answer must mention both the yield effect and the rate effect, and state explicitly that a compromise is being made.
The catalyst
An iron catalyst is used. It increases the rate of reaction by providing an alternative pathway with a lower activation energy, so a greater proportion of collisions are successful.
Two points about catalysts are routinely tested here. A catalyst does not change the position of equilibrium and therefore does not affect the yield; it only allows equilibrium to be reached sooner. And a catalyst is not used up, so it can be recovered unchanged and reused, which is why the cost of the catalyst is spread over a long period.
The practical benefit is that the catalyst allows an acceptable rate to be achieved at a lower temperature than would otherwise be needed, which in turn protects the yield.
Separating the ammonia and recycling
The mixture leaving the reaction vessel contains ammonia together with unreacted nitrogen and hydrogen.
The mixture is cooled, and because ammonia has a much higher boiling point than nitrogen or hydrogen, the ammonia condenses to a liquid while the other two remain gases. The liquid ammonia is drained off and stored.
The unreacted nitrogen and hydrogen are recycled back into the reaction vessel. This is economically essential: the hydrogen in particular is expensive to produce, and discarding it would make the process uneconomic. Recycling means that although the yield at equilibrium may be only around 15 per cent at the conditions used, almost all of the nitrogen and hydrogen is eventually converted to ammonia.
This point resolves a puzzle candidates often raise about the low percentage yield. The yield per pass is low, but the overall conversion is high because nothing is wasted.
Uses of ammonia
Ammonia is used principally to make fertilisers. It reacts with nitric acid to give ammonium nitrate, with sulfuric acid to give ammonium sulfate, and with phosphoric acid to give ammonium phosphate. All are soluble, which is what allows plants to absorb them.
Some ammonia is also oxidised to make nitric acid, which is itself then used to make ammonium nitrate, so the process feeds into itself.
Ammonia is additionally used in the manufacture of explosives, dyes and cleaning products.
Worked examples
Example 1: Explaining the temperature compromise (4 marks)
Explain why a temperature of 450 degrees Celsius is used in the Haber process rather than a much lower temperature.
The forward reaction is exothermic, so decreasing the temperature would shift the position of equilibrium towards the products and increase the equilibrium yield of ammonia. On yield alone a lower temperature would therefore be better.
However, a lower temperature would also decrease the rate of reaction, because the particles would have less energy, collide less frequently, and fewer collisions would have energy equal to or greater than the activation energy. Equilibrium would be reached very slowly and little ammonia would be produced in a working day.
The temperature of 450 degrees Celsius is a compromise that accepts a lower yield in return for a commercially acceptable rate.
Example 2: Predicting the effect of a pressure change (3 marks)
Predict and explain the effect on the yield of ammonia of increasing the pressure from 200 to 400 atmospheres.
There are four molecules of gas on the left of the equation and two on the right. Increasing the pressure shifts the position of equilibrium towards the side with fewer molecules of gas, which is the right-hand side, so the yield of ammonia would increase.
However, this higher pressure is not used industrially because the stronger reaction vessels and pipes required would be considerably more expensive to build and maintain, the energy needed to compress the gases would be greater, and operating at such pressures carries increased safety risks. The additional yield does not justify these costs.
Example 3: Explaining the role of recycling (3 marks)
The percentage yield of ammonia at the conditions used is only about 15 per cent. Explain why the process is nevertheless economic.
The mixture leaving the reactor is cooled so that the ammonia, which has a much higher boiling point, condenses and can be removed as a liquid, while the unreacted nitrogen and hydrogen remain as gases.
These unreacted gases are returned to the reaction vessel rather than being discarded, so they have further opportunities to react. Over many passes, almost all of the nitrogen and hydrogen is eventually converted into ammonia. The overall conversion is therefore high even though the yield on any single pass is low, and no expensive hydrogen is wasted.
Common mistakes and how to avoid them
The most frequent error is stating that 450 degrees Celsius gives the highest yield. It does not; it gives a lower yield than a cooler temperature would, and is chosen for rate.
Students often claim that the catalyst increases the yield. A catalyst affects only the rate and the time taken to reach equilibrium, never the position of equilibrium.
Another common slip is saying that high pressure is not used because it is dangerous, without mentioning cost. Both the expense of the equipment and the energy of compression are usually required.
Many candidates omit the source of the raw materials. Nitrogen from the air and hydrogen from natural gas are frequently worth a mark each.
Finally, answers about separation sometimes say the ammonia is filtered off. It is separated by cooling and condensation, which works because of the difference in boiling points.
Exam technique for "The Haber process and industrial conditions"
Before answering any equilibrium question, note two things: which direction is exothermic, and how many gas molecules sit on each side. Those two facts answer every temperature and pressure question in the topic.
For compromise questions, always give three elements — the effect on yield, the effect on rate, and the statement that a compromise is being struck. Answers giving only one of the first two cap out quickly.
Distinguish clearly between the effect on yield and the effect on rate. Pressure improves both; temperature improves rate but reduces yield; the catalyst improves rate only.
When a question asks why a particular value is used, the answer usually involves cost. Be specific about what costs money: equipment, energy for compression, or energy for heating.
Quick revision summary
The Haber process makes ammonia from nitrogen obtained by fractional distillation of liquefied air and hydrogen obtained from natural gas, in a reversible reaction with one nitrogen and three hydrogen molecules giving two ammonia molecules, the forward direction being exothermic. Industrial conditions are about 450 degrees Celsius, about 200 atmospheres, and an iron catalyst. Higher pressure increases both yield, because there are fewer gas molecules on the product side, and rate, but is limited by the cost of strong equipment, the energy of compression and safety. Higher temperature increases rate but decreases yield because the forward reaction is exothermic, so 450 degrees is a compromise accepting lower yield for acceptable speed. The iron catalyst increases rate by lowering activation energy but does not change the yield and is not used up. The ammonia is separated by cooling and condensing, and unreacted gases are recycled, so overall conversion is high despite a yield of only about 15 per cent per pass. Ammonia is used chiefly to make ammonium nitrate, sulfate and phosphate fertilisers, and also nitric acid, explosives and dyes.