The Haber process — AQA GCSE Chemistry
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The Haber process explained
The Haber process covers 5 specification points, including The Haber process is used to manufacture… and The raw materials for the Haber process are….
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Open a point below for its explanation and revision guidance.
The Haber process is used to manufacture ammonia, which can be used to produce nitrogen-based fertilisers.
The Haber process reacts nitrogen from air with hydrogen to make ammonia. The ammonia produced is the starting point for nitrogen-based fertilisers such as ammonium nitrate and ammonium sulfate. Fertilisers supply nitrogen compounds that plants need for making proteins and DNA, so crops grow larger and yields increase. In the process, the gases are passed over an iron catalyst at a high temperature and pressure, and ammonia is formed as a gas and then cooled and liquefied for collection. Unreacted nitrogen and hydrogen are recycled. The link to fertilisers matters because ammonia itself is not spread on fields; it is reacted with acids to form soluble salts that release nitrate or ammonium ions into the soil.
The raw materials for the Haber process are nitrogen and hydrogen.
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.
Students should be able to recall a source for the nitrogen and a source for the hydrogen used in the Haber process.
The Haber process makes ammonia from nitrogen and hydrogen, so you must be able to name where each reactant comes from. Nitrogen is obtained from the air, which is about 78% nitrogen by volume; fractional distillation of liquid air separates it from oxygen and other gases. Hydrogen is usually obtained from natural gas (methane) by reaction with steam, though other sources exist. A useful memory hook is that nitrogen comes from the sky and hydrogen comes from fossil fuel. In an exam, a question may ask you to state one source for each gas, so learn one reliable source for nitrogen and one for hydrogen, and be ready to name the raw material rather than just the element.
The purified gases are passed over a catalyst of iron at a high temperature (about 450°C) and a high pressure (about 200 atmospheres). Some of the hydrogen and nitrogen reacts to form ammonia. The reaction is reversible so some of the ammonia produced breaks down into nitrogen and hydrogen: nitrogen + hydrogen ammonia On cooling, the ammonia liquefies and is removed. The remaining hydrogen and nitrogen are recycled.
In the Haber process, purified nitrogen and hydrogen are passed over an iron catalyst at about 450°C and about 200 atmospheres. Some nitrogen and hydrogen react to form ammonia, but because the reaction is reversible, some ammonia breaks down again into nitrogen and hydrogen, so an equilibrium mixture leaves the reactor. The mixture is cooled; ammonia liquefies and is removed, while unreacted nitrogen and hydrogen remain as gases and are recycled back into the reactor. Recycling improves the overall yield because it gives unreacted gases further chances to react. The word equation is nitrogen + hydrogen ⇌ ammonia, and the balanced symbol equation is N₂ + 3H₂ ⇌ 2NH₃.
(HT only) Students should be able to: interpret graphs of reaction conditions versus rate apply the principles of dynamic equilibrium in Reversible reactions and dynamic equilibrium (page 59) to the Haber process explain the trade-off between rate of production and position of equilibrium explain how the commercially used conditions for the Haber process are related to the availability and cost of raw materials and energy supplies, control of equilibrium position and rate. AQA GCSE Chemistry 8462. GCSE ex
Higher tier students must connect reaction conditions to both rate and equilibrium position. Raising temperature increases rate but shifts the exothermic forward reaction backwards, lowering yield, so about 450°C is a compromise. Raising pressure increases rate and shifts equilibrium towards fewer gas molecules, favouring ammonia, but very high pressure is expensive and hazardous, so about 200 atmospheres is used. The iron catalyst raises rate without changing equilibrium position. Graphs of rate against temperature or pressure show rate rising, while yield graphs show the equilibrium trade-off. Commercial choices also depend on the cost and availability of raw materials and energy, so the conditions balance yield, speed and expense.
Your focus
- State the purpose of the Haber process and the product formed.
- Describe how ammonia is used to produce nitrogen-based fertilisers.
- Explain why plants need nitrogen compounds supplied by fertilisers.
Show all 15 objectives
- Name the raw materials used in the Haber process.
- State the source of each raw material.
- Write the word equation for the formation of ammonia using the reversible arrow.
- State that nitrogen is obtained from air.
- State that hydrogen is commonly obtained from natural gas.
- Link each named source to the correct reactant in the Haber process.
- Describe the conditions and catalyst used in the Haber process.
- Explain how ammonia is separated and how unreacted gases are recycled.
- Write the word and balanced symbol equations for the Haber process using the reversible arrow.
- Interpret graphs showing how rate and yield change with temperature and pressure.
- Explain the trade-off between rate of production and position of equilibrium in the Haber process.
- Justify the commercial conditions using equilibrium, rate, cost and safety arguments.
The Haber process exam tips
Marking Points
- States that the Haber process manufactures ammonia.
- Names nitrogen and hydrogen as the reacting gases.
- Identifies ammonia as the raw material or intermediate for making nitrogen-based fertilisers.
- Names a nitrogen-based fertiliser, such as ammonium nitrate or ammonium sulfate.
- Explains that nitrogen from fertilisers is needed by plants for growth, for example for making proteins.
- Describes the industrial conditions in outline, including an iron catalyst and a high temperature and pressure.
- 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.
- Nitrogen is obtained from air, which is approximately 78% nitrogen by volume.
- Nitrogen can be separated from air by fractional distillation of liquid air.
- Hydrogen is commonly obtained from natural gas, which is mainly methane.
- Hydrogen can be produced by reacting methane with steam (steam reforming).
- The Haber process combines nitrogen and hydrogen to produce ammonia.
- A named raw material, such as air or natural gas, is a valid source in an exam answer.
- Purified nitrogen and hydrogen are passed over an iron catalyst.
- The temperature used is about 450°C.
- The pressure used is about 200 atmospheres.
- The reaction is reversible, so ammonia can break down into nitrogen and hydrogen.
- Cooling the mixture liquefies ammonia, which is then removed.
- Unreacted nitrogen and hydrogen are recycled back into the reactor.
- The word equation is nitrogen + hydrogen ⇌ ammonia.
- The balanced symbol equation is N₂ + 3H₂ ⇌ 2NH₃.
- Increasing temperature increases rate but shifts the exothermic equilibrium backwards, reducing ammonia yield.
- Increasing pressure increases rate and shifts equilibrium towards the side with fewer gas molecules, increasing ammonia yield.
- A catalyst increases the rate of both forward and reverse reactions equally and does not change the position of equilibrium.
- The chosen conditions are a compromise between rate of production, equilibrium yield, safety and cost.
- Graphs of rate against temperature or pressure show rate increasing as the variable increases.
- Graphs of yield against temperature show yield decreasing as temperature rises for this exothermic reaction.
- Raw material and energy costs influence the commercial conditions chosen.
- The Haber process uses about 450°C and about 200 atmospheres with an iron catalyst.
Examiner Tips
- 💡Learn the word equation nitrogen + hydrogen ⇌ ammonia and use it to anchor your answer.
- 💡When asked about fertilisers, name a specific compound and state that it supplies nitrogen to plants.
- 💡Keep the process conditions separate from the fertiliser use so each part of the answer is clear.
- 💡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.
- 💡Learn one named source for each gas rather than several, so recall is quick and accurate.
- 💡Use the phrase 'from air' for nitrogen and 'from natural gas' for hydrogen as a minimum secure answer.
- 💡If asked to explain, link fractional distillation to separating nitrogen from liquid air, and steam reforming to producing hydrogen from methane.
- 💡Quote the conditions as about 450°C and about 200 atmospheres, including the word 'about' because they are approximate.
- 💡Name iron as the catalyst and state that it speeds up the reaction without being used up.
- 💡When describing the process, use the sequence react, cool, liquefy, remove, recycle to keep your answer ordered.
- 💡Use Le Chatelier reasoning explicitly: state the change, then the direction the equilibrium shifts, then the effect on yield.
- 💡When interpreting a graph, read the axes first and quote a trend, for example rate rises with temperature while yield falls.
- 💡Link each commercial condition to a reason involving rate, equilibrium, cost or safety to reach the higher tariff.
Common Mistakes
- Saying the Haber process makes nitric acid; the correction is that it makes ammonia, which can then be used to make fertilisers.
- Confusing the raw materials with the product; the correction is that nitrogen and hydrogen are the raw materials and ammonia is the product.
- Claiming ammonia is applied directly as a fertiliser; the correction is that ammonia is reacted to form soluble nitrogen-containing salts used as fertilisers.
- Listing ammonia as a raw material; the correction is that ammonia is the product, while nitrogen and hydrogen are the raw materials.
- Saying nitrogen comes from the soil or from fertiliser; the correction is that nitrogen is obtained from air.
- Writing the equation with an equals sign; the correction is to use the reversible arrow ⇌ to show the reaction can go both ways.
- Saying nitrogen comes from nitrogen fertiliser: the error is confusing a use of nitrogen compounds with the industrial source; the correction is that nitrogen is obtained from air.
- Saying hydrogen comes from water with no named process: the error is giving a vague source; the correction is to name natural gas or methane reacted with steam.
- Writing that nitrogen is made in the Haber process: the error is reversing reactant and product; the correction is that nitrogen is a reactant obtained from air, while ammonia is the product.
- Writing the equation with a single arrow instead of the reversible arrow: the error is ignoring reversibility; the correction is to use ⇌ in the equation.
- Saying ammonia is removed by filtration: the error is choosing the wrong separation method; the correction is that ammonia is liquefied by cooling and then removed as a liquid.
- Forgetting to recycle unreacted gases: the error is treating the reactor as a one-pass complete reaction; the correction is that remaining nitrogen and hydrogen are returned to the reactor.
- Claiming a catalyst increases yield: the error is confusing rate with equilibrium position; the correction is that a catalyst speeds up both directions equally and leaves the equilibrium position unchanged.
- Saying higher temperature always gives more ammonia: the error is ignoring the exothermic nature of the forward reaction; the correction is that higher temperature lowers ammonia yield at equilibrium.
- Stating that pressure has no effect on equilibrium: the error is forgetting the mole ratio of gases; the correction is that increasing pressure favours the side with fewer gas molecules, which is the ammonia side.