Haber manufacture and NPK fertilisers: ratios, conditions and salts
Introduced| English | Meaning |
|---|---|
| NPK fertiliser/ˌen piː ˈkeɪ ˈfɜːtəlaɪzə/ | A formulation of salts supplying three nutrient elements. |
| Haber process/ˈheɪbə ˈprəʊses/ | The industrial production of ammonia by reacting nitrogen with hydrogen under controlled temperature and pressure using a catalyst. |
Does recycling gases make fertiliser resource-free?
- Returning unreacted gases improves feedstock use. It does not eliminate the energy needed to heat and compress gases or the resources needed to make salts.
- This Separate Chemistry lesson covers Haber manufacture and NPK fertilisers 氮磷钾化肥. The process loop is required at both tiers; equilibrium/rate evaluation is explicitly Higher tier (HT).
Follow the Haber loop
- The Haber process 哈伯法 manufactures ammonia. Nitrogen comes from air; hydrogen can come from natural gas reacted with steam. Purified gases pass over iron at about 450 °C and about 200 atmospheres: $N_2+3H_2\rightleftharpoons2NH_3$.
- Cooling liquefies ammonia for removal. Unreacted nitrogen and hydrogen return to the reactor. The reaction is reversible, so per-pass conversion is different from overall conversion after repeated recycling.
Haber process: pressure and temperature choices
N₂ + 3H₂ ⇌ 2NH₃ has fewer gas molecules on the right, so high pressure favours ammonia; the forward reaction is exothermic, so a lower temperature would too, but 450 °C keeps the rate acceptable.
Which stream is returned to the Haber reactor after cooling?
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Cooling removes liquefied ammonia; unreacted nitrogen and hydrogen return. The recycle improves feedstock use but heating and compression still need energy.
Use the coefficient ratio on one pass
- Feed contains 2 mol nitrogen and 6 mol hydrogen. If only 1 mol nitrogen reacts, it consumes 3 mol hydrogen and forms 2 mol ammonia. Remaining gases are 1 mol nitrogen and 3 mol hydrogen.
- If all the original feed converted, the theoretical maximum would be 4 mol ammonia. Do not use this maximum as the amount actually made on the stated pass. The balanced equation conserves nitrogen and hydrogen atoms.
HT: balance rate, yield and cost
- The forward reaction is exothermic. Lower temperature favours equilibrium ammonia yield but slows reaction; higher temperature increases rate but lowers equilibrium yield. Interpret rate and equilibrium graphs as different quantities.
- Higher pressure favours ammonia because four reactant gas molecules become two product molecules, but compression and strong equipment cost energy and money. Iron speeds both directions and approach to equilibrium without changing its position. The industrial conditions are a compromise, not maximum possible equilibrium yield.
At unchanged temperature and pressure, iron increases the equilibrium yield of ammonia.
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Iron speeds both forward and reverse reactions, reaching equilibrium sooner without changing its position. This HT distinction separates rate from equilibrium yield.
Supply nitrogen, phosphorus and potassium
- NPK fertiliser is a formulation of salts supplying three nutrient elements. Ammonia can make ammonium salts and nitric acid. Neutralisation gives ammonium nitrate: $NH_3+HNO_3\rightarrow NH_4NO_3$.
- Potassium chloride, potassium sulfate and phosphate rock are mined. Phosphate rock is not directly suitable as fertiliser; treatment makes usable soluble salts. Phosphoric acid supplies no potassium by itself.
What ammonium salt forms when ammonia neutralises nitric acid?
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NH₃ +HNO₃ →NH₄NO₃, ammonium nitrate. This supplies nitrogen, while a complete NPK formulation also supplies phosphorus and potassium.
Distinguish three phosphate-rock treatments
- Nitric acid treatment produces calcium nitrate and phosphoric acid. Sulfuric acid makes single superphosphate containing calcium dihydrogen phosphate with calcium sulfate.
- Phosphoric acid makes triple superphosphate containing calcium dihydrogen phosphate without that sulfate component. Do not call every acid treatment simply “superphosphate”, and do not confuse a phosphorus source with a potassium source.
Match the term with its precise meaning.
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Use the chemical structure, process or comparison boundary to distinguish the terms.
Compare the same salt in lab and industry
- Carefully controlled laboratory neutralisation/titration followed by evaporation and crystallisation can produce a pure solid. A laboratory titration does not inherently give an impure product.
- Industry can integrate ammonia/acid production, recover heat and sustain large output, but needs equipment and emissions control. Compare supplied energy per unit, purity, scale, equipment, waste and costs; “industry is always better” is not an evaluation.
Which two claims correctly limit a manufacturing comparison?
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A lab titration is not inherently impure. Industry offers scale and integration but still needs energy and equipment; evaluate the actual criteria and data.
Apply the method to a new feed
- Independent feed: 3 mol nitrogen and 9 mol hydrogen. If 2 mol nitrogen reacts on a pass, 6 mol hydrogen reacts, 4 mol ammonia forms and 1 mol nitrogen plus 3 mol hydrogen remain.
- A catalyst can reach equilibrium sooner but cannot increase the equilibrium amount at unchanged temperature and pressure. Give a reason based on rate or equilibrium, then a cost/condition that limits the industrial claim.
Follow the process, use a balanced ratio, distinguish rate from equilibrium yield, and identify the actual nutrient salts and manufacturing comparison.
A feed has 3 mol nitrogen and 9 mol hydrogen. If 2 mol nitrogen reacts on one pass, how many mol ammonia form?
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N₂ + 3H₂ ⇌ 2NH₃. Two mol nitrogen consumes 6 mol hydrogen and forms 4 mol ammonia, leaving 1 mol nitrogen and 3 mol hydrogen. The all-feed theoretical maximum of 6 mol is not the stated per-pass amount.