Consider the following statements : 1. The circulation of nitrogen from the inorganic compounds in the soil to form protein in plants and animals and back again is known as the nitrogen cycle. 2. The preparation of ammonia by Haber process is an endothermic reaction. 3. Urea is synthetically made by the action of ammonia and carbon dioxide under high temperature and high pressure. Which of the statements given above is/are not correct?

Correct — C, 2 only. The question asks which statements are not correct, and only the second fails. The Haber process is exothermic, not endothermic: nitrogen and hydrogen combine to give ammonia with a release of about 92 kilojoules per mole of nitrogen, which is exactly why the plant is run at a compromise temperature — raising the temperature speeds the reaction up but pushes the equilibrium back towards the reactants. Statement 1 is a fair description of the nitrogen cycle, the circulation of nitrogen from soil compounds into plant and animal protein and back to the soil. Statement 3 is also correct: urea is manufactured from ammonia and carbon dioxide at high pressure and high temperature.

• (a)1 and 3 — Marks the two statements that are in fact correct and lets the false one pass. Both the description of the nitrogen cycle and the industrial route to urea are standard.
• (b)2 and 3 — Rightly rejects the Haber statement but wrongly adds the urea one. Urea really is made by reacting ammonia with carbon dioxide under high pressure and temperature, in the two-step Bosch-Meiser route that first forms ammonium carbamate and then dehydrates it.
• (d)1 only — Rejects the description of the nitrogen cycle, which is accurate, and accepts the Haber statement, which is not. It gets the item exactly the wrong way round.

Nitrogen makes up about 78 per cent of the atmosphere but is almost inert, so life depends on fixing it into usable compounds. Biological fixation by Rhizobium and free-living bacteria, and lightning, do this naturally; industrially the Haber-Bosch process combines nitrogen and hydrogen over an iron catalyst at roughly 200 atmospheres and 700 K. That reaction is exothermic and proceeds with a decrease in the number of gas molecules, so by Le Chatelier's principle high pressure and low temperature favour the yield, while kinetics demands heat — hence the compromise conditions. The ammonia then feeds the urea plant, where it reacts with carbon dioxide under pressure.

This is a not-correct item, so the statements have to be judged for falsity and the tempting error is to mark the ones that are true. Only statement 2 is wrong, and it is wrong in a way worth remembering: candidates often reason that because the Haber process needs heating it must absorb heat. Heating is supplied to make the reaction go fast enough, not because the reaction consumes energy overall, and the same applies to the urea plant. This card marks a different letter from the answer we have on file; the statement-by-statement working above is the reason.

• The Haber process is exothermic, releasing about 92 kilojoules per mole of nitrogen converted.
• It runs at roughly 200 atmospheres and about 700 K over an iron catalyst — a compromise between yield and rate.
• Urea is made industrially from ammonia and carbon dioxide at high pressure and temperature.
• Nitrogen is about 78 per cent of the atmosphere by volume but is chemically inert because of its triple bond.
• In the nitrogen cycle, nitrogen moves from soil compounds into plant and animal protein and returns through decay and denitrification.

• Assuming a reaction that requires heating must be endothermic.
• In a not-correct item, marking the statements that are true.
• Doubting the urea statement because it sounds specific; it is the standard industrial route.

A three-statement not-correct item whose difficulty lies entirely in the direction of the question rather than in the chemistry.

• practice — not a real PYQ

  The synthesis of ammonia by the Haber process is favoured by

  • (a)high pressure and low temperature
  • (b)low pressure and high temperature
  • (c)low pressure and low temperature
  • (d)high pressure and very high temperature

  Answer(a) high pressure and low temperature — the reaction is exothermic and reduces the number of gas molecules, though a moderate temperature is used in practice for speed.

• practice — not a real PYQ

  The catalyst used in the Haber process for the manufacture of ammonia is

  • (a)platinum
  • (b)iron
  • (c)nickel
  • (d)vanadium pentoxide

  Answer(b) iron — finely divided iron with promoters is the industrial catalyst; platinum belongs to the Ostwald process and vanadium pentoxide to the contact process.