A science club is testing a small sealed reactor used to demonstrate the manufacture of ammonia. Nitrogen and hydrogen are passed over an iron catalyst at 4...
Question 1 Report
A science club is testing a small sealed reactor used to demonstrate the manufacture of ammonia. Nitrogen and hydrogen are passed over an iron catalyst at 450 degrees C. The reaction is reversible, so the contents eventually reach a mixture in which the amounts no longer change. Fig. 1 represents the numbers of molecules in the vessel before heating and after equilibrium is reached. Each formula shown represents one molecule. The club uses a pressure gauge because the reaction changes the number of gas molecules. Ammonia is an important compound used in fertiliser production.
The reactor is fitted with strong walls. This is needed because a higher pressure can improve the amount of ammonia made, although operating equipment at very high pressure requires more energy.
(a) Write the balanced symbol equation for the reaction of nitrogen with hydrogen to form ammonia. [2]
(b) Use Fig. 1 to work out the number of nitrogen molecules that reacted. Show how you obtained your answer. [2]
(c) Describe two effects of adding an iron catalyst to this reaction. [2]
(d) Explain why raising the temperature far above 450 degrees C can make the reaction faster but give less ammonia at equilibrium. [3]
(e) Describe how increasing pressure changes the equilibrium yield of ammonia. [2]
Answer Details
(a) \[\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}\] The formulae and balancing are both needed. [2]
(b) The diagram shows 3 nitrogen molecules before heating and 1 at equilibrium:
\[3 - 1 = 2\]
2 nitrogen molecules reacted. [2]
(c) An iron catalyst increases reaction rate by providing an alternative pathway with lower activation energy. It does not change the equilibrium position or equilibrium yield. Any two points are required. [2]
(d) Higher temperature gives particles more kinetic energy, causing more successful collisions, so the reaction becomes faster. However, ammonia formation is exothermic. Increasing temperature shifts equilibrium towards nitrogen and hydrogen, reducing the equilibrium yield of ammonia. [3]
(e) Increasing pressure increases ammonia yield because the ammonia side has fewer gas molecules. Equilibrium shifts towards the side with fewer gas molecules. [2]
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