Ammonia is manufactured by the Haber process. Fe catalyst N2 + 3H2 ⇌ 2NH3 ΔH = -92.4kJ/mol The table below shows how the percentage yield of ammonia at equilibrium varies with both temperature and pressure. pressure /atmospheres | % yield at 200 °C | % yield at 300°C | % yield at 400 °C | % yield at 500 °C ---|---|---|---|--- 30 | 68 | 32 | 11 | 4 100 | 81 | 51 | 25 | 10 200 | 86 | 63 | 36 | 18 300 | 88 | 69 | 40 | 24

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

1. [1] B9 (a) decreases with increase in temperature (1)
2. [1] B9 (a) reaction is exothermic/increasing temperature favours reaction which absorbs heat (1)
3. [1] B9 (b) increases with increasing pressure (1)
4. [1] B9 (b) increasing pressure causes reaction to go in direction of decreasing number of moles/smaller volume (1)
5. [1] B9 (c) low(er) temperature makes reaction rate too slow (1)
6. [1] B9 (c) high(er) temperature decreases percentage yield (1)
7. [1] B9 (c) low(er) temperature increases percentage yield (1)
8. [1] B9 (c) this temperature (i.e. 350–450) gives a (relatively) high rate and low yield (1)
9. [1] B9 (c) low(er) pressure gives poor yield (1)
10. [1] B9 (c) high(er) pressure increases yield (1)
11. [1] B9 (c) high(er) pressure expends too much energy (1)
12. [1] B9 (c) high a pressure too expensive (1)
13. [1] B9 (c) high(er) pressure gives a higher rate (1)
14. [1] B9 (c) high pressure a safety risk (1)
15. [1] B9 (c) this pressure (i.e. 200–300) gives a high yield and high rate (1)
16. [1] B9 (d) speeds up the reaction/lowers the activation energy (1)
17. [1] B9 (d) lowers energy costs/less energy used (1)
18. [1] B9 (e) molar mass of (NH4)3PO4 = 149 (1)
19. [1] B9 (e) 42/149 × 100 = 28.19%/28.2% (1)