For the reversible synthesis of ammonia, N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g)
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
For the reversible synthesis of ammonia,
N₂(g) + 3 H₂(g) ⇌ 2 NH₃(g),
a catalytic reactor is charged with the stoichiometric mixture of N₂ and H₂ at 500 K and total pressure 200 kPa. A process‐control computer records the forward and reverse rates every 10 s. A portion of the data is shown.
Time (s) | Rate forward (kPa·s⁻¹) | Rate reverse (kPa·s⁻¹)
0 | 4.0 | 0.00
10 | 2.8 | 1.05
20 | 2.1 | 1.60
30 | 1.6 | 1.95
40 | 1.3 | 2.15
50 | 1.1 | 2.25
60 | 1.0 | 2.30
70 | 0.98 | 2.31
Assume the trends continue smoothly after 70 s.
(a) Using the tabulated values, estimate to the nearest second the time at which the reactor first reaches equilibrium. Justify your method.
(b) On the same set of axes, sketch smooth curves that represent the complete forward‐ and reverse‐rate functions from 0 s to 90 s. Clearly label both curves and the equilibrium time.
(c) Determine the approximate common value of the forward and reverse rates at equilibrium, expressing your answer to two significant figures in kPa·s⁻¹.
(d) Explain why the common rate value you obtained in part (c) would change if the reactor temperature were raised while keeping total pressure constant.