A sealed 1.0 L reaction vessel initially contains only colorless gas A at 800 K
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
A sealed 1.0 L reaction vessel initially contains only colorless gas A at 800 K. The gas slowly decomposes to form gases B and C according to the balanced equation A ⇌ B + C.
A chemist records the total pressure in the vessel every minute for 20 minutes. The pressure readings (in kPa) are:
0 min — 101.3
5 min — 115.4
10 min — 123.8
15 min — 124.0
20 min — 124.0
Assume the temperature remains constant and the gases behave ideally.
(1) Using the pressure data, identify the earliest time at which the system can reasonably be considered to have reached equilibrium. Cite specific evidence from the data.
(2) Explain, in terms of molecular-level events, why the pressure no longer changes after this time even though the decomposition reaction continues to occur.
(3) Suppose the temperature of the vessel is suddenly increased to 850 K at 20 min while the volume remains constant. Predict qualitatively how (i) the total pressure and (ii) the relative rates of the forward and reverse reactions will change immediately after the temperature jump, and justify each prediction.