The gas-phase decomposition of dinitrogen pentoxide was studied in a rigid, 2.00 L…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
The gas-phase decomposition of dinitrogen pentoxide was studied in a rigid, 2.00 L flask at 45 °C.
2 N2O5(g)→4NO2(g)+O2(g)
The total pressure inside the flask was monitored, and selected values are provided below.
Time (s)
Total Pressure (kPa)
0
48.0
120
81.6
240
94.5
360
101
480
104
600
105
720
105
At 720 s a stopcock is opened briefly, allowing some of the gas mixture to escape rapidly into the laboratory atmosphere (1 atm, 101 kPa), after which the stopcock is closed and the flask is left to stand at 45 °C.
(1) Part 1: (a) Using the data in the table, determine whether the system had reached a dynamic equilibrium by 600 s. Cite specific evidence from the pressure measurements.
(2) Part 2: (b) Classify each of the following individual operations as reversible or irreversible under the stated conditions and defend your choice with chemical reasoning.
(i) The gas-phase decomposition of N2O5 occurring inside the closed flask.
(ii) The rapid release of gas when the stopcock is opened to the laboratory atmosphere.
(3) Part 3: (c) After the stopcock is closed at 720 s, predict qualitatively how the total pressure inside the flask will change with time. Explain your reasoning in terms of forward and reverse processes.
(4) Part 4: (d) Suppose instead that the stopcock is connected to a second evacuated flask at 45 °C that has the same volume as the reaction flask, and the stopcock is opened very slowly so that the pressure difference across the valve never exceeds 0.10 kPa at any moment. Describe how this alternative procedure would influence the reversibility of the gas transfer compared with the rapid venting described in part (b)(ii).