The decomposition of phosphorus pentachloride is represented by PCl5(g) ⇌ PCl3(g) +…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
The decomposition of phosphorus pentachloride is represented by
PCl5(g) ⇌ PCl3(g) + Cl2(g)
A sealed 2.0 L container at constant temperature initially contains 50 PCl5 molecules and no products. The particle counts (which are proportional to molar concentrations) are recorded at two later times:
Time t2: 35 PCl5, 20 PCl3, 20 Cl2
Time t3: 33 PCl5, 21 PCl3, 21 Cl2
A student, believing the reaction has gone to completion, draws a final particulate diagram that shows only PCl3 and Cl2 molecules and no PCl5.
(1) Using the particle counts at t2 and t3, determine whether the system has reached equilibrium by t3. Support your answer with quantitative reasoning.
(2) Calculate an approximate value for Kc at t3, treating the particle counts as directly proportional to concentration in the 2.0 L volume.
(3) Critique the student’s final diagram that omits PCl5 in light of your conclusion in part (a) and the Kc value from part (b).
(4) Propose a revised set of particle counts (total particles equal to 75, the same as the initial total) that would accurately represent the equilibrium mixture. Justify how your proposed diagram is consistent with the calculated equilibrium constant.