A sealed reaction vessel contains the system C(g) ⇌ D(g) + E(g) at constant temperature
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 reaction vessel contains the system C(g) ⇌ D(g) + E(g) at constant temperature. A chemist records the concentrations of all three species every minute for 30 min. A concise verbal description of the resulting concentration–versus–time curves is provided.
• From t = 0 min to t = 10 min, [C] falls from 0.30 M to 0.20 M, while [D] and [E] each rise from 0.20 M to 0.35 M.
• Between t = 10 min and t = 15 min, all concentrations remain constant.
• At t = 15 min, a sudden spike in [E] to 0.50 M is observed; [C] and [D] are unaffected instantaneously.
• After t = 15 min, [C] slowly increases to 0.25 M, [D] decreases to 0.30 M, and [E] decreases to 0.40 M, after which all concentrations remain steady.
Use these observations to answer the following.
(a) Identify the two distinct time intervals during which the system is at equilibrium, and justify your choices based on the behaviour of the concentration curves.
(b) Explain, without invoking any mathematical expressions, why the equilibrium shifts in the direction observed after the spike in [E].
(c) Calculate the change in concentration of C(g) between the first and second equilibrium positions, and relate this change quantitatively to the stoichiometry of the balanced reaction.
(d) Predict qualitatively how the magnitude of the concentration change for D(g) would differ if the reaction had stoichiometry C(g) ⇌ 2 D(g) + E(g) instead, assuming the same disturbance in [E] at t = 15 min. Provide a brief justification based on mole ratios.