A closed container initially holds only species A at a concentration of 1.00 M. The reversible reaction A ⇌ B proceeds at constant temperature. A researcher monitors the system and records the following qualitative description of a concentration–versus–time graph.
• At t = 0 min, [A] = 1.00 M and [B] = 0 M.
• [A] decreases smoothly, while [B] increases correspondingly.
• Both curves become flat and remain horizontal after roughly 8–10 min; the plateau values are [A] ≈ 0.40 M and [B] ≈ 0.60 M.
Use this information to analyze the behaviour of the system.
(a) Estimate the time interval during which the system can be considered to have reached equilibrium. Describe the specific feature of the graph that justifies your estimate.
(b) Explain, in terms of the relative magnitudes of the forward and reverse reaction rates, why the concentrations remain constant after equilibrium is attained even though the reaction is reversible.
(c) On the particulate (molecular) level, describe what is happening to molecules of A and B in the system once the plateaus have been reached.
(d) Suppose the same reaction were repeated at a higher temperature. Predict qualitatively how the time required to reach equilibrium would change, assuming all other conditions remain identical, and provide a chemical explanation for your prediction.