CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
For the gas-phase reaction D₂ ⇌ 2D in a rigid, temperature-controlled container, a student monitors the forward and reverse reaction rates by sampling the mixture at equal time intervals. The measured rates are shown in the table.
Time (s) | Rate_fwd (mol L⁻¹ s⁻¹) | Rate_rev (mol L⁻¹ s⁻¹)
0 | 4.0 | 0.0
20 | 2.5 | 1.0
40 | 1.6 | 1.6
60 | 1.6 | 1.6
80 | 1.6 | 1.6
(1) Determine the time at which dynamic equilibrium is first established. Explain your reasoning with reference to the data.
(2) At 40 s the concentration of D₂ is measured to be 0.20 mol L⁻¹. Without performing any additional experiments, determine the concentration of D at the same moment, and justify your answer using stoichiometric reasoning.
(3) Immediately after 80 s, the volume of the container is halved at constant temperature. Predict qualitatively how the forward and reverse reaction rates will change relative to the values at 80 s. Explain your reasoning in terms of collision frequency.