CHEMISTRY · Single correct · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
In a closed 1.0 L vessel, the reaction X(g) ⇌ Y(g) is monitored at constant temperature. At t = 0 s, the forward reaction rate is 0.25 mol·L⁻¹·s⁻¹ and the reverse reaction rate is 0.10 mol·L⁻¹·s⁻¹. After 20 s, both rates are 0.13 mol·L⁻¹·s⁻¹. Which statement most accurately describes the changes in concentration of X and Y during the first 20 s?
(a) [X] increases while [Y] decreases until equilibrium is reached at approximately 20 s.
(b) [X] decreases and [Y] increases until both concentrations become constant at approximately 20 s.
(c) Both [X] and [Y] remain essentially constant during the 20 s; only the reaction rates change until they match.
(d) [X] decreases and [Y] increases until about 20 s, when both reactions stop because equilibrium has been achieved.