A student monitors the forward and reverse rates for the reversible reaction CO(g) +…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
A student monitors the forward and reverse rates for the reversible reaction
CO(g) + Cl₂(g) ⇌ COCl₂(g)
inside a sealed, rigid container at 450 K. The student uses a pressure sensor connected to software that continuously calculates instantaneous rates (in kPa·s⁻¹) for both directions.
At t = 0 s, only CO and Cl₂ are present, each at a partial pressure of 30 kPa. The software reports the following qualitative behavior:
• In the first 25 s the forward rate decreases sharply while the reverse rate increases from zero.
• Between 25 s and 70 s both rates change more gradually.
• After about 70 s both rates are constant and numerically equal within the precision of the sensor.
Assume temperature remains constant and no gases escape the container.
(a) On clearly labeled axes, sketch qualitative curves of forward rate and reverse rate as a function of time from t = 0 s to t = 100 s. Indicate the approximate time at which equilibrium is first attained.
(b) Describe how the shapes of the two curves illustrate that equilibrium is a dynamic rather than a static state.
(c) Explain, at the particulate level, why the forward rate decreases while the reverse rate increases over the first 25 s.
(d) The student claims that the constant value of each rate after 70 s proves that all molecular collisions stop at equilibrium. Refute this claim using evidence from the graph you sketched.