CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
Phosgene, COCl₂(g), is synthesized from carbon monoxide and chlorine gas via
CO(g) + Cl₂(g) ⇌ COCl₂(g)
A chemist starts with 0.100 mol of CO and 0.100 mol of Cl₂ in a 1.00-L reaction vessel at 600 K and no COCl₂. The chemist records concentration-versus-time data until equilibrium is achieved and notes the following:
• [CO] and [Cl₂] decrease rapidly at first and then level off at 0.032 M.
• [COCl₂] increases from 0 to 0.068 M and then becomes constant.
Answer the following about the system’s approach to equilibrium.
(a) On clearly labeled axes, sketch qualitative curves of the forward rate and the reverse rate as functions of time. Indicate the point at which equilibrium is reached.
(b) Explain, with reference to molecular collisions, why the two rate curves you sketched eventually intersect and then coincide.
(c) Immediately after a catalyst is added at 400 s, both rates are observed to increase but [CO], [Cl₂], and [COCl₂] remain at their equilibrium values. Provide a mechanistic explanation for this observation.
(d) Using the stoichiometry of the reaction, justify how equal forward and reverse rates can keep all three concentrations constant even though one mole of COCl₂ is formed from two separate reactant molecules.