The dimerization equilibrium 2 NO₂(g) ⇌ N₂O₄(g) is established in a gas-tight syringe…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
The dimerization equilibrium
2 NO₂(g) ⇌ N₂O₄(g)
is established in a gas-tight syringe at 298 K. Initially, only NO₂ at a partial pressure of 1.00 atm is present. The system reaches equilibrium after 5 min. At 7 min, an additional pulse of NO₂ is injected so that the total pressure of the mixture instantly doubles to 2.00 atm, while the temperature and volume remain constant.
Analyze the sequence of events that follows the injection.
(a) Immediately after the injection (but before any reaction occurs), compare the forward rate to the reverse rate. Explain your reasoning.
(b) Describe qualitatively how the forward and reverse rates change over time until a new equilibrium is reached. Include a discussion of molecular collisions and changes in partial pressures.
(c) Predict the direction in which the net reaction proceeds and explain how the partial pressures of NO₂ and N₂O₄ evolve on their way to the new equilibrium.
(d) A student asserts, "Because the concentration of NO₂ was permanently doubled, the forward rate will always exceed the reverse rate." Construct a reasoned argument evaluating this assertion, making explicit reference to the definition of equilibrium and the observed constancy of macroscopic properties at long times.